Pre main Sequence Stars

We present a model of a dusty disk with an inner hole which accounts for the Spitzer Space Telescope Infrared Spectrograph observations of the low-mass pre-main sequence star CoKu Tau/4. We have modeled the mid-IR spectrum (between 8 and 25 µm) as arising from the inner wall of a disk. Our model disk has an evacuated inner zone of radius ∼ 10 AU, with a dusty inner "wall", of half-height ∼ 2 AU, that is illuminated at normal incidence by the central star. The radiative equilibrium temperature decreases from the inner disk edge outward through the optically-thick disk; this temperature gradient is responsible for the emission of the silicate bands at 10 and 20 µm. The observed spectrum is consistent with being produced by Fe-Mg amorphous glassy olivine and/or pyroxene, with no evidence of a crystalline component. The mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older star TW Hya, where it has been suggested that the significant clearing of its inner disk is due to planet formation. However, no inner disk remains in CoKu Tau/4, consistent with the star being a weak-emission (non-accreting) T Tauri star. The relative youth of CoKu Tau/4 (∼ 1 Myr) may indicate much more rapid planet formation than typically assumed.

We present an analysis of the classical T Tauri star RW Aur A, based on 77 echelle spectra obtained at Lick Observatory over a decade of observations. RW Aur, which has a higher than average mass accretion rate among T Tauri stars, exhibits permitted (Hα, Hβ, Ca II, He I, NaD) and forbidden ([OI]6300Å) emission lines with strong variability. The permitted lines display multiple periodicities over the years, often with variable accretion (redshifted) and outflow (blueshifted) absorption components, implying that both processes are active and changing in this system. The broad components of the different emission lines exhibit correlated behavior, indicating a common origin for all of them. We compute simple magnetospheric accretion and disk-wind Hα, Hβ and NaD line profiles for RW Aur. The observed Balmer emission lines do not have magnetospheric accretion line profiles. Our modeling indicates that the wind contribution to these line profiles is very important and must be taken into account. Our results indicate that the Hα, Hβ and NaD observed line profiles of RW Aur are better reproduced by collimated disk-winds starting from a small region near the disk inner radius. Calculations were performed in a region extending out to 100 R . Within this volume, extended winds originating over many stellar radii along the disk are not able to reproduce the three lines simultaneously. Strongly open-angled winds also generate profiles that do not look like the observed ones. We also see evidence that the outflow process is highly dynamic -the low-and high-velocity components of the [OI](6300Å) line vary independently on timescales of days. The apparent disappearance from December 1999 to December 2000 of the [OI](6300Å) low velocity component, which is thought to come from the disk-wind, shows that the the slow wind can exhibit dramatic variability on timescales of months (placing limits on how extended it can be). There is no comprehensive explanation yet for the behavior of RW Aur, which may in part be due to complications that would be introduced if it is actually a close binary.

Period changes of Delta Scuti stars have been collected or redetermined from the available observations and are compared with values computed from evolutionary models with and without convective core overshooting. For the radial pulsators of Pop. I, the observations indicate (1/P) dP/dt values around 10^-7 year^-1 with equal distribution between period increases and decreases. The evolutionary models, on the other hand, predict that the vast majority should show increasing periods. This increase should be a factor of about ten times smaller than observed. For nonradial Delta Scuti pulsators of Pop. I, the discrepancies are even larger. The behavior suggests that for these relatively unevolved stars the rate of evolution cannot be deduced from the period changes. The period changes of most Pop. II Delta Scuti (SX Phe) stars are characterized by sudden jumps of the order of Delta P/P approx. 10^-6. However, at least one star, BL Cam, shows a large, continuous period increase. The variety of observed behavior also seems to exclude an evolutionary origin of the changes. Model calculations show that the evolutionary period changes of pre-MS Delta Scuti stars are a factor of 10 to 100 larger than those of MS stars. Detailed studies of selected pre-MS Delta Scuti stars are suggested.

We have investigated the infrared spectra of all 46 Herbig Ae/Be stars for which spectroscopic data is available in the ISO data archive. Our quantitative analysis of these spectra focusses on the emission bands linked to polycyclic aromatic hydrocarbons (PAHs), the amorphous 10 micron silicate band and the crystalline silicate band at 11.3 micron. We have detected PAH emission in 57% of the Herbig stars in our sample. Clear examples of differences in the PAH spectra are present within our sample, indicating differences in PAH size, chemistry and/or ionization. Amorphous silicate emission was detected in the spectra of 52% of the sample stars, amorphous silicate absorption in 13%. We have detected crystalline silicate emission in 11 stars (24% of our sample), of which four (9%) also display strong PAH emission. We have classified the sample sources according to the strength of their mid-IR energy distribution. The systems with stronger mid-infared (20-100 um) excesses relative to their near-infrared (1-5 um) excess display significantly more PAH emission than those with weaker mid-infrared excesses. This provides strong observational support for the disk models by Dullemond (2002), in which systems with a flaring disk geometry display a strong mid-infrared excess, whereas those with disks that are strongly shadowed by the puffed-up inner rim of the disk only display modest amounts of mid-infrared emission. The PAH emission is expected to be produced mainly in the part of the disk atmosphere that is directly exposed to radiation from the central star. In this model, self-shadowed disks should display weaker PAH emission than flared disks, consistent with our observations.

We present optical lightcurves of 428 periodic variable stars coincident with ROSAT X-ray sources, detected using the first run of the SuperWASP photometric survey. Only 68 of these were previously recognised as periodic variables. A further 30 of these objects are previously known pre-main sequence stars, for which we detect a modulation period for the first time. Amongst the newly identified periodic variables, many appear to be close eclipsing binaries, their X-ray emission is presumably the result of RS CVn type behaviour. Others are probably BY Dra stars, pre-main sequence stars and other rapid rotators displaying enhanced coronal activity. A number of previously catalogued pulsating variables (RR Lyr stars and Cepheids) coincident with X-ray sources are also seen, but we show that these are likely to be misclassifications. We identify four objects which are probable low mass eclipsing binary stars, based on their very red colour and light curve morphology.

The aim of this paper is to characterise the star formation activity in the poorly studied embedded cluster Serpens/G3-G6, located ∼ 45 ′ (3 pc) to the south of the Serpens Cloud Core, and to determine the luminosity and mass functions of its population of Young Stellar Objects (YSOs). Multi-wavelength broadband photometry was obtained to sample the near and mid-IR spectral energy distributions to separate YSOs from field stars and classify the YSO evolutionary stage. ISOCAM mapping in the two filters LW2 (5-8.5 µm) and LW3 (12-18 µm) of a 19 ′ × 16 ′ field was combined with JHK S data from 2MASS, K S data from Arnica/NOT, and L ′ data from SIRCA/NOT. Continuum emission at 1.3 mm (IRAM) and 3.6 cm (VLA) was mapped to study the cloud structure and the coldest/youngest sources. Deep narrow band imaging at the 2.12 µm S(1) line of H 2 from NOTCam/NOT was obtained to search for signs of bipolar outflows. We have strong evidence for a stellar population of 31 Class II sources, 5 flat-spectrum sources, 5 Class I sources, and two Class 0 sources. Our method does not sample the Class III sources. The cloud is composed of two main dense clumps aligned along a ridge over ∼ 0.5 pc plus a starless core coinciding with absorption features seen in the ISOCAM maps. We find two S-shaped bipolar collimated flows embedded in the NE clump, and propose the two driving sources to be a Class 0 candidate (MMS3) and a double Class I (MMS2). For the Class II population we find a best age of ∼ 2 Myr and compatibility with recent Initial Mass Functions (IMFs) by comparing the observed Class II luminosity function (LF), which is complete to 0.08 L ⊙ , to various model LFs with different star formation scenarios and input IMFs.

Aims. We obtain accurate estimates for the individual masses of the components of the tight binary system T Tau S to settle the ongoing debate on the nature of T Tau Sa, a so-called infrared companion. Methods. We take advantage of the fact that T Tau S belongs to a triple system composed of two hierarchical orbits to simultaneously analyze the motion of T Tau Sb in the rest frames of T Tau Sa and T Tau N. With this method, it is possible to pinpoint the location of the center of mass of T Tau S and, thereby, to determine individual masses for T Tau Sa and T Tau Sb with no prior assumption about the mass/flux ratio of the system. This improvement over previous studies of the system results in much better constraints on orbital parameters. Results. We find individual masses of 2.73 ± 0.31 M for T Tau Sa and of 0.61 ± 0.17 M for T Tau Sb (in agreement with its early-M spectral type), including the uncertainty on the distance to the system. These are among the most precise estimates of the mass of any Pre-Main Sequence star, a remarkable result since this is the first system in which individual masses of T Tauri stars can be determined from astrometry only. This model-independent analysis confirms that T Tau Sa is an intermediate-mass star, presumably a very young Herbig Ae star, that may possess an almost edge-on disk.

Variable Star Network (VSNET) is a global professional-amateur network of researchers in variable stars and related objects, particularly in transient objects, such as cataclysmic variables, black hole binaries, supernovae and gamma-ray bursts. The VSNET has been playing a pioneering role in establishing the field of transient object astronomy, by effectively incorporating modern advance in observational astronomy and global electronic network, as well as collaborative progress in theoretical astronomy and astronomical computing. The VSNET is now one of the best-featured global networks in this field of astronomy. We review on the historical progress, design concept, associated technology, and a wealth of scientific achievements powered by the VSNET.

We present an analytic calculation of the thermonuclear depletion of lithium in contracting, fully convective, pre-main sequence stars of mass M ∼ < 0.5M ⊙ . Previous numerical work relies on still-uncertain physics (atmospheric opacities and convection, in particular) to calculate the effective temperature as a unique function of stellar mass. We assume that the star's effective temperature, T eff , is fixed during Hayashi contraction and allow its actual value to be a free parameter constrained by observation. Using this approximation, we compute lithium burning analytically and explore the dependence of lithium depletion on T eff , M , and composition. Our calculations yield the radius, age, and luminosity of a pre-main sequence star as a function of lithium depletion. This allows for more direct comparisons to observations of lithium depleted stars. Our results agree with those numerical calculations that explicitly determine stellar structure during Hayashi contraction. In agreement with , we show that the absence of lithium in the Pleiades star HHJ 3 implies that it is older than 100 Myr. We also suggest a generalized method for dating galactic clusters younger than 100 Myr (i.e., those with contracting stars of M ∼ > 0.08M ⊙ ) and for constraining the masses of lithium depleted stars.

We present pre- and post-outburst observations of the new FU Orionis-like young stellar object PTF 10qpf (also known as LkHa 188-G4 and HBC 722). Prior to this outburst, LkHa 188-G4 was classified as a classical T Tauri star on the basis of its optical emission-line spectrum superposed on a K8-type photosphere, and its photometric variability. The mid-infrared spectral index of LkHa 188-G4 indicates a Class II-type object. LkHa 188-G4 exhibited a steady rise by ~1 mag over ~11 months starting in Aug. 2009, before a subsequent more abrupt rise of > 3 mag on a time scale of ~2 months. Observations taken during the eruption exhibit the defining characteristics of FU Orionis variables: (i) an increase in brightness by > 4 mag, (ii) a bright optical/near-infrared reflection nebula appeared, (iii) optical spectra are consistent with a G supergiant and dominated by absorption lines, the only exception being Halpha which is characterized by a P Cygni profile, (iv) near-infrared spectra resemble those of late K--M giants/supergiants with enhanced absorption seen in the molecular bands of CO and H_2O, and (v) outflow signatures in H and He are seen in the form of blueshifted absorption profiles. LkHa 188-G4 is the first member of the FU Orionis-like class with a well-sampled optical to mid-infrared spectral energy distribution in the pre-outburst phase. The association of the PTF 10qpf outburst with the previously identified classical T Tauri star LkHa 188-G4 (HBC 722) provides strong evidence that FU Orionis-like eruptions represent periods of enhanced disk accretion and outflow, likely triggered by instabilities in the disk. The early identification of PTF 10qpf as an FU Orionis-like variable will enable detailed photometric and spectroscopic observations during its post-outburst evolution for comparison with other known outbursting objects.

Aims. We present the first results of an ambitious ground-based observation programme conducted on 1-4 meter class telescopes. Our sample consists of 1097 active and presumably young stars, all of them being optical (Tycho Catalogue) counterparts of ROSAT All-Sky Survey X-ray sources in the northern hemisphere. In this paper, we concentrate on the optically brightest (V T 9. m 5) candidates (704 objects). We acquired high-resolution optical spectroscopy in the Hα and/or lithium spectral regions for 426 of such stars without relevant data in the literature. We describe the star sample and the observations and we start to discuss the physical properties of the investigated stars. Methods. We used a cross-correlation technique and other tools developed by us to derive accurate radial and rotational velocities and to perform an automatic spectral classification for both single stars and double-lined systems. The spectral subtraction technique was used to derive chromospheric activity levels and lithium abundances. We estimated the fraction of young single stars and multiple systems in stellar soft X-ray surveys and the contamination by more evolved systems, like RS CVn binaries. We classified stars on the basis of their lithium abundance and give a glimpse of their sky distribution. Results. The sample appears to be a mixture of quite young Pleiades-like and Hyades-like stars plus an older lithium-poor population probably born within the last 1-2 Gyr. Seven stars with a lithium abundance compatible with the age of IC 2602 (about 30 Myr) or even younger were detected as well, although two appear to be lithium-rich giants.The discovery of a large number of highly or moderately lithium-rich giants is another outcome of the present survey.

Context. The open cluster NGC 1893, illuminating the H  region IC 410, contains a moderately large population of O-type stars and is one of the youngest clusters observable in the optical range. It is suspected to harbour a large population of pre-main-sequence (PMS) stars. Aims. We have probed the stellar population of NGC 1893 in an attempt to determine its size and extent. In particular, we look for signs of sequential star formation. Methods. We classify a large sample of cluster members with new intermediate resolution spectroscopy. We use Hα slitless spectroscopy of the field to search for emission line objects, identifying 18 emission-line PMS stars. We then combine existing optical photometry with 2MASS JHK S photometry to detect stars with infrared excesses, finding close to 20 more PMS candidates.

We present an optical/infrared study of the dense molecular cloud, L935, dubbed "The Gulf of Mexico", which separates the North America and the Pelican nebulae, and we demonstrate that this area is a very active star forming region. A wide-field imaging study with interference filters has revealed 35 new Herbig-Haro objects in the Gulf of Mexico. A grism survey has identified 41 Hα emissionline stars, 30 of them new. A small cluster of partly embedded pre-main sequence stars is located around the known LkHα 185-189 group of stars, which includes the recently erupting FUor HBC 722.

In the quest of understanding how star formation occurs and propagates in the low metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (∼V), and F814W (∼I) HST/ACS images of the young and massive star forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ≈10 Gyr in the past until ≈ 150 Myr in the field of the SMC. The youngest stellar population (∼ 3 ± 1 Myr) is associated with the NGC 346 cluster. It includes a rich component of low mass pre-main sequence stars mainly concentrated in a number of sub-clusters, spatially colocated with CO clumps previously detected by Rubio et al. (2000). Within our analysis uncertainties, these sub-clusters appear coeval with each other. The most massive stars appear concentrated in the central sub-clusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate age star cluster, BS90, formed ∼ 4.3 ± 0.1 Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.

We report the serendipitous discovery of a population of low mass, pre-main sequence stars (PMS) in the direction of the Wolf-Rayet/O-star binary system γ 2 Vel and the Vela OB2 association. We argue that γ 2 Vel and the low mass stars are truly associated, are approximately coeval and that both are at distances between 360-490 pc, disagreeing at the 2σ level with the recent Hipparcos parallax of γ 2 Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the γ 2 Vel system, but also offer an exciting opportunity to investigate the influence of high mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.

The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) is a large program designed to systematically investigate the X-ray properties of young stellar/substellar objects in the Taurus star forming region. This paper summarizes selected key findings from XEST.

We present the results of a multi-technique investigation of the M4.5Ve flare star AP Col, which we discover to be the nearest pre-main-sequence star. These include astrometric data from the CTIO 0.9m, from which we derive a proper motion of 342.0±0.5 mas yr −1 , a trigonometric parallax of 119.21±0.98 mas (8.39±0.07 pc), and photometry and photometric variability at optical wavelengths. We also provide spectroscopic data, including radial velocity (22.4±0.3 km s −1 ), lithium Equivalent Width (EW) (0.28±0.02Å), Hα EW (−6.0 to −35Å), vsini (11 ± 1 km s −1 ), and gravity indicators from the Siding Spring 2.3-m WiFeS, Lick 3-m Hamilton echelle, and Keck-I HIRES echelle spectrographs. The combined observations demonstrate that AP Col is the closer of only two known systems within 10 pc of the Sun younger than 100 Myr. Given its space motion and apparent age of 12-50 Myr, AP Col is likely a member of the recently proposed ∼40 Myr old Argus/IC 2391 association. 1 www.recons.org 2 The central wavelengths for V J , R KC , and I KC are 5475, 6425, and 8075Å, respectively. The subscript

Context. NGC 6611 and its parental cloud, the Eagle Nebula (M16), are well-studied star-forming regions, thanks to their large content of both OB stars and stars with disks and the observed ongoing star formation. In our previous studies of the Eagle Nebula, we identified 834 disk-bearing stars associated with the cloud, after detecting their excesses in NIR bands from J band to 8.0µm. Aims. In this paper, we study in detail the nature of a subsample of disk-bearing stars that show peculiar characteristics. They appear older than the other members in the V vs. V − I diagram, and/or they have one or more IRAC colors at pure photospheric values, despite showing NIR excesses, when optical and infrared colors are compared. Methods. We confirm the membership of these stars to M16 by a spectroscopic analysis. The physical properties of these stars with disks are studied by comparing their spectral energy distributions (SEDs) with the SEDs predicted by models of T-Tauri stars with disks and envelopes. Results. We show that the age of these stars estimated from the V vs. V − I diagram is unreliable since their V − I colors are altered by the light scattered by the disk into the line of sight. Only in a few cases their SEDs are compatible with models with excesses in V band caused by optical veiling. Candidate members with disks and photospheric IRAC colors are selected by the used NIR disk diagnostic, which is sensitive to moderate excesses, such as those produced by disks with low masses. In 1/3 of these cases, scattering of stellar flux by the disks can also be invoked. Conclusions. The photospheric light scattered by the disk grains into the line of sight can affect the derivation of physical parameters of Class II stars from photometric optical and NIR data. Besides, the disks diagnostic we defined are useful for selecting stars with disks, even those with moderate excesses or whose optical colors are altered by veiling or photospheric scattered light.

We present broad band spectra of a sample of 21 low luminosity sources in the Trapezium Cluster, with masses in the range 0.008 -0.10 M ⊙ (assuming an age of 1 Myr). These were selected for low extinction in most cases and are located west of the brighter nebulosity. The spectra are in the H bandpass (1.4-1.95 µm) and K bandpass (1.9-2.5 µm) also for most of the brighter sources, with a resolution of 50 nm. They were taken with the United Kingdom Infrared Telescope (UKIRT) using the CGS4 spectrometer. Absorption by water vapour bands is detected in all the substellar candidates except one, which is a highly reddened object with strong H 2 emission and an anomalously blue (I-J) colour, implying that it is a very young cluster member with circumstellar matter. The observation of prominent water vapour bands confirms the low Effective Temperatures implied by our (I-J) colour measurements in an earlier paper and would imply late M or L spectral types if these were older field dwarfs. However, the profiles of the H bandpass spectra are very different from those of field dwarfs with similar water absorption strength, demonstrating that they are not foreground or background objects. In addition, the CO absorption bands at 2.3 µm and the NaI absorption feature at 2.21 µm are very weak for such cool sources. All these features are quite well reproduced by the AMES-Dusty-1999 model atmospheres of , and arise from the much lower gravities predicted for the Trapezium sources (3.5 < log(g) < 4.0) compared to evolved objects (log g ∼ 5.5), This represents a new proof of the substellar status of our sources, independent of the statistical arguments for low contamination, which are reexamined here. The very late spectral types of the planetary mass objects and very low mass brown dwarfs demonstrate that they are cluster members, since they are too luminous to be field dwarfs in the background. We also present additional UKIRT photometry of a small region in the south of the Trapezium cluster where the extinction and nebular brightness are low, which permitted the detection of objects with 1 Myr masses slightly lower than our previous least massive source at 8 M Jup . Following a minor update to our previous J band photometry, due to a new UKIRT filter calibration, there are ∼ 15 planetary mass candidates in the full dataset.

Cool magnetohydrodynamics (MHD) disc wind physics is reviewed by means of a self-similar analytical model, putting special emphasis on the mathematical aspects of the solution. It is found that the key parameter of the theory (μ) measures the relation between magnetic and tidal forces. The generation of MHD winds from accretion discs requires a subtle tuning between both stresses because only a narrow range of μ values is allowed; this range is, indeed, close to the cut-off of the magnetic turbulence induced by the development of the Balbus-Hawley instability. The space of solutions can be separated into two quite distinct classes: low-μsolutions generate magnetically dominated outflows and display a characteristic density change from horizontal to vertical stratification, while in high-μsolutions the density decreases without any intermediate enhancement as the rotation axis is approached. These theoretical (dynamical) results have been used to study the properties of the base of the wind. Density and velocity laws have been derived directly from the dynamics. The effect of the propagation of the stellar X-ray radiation through the wind has been analysed to determine the temperature law at the base of the wind (polar angles θ > 45°). It is shown that a cocoon of photoionized gas is generated around the star. The extent of the photoionized region is small (tenths of au) in dense outflows and close to the disc plane; however, it may cover the whole wind extent in diffuse winds, e.g. disc winds generated by small accretion rates (<=10-9 Msolar yr-1). Photoionization also modifies the electron density in the plasma. As a consequence, the ambipolar diffusion heating decreases in the inner part of the wind by roughly one order of magnitude with respect to that derived by other authors. In fact, radiative heating controls the thermal properties of the inner 0.3 and 1 au of the disc wind for accretion rates of 10-7 and 10-8 Msolar yr-1, respectively. The temperature of the densest region (base) of the wind is, at most, ~=10 000 K. Therefore, although densities as high as ~109 cm-3 can be achieved by disc winds, the temperature is significantly smaller than the ~5 × 105-8 × 105 K derived from the ultraviolet (UV) observations of the base of the optical jets. Also, it is shown that densities as high as ~109 cm-3 cannot be achieved at the jet recollimation point for the accretion rates observed in the T Tauri stars. In summary, we conclude that the flow traced by the UV semiforbidden lines is not associated with cold disc winds but, most likely, it is tracing the hot inner jet, postulated in cold disc wind theory, which prevents the radial collapse of the wind.

Aims: By using kinematical information, we test the membership of the new Lupus candidate members proposed by the Cores to Disks (c2d) Spitzer Legacy Program program and by a complementary optical survey. We also investigate the relationship between the proper motions (pm) and other properties, in order to get some clues about their formation and early evolution. Methods: We compiled a list of members and possible members of Lupus 1, 3, and 4, together with all information on their spectral types, disks, and physical parameters. Using VO-tools, we cross-matched this list with the astrometric catalogues to get pm. Our final sample contains sources with magnitudes I<16mag and estimated masses >~0.1Msun. Results: According to the kinematic information, our sources can be divided into two main groups. The first one contains sources with higher pm in agreement with other Gould Belt populations and with spatial distribution, optical and near-infrared colours, and disk composition consistent with these objects belonging to the Lupus clouds. In the second group, sources have lower pm with random orientations, and they are mostly located outside the cloud cores, making their association with the Lupus complex more doubtful. We investigate the properties of the higher pm group, but cannot find any correlations with spatial location, binarity, the presence of a circumstellar disk, or with physical properties such as effective temperature, luminosity, mass, or age. Conclusions: We conclude that the lower pm group probably represents a background population or mixture of populations unrelated to the Lupus clouds. The higher pm group, on the other hand, has properties consistent with it being a genuine population of the Lupus star-forming region. More accurate pm and/or radial velocity information are required for a more detailed study of the kinematic properties of the Lupus stellar members.

We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 micron data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (Ic) and/or near-infrared (JKs) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence (PMS) eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 & 4.5 micron variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009, and we highlight our light curves for AA-Tau analogs - YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.

HH 30 in Taurus has been imaged with the Hubble Space Telescope WFPC2. The images show in reflected light a flared disk with a radius of about 250 AU that obscures the protostar. The disk resembles detailed accretion disk models that constrain its density distribution and show that its inclination is less than 100. There are bipolar emission-line jets perpendicular to the disk, a very clear demonstration of the standard paradigm for accretion disk and jet systems. However, asymmetries in the light distribution show that the disk has not completely settled into a quasi-equilibrium accretion state, or that some of the observed scattering is from an asymmetric envelope. The emission-line jet itself is resolved into a number of knots with typical lengths and separations of 0".4, much smaller and more numerous than indicated by lower resolution ground-based studies. There are indications of still finer structures in the jet all the way to the resolution limit of 0".1. The knots have proper motions ranging from 100 to 300 km s-1 and are therefore generated at the surprisingly high rate of about 0.4 knots per jet per year. The jet appears to be collimated within a cone of opening angle 3° and can be seen to within 30 AU of the star. Both single- and multiple-scattering disk models have a range of possible solutions, but by requiring pressure support and temperature equilibrium, a self-consistent model emerges. There is evidence for pressure support because the disk appears to have a Gaussian height profile. The temperature at each point in the disk is determined by the disk geometry, which in turn fixes the temperature in a self- consistent manner. The extinction to the protostar is unknown but constrained to be greater than 24 mag. The optical properties of the scattering grains in the disk are determined and found to imply a large scattering asymmetry, but they seem to follow the interstellar reddening law. The absolute magnitude and colors of the unseen protostar, which has a brightness in the I bandpass of about 0.16 times solar and is very red, are obtained. The disk mass is about 0.006 times solar and has an expected lifetime of about 105 yr.

We report differential I-band and J-band photometry of S Ori 45, a cool (spectral type M8.5), young (1-8 Myr) brown dwarf of the σ Orionis cluster with a likely mass estimated at around 20 times the mass of Jupiter. We detect variability (amplitudes ranging from 34 to 81 mmag) and observe a modulation at a period of 2.5-3.6 h in both optical and near-infrared light curves. The most recent optical data set, however, presents a modulation at the very short period of 46.4 ± 1.5 min, which remains a mystery. The origin of the 2.5-3.6 h modulation is analized in terms of various scenarios: inhomogeneous features (dust clouds or magnetically induced dark spots) co-rotating with the object's surface, and presence of an unseen very low-mass companion that is steadily transferring mass to the primary. Because of the very young age of the object and its persistent strong Hα emission, the possible presence of an accreting disk is also discussed. If the period of a few hours is related to rotation, our results suggest that σ Orionis low-mass brown dwarfs are rotating faster than more massive cluster brown dwarfs at a rate consistent with their theoretically inferred masses and radii, implying that all of these objects have undergone similar angular momentum evolution.

Multiple-epoch phase-referenced VLBI observations of 11 radio-emitting stars have been conducted as part of an astrometric program to link the Hipparcos optical reference frame to the radio extragalactic reference frame. We present the VLBI positions, proper motions and trigonometric parallaxes from this program in the ICRF (International Celestial Reference Frame). These astrometric parameters are absolute because they are directly measured relative to the distant quasars used as VLBI phase reference calibrators. The mean astrometric precision achieved relative to the calibrators is 0.36 milliarcsecond and the highest precision is for the RS CVn close binary σ 2 CrB with formal uncertainties of 0.12 milliarcsecond for its relative position, 0.05 milliarcsecond for its annual proper motion and 0.10 milliarcsecond for its trigonometric parallax.

In January 2003, the Chandra Orion Ultradeep Project (COUP) detected about 1400 young stars during a 13.2 day observation of the Orion Nebula Cluster (ONC). This paper is a study of the X-ray properties of a well-defined sample of 28 solar-mass ONC stars based on COUP data. Our goals are to characterize

At a distance of 450 pc OMC2/OMC3 is one of the most active sites of ongoing star formation known. Twenty-one protostellar objects have been detected in this region by sub-millimeter surveys (Chini et al. 1997) suggesting that it may be at the early stages of cluster formation. We present a progress report of a detailed investigation into the young pre-main

We present a detailed study of the young T Tauri star Par 1724, located 15 arc min north of the Trapezium cluster in Orion. Our extensive VRI photometric measurements confirm the rotational period to be 5.7 days. Repeated high-resolution spectra show variability in the radial velocity with the same period. A Doppler imaging analysis based on high-S/N highresolution spectra yields an image showing a pronounced dark feature (spot) at relatively low latitude, which is responsible for most or all of the observed variability. Our high-resolution spectra yield a rotational velocity of v • sin i 71 km s −1 , a surface gravity of log g 3, and a mean heliocentric radial velocity of ∼ 23 km s −1 , the latter being consistent with membership to the Orion association. The equivalent width of the lithium 6708Å line is variable, consistent with rotational modulation. The line is stronger when the spot is on the front side; the lithium abundance observed when the spot is on the back side is consistent with the primordial value. Many ROSAT X-ray observations show that Par 1724 is a strong and variable X-ray source. It has shown one of the most powerful X-ray flares. Our deep infrared Send offprint requests to: Ralph Neuhäuser Partly based on observations with the Multiple Mirror Telescope, a joint facility of the Smithsonian Institution and the University of Arizona; the 2.1m telescope at McDonald Observatory; the European Southern Observatory 3.6m, 1.52m, and 0.91m telescopes under programs 57.E-0250, 57.E-0646, and 56.E-0601; the Calar Alto Observatory 3.5m, 2.2m, and 1.2m telescopes; and the Cerro Tololo Inter-American and Kitt Peak National Observatories, NOAO, which are operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.

Aims. The main purpose of our investigation is to construct the long-time light curve of the PMS star V 1184 Tau. We consider the study of the photometrical variability of PMS stars as very important for understanding stellar evolution. The unusual photometric variability of V 1184 Tau was reported in previous studies as well, but the nature of the observed deep minima is still under discussion. Methods. We present recent data from CCD photometry and from archival photographic plates. The photometric BVRI data presented in this paper were collected from November 2005 to November 2007. To construct the historical light curve of V 1184 Tau, a search for archived photographic observations in the Wide-Field Plate Database (WFPDB) was made. As a result, 412 photographic plates were found containing the field of V 1184 Tau. A part of the plates were scanned at our request and a magnitude estimation was made of V 1184 Tau. Results. Our recent photometric data suggest that the star brightness is still near the minimum. Thus the period of strong light variations that started in 2003 continues up to the present, hence more than 4 years. Our data from the archival photographic plates suggest that an unknown minimum of brightness exists in the approximate period 1980−1985. Taking all available photometric and spectroscopic data into account we must reject the hypothesis that V 1184 Tau is an FU Orionis type object. V 1184 Tau is a G type low-mass star whose spectrum is similar to WTT stars, but its photometric behavior is typical of the UX Orionis variable stars. Assuming the obscuration from orbiting dust clouds as a reason for a deep minimum, we estimate the approximate period of obscurations and the interval between two deep minima. Our calculations give a 25−28 year period between the two minima and approximately 8−10 year duration of the minima.

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We Ðnd that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the postÈT Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why postÈT Tauris have been so elusive. HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

We present a survey of mid-infrared gas-phase lines toward a sample of 76 circumstellar disks around low mass pre-main sequence stars from the Spitzer "Cores to Disks" legacy program. We report the first detections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20% respectively ~9% of our sources. The observed [Ne II] line fluxes and upper limits are consistent with [Ne II] excitation in an X-ray irradiated disk around stars with X-ray luminosities L_X=10^{29}-10^{31} erg s^{-1}. [Fe I] is detected at ~10^-5-10^-4 L_Sun, but no [S I] or [Fe II] is detected down to ~10^{-6} L_Sun. The [Fe I] detections indicate the presence of gas-rich disks with masses of >~0.1 M_J. No H_2 0-0 S(0) and S(1) disk emission is detected, except for S(1) toward one source. These data give upper limits on the warm (T~100-200K) gas mass of a few Jovian masses, consistent with recent T Tauri disk models which include gas heating by stellar radiation. Compact disk emission of hot (T>~500K) gas is observed through the H_2 0-0 S(2) and/or S(3) lines toward ~8% of our sources. The line fluxes are, however, higher by more than an order of magnitude than those predicted by recent disk models, even when X-ray and excess UV radiation are included. Similarly the [Ne II]/H_2 0-0 S(2) ratios for these sources are lower than predicted, consistent with the presence of an additional hot molecular gas component not included in current disk models. Oblique shocks of stellar winds interacting with the disk can explain many aspects of the hot gas emission, but are inconsistent with the non-detection of [S I] and [Fe II] lines.

For several hours on 2008 April 19, the T Tauri spectroscopic binary DQ Tau was observed to brighten, reaching a maximum detected flux of 468 mJy and likely making it (briefly) the brightest object at 3 mm in the Taurus star-forming region. We present the light curve of a rarely before observed millimeter flare originating in the region around a pre-main-sequence star, and the first from a classical T Tauri star. We discuss the properties and nature of the flaring behavior in the context of pulsed accretion flows (the current picture based largely on studies of this object's optically variable spectrum), as well as magnetospheric re-connection models (a separate theory that predicts millimeter flares for close binaries of high orbital eccentricity). We believe that the flare mechanism is linked to the binary orbit, and therefore periodic. DQ Tau makes a strong case for multi-wavelength follow-up studies, performed in parallel, of future flares to help determine whether magnetospheric and dynamical interactions in a proto-binary system are independent.

Aims. In order to gain insights into the variability behaviour of the circumstellar (CS) atomic gas, we have analysed 337 multi-epoch optical spectra of 38 Herbig Ae/Be (HAeBe) stars. Methods. Equivalent widths (EWs) and line fluxes of the Hα, [O i]6300, He i5876 and Na iD lines were obtained for each spectrum; the Hα line width at 10% of peak intensity (W 10 ) and profile shapes were also measured and classified. The mean line strengths and relative variabilities were quantified for each star. Simultaneous optical photometry was used to estimate the line fluxes. Results. We present a homogeneous spectroscopic database of HAeBe stars. The lines are variable in practically all stars and timescales, although 30 % of the objects show a constant EW in [O i]6300, which is also the only line that shows no variability on timescales of hours. The He i5876 and Na iD EW relative variabilities are typically the largest, followed by those in [O i]6300 and Hα. The EW changes can be larger than one order of magnitude for the He i5876 line, and up to a factor 4 for Hα. The [O i]6300 and Hα EW relative variabilities are correlated for most stars in the sample. The Hα mean EW and W 10 are uncorrelated, as are their relative variabilities. The Hα profile changes in ∼ 70 % of the objects. The massive stars in the sample (M * > 3 M ⊙ ) usually show more stable Hα profiles with blueshifted self-absorptions and less variable 10% widths. Conclusions. Our data suggest multiple causes for the different line variations, but the [O i]6300 and Hα variability must share a similar origin in many objects. The physical mechanism responsible for the Hα line broadening does not depend on the amount of emission; unlike in lower-mass stars, physical properties based on the Hα luminosity and W 10 would significantly differ. Our results provide additional support to previous works that reported different physical mechanisms in Herbig Ae and Herbig Be stars. The multi-epoch observations we present are a useful tool for understanding the origin of the CS lines and their variability, and to establish distinctions in the physical processes operating in pre-main sequence stars.

Context. Emission and absorption line profiles which are formed in the interaction of pre-main sequence (PMS) stars and their circumstellar environment are found to be variable at various timescales. Aims. We investigate the patterns and timescales of temporal line profile variability in order to explore the dynamical circumstellar environment of the PMS Herbig Ae star V351 Ori. Methods. We obtained 45 high-resolution (R∼28,000) spectra of V351 Ori at timescales of hours, days, and months. We analysed the Hα line profiles and also examined the Hβ, NaD1 and NaD2 line profiles to explore the nature of the spectroscopic variability. Results. The Hα line profiles showed strong variations over all timescales. The shape of the profiles changed over timescales of a day. Single as well as simultaneous event(s) of blue-shifted and red-shifted transient absorption components (TACs), i.e. signatures of outflow and infall, were also observed in the Hα line profiles. The shortest period of variation in the TACs was ≤ 1 hour. All transient absorption events were found to decelerate with a rate of a few to fractions of m s −2 . The depth and width of the TACs were also changing with time. The presence of elongated red-shifted components at some epochs supports the episodic nature of accretion. Conclusions. Variable emission and absorption components detected in Hα line profiles show the dynamic nature of interaction between V351 Ori and its circumstellar environment. The Hα non-photospheric profiles of the star most probably originate in the disk wind. Episodic accretion of gaseous material at a slow rate and outflow of clumpy gaseous material are still occurring in V351 Ori at an age of ∼6.5 Myr. Dynamic magnetospheric accretion and disk wind emerge as the most satisfactory model for interpreting the observed line profile variations of V351 Ori.

We present a study on the shells (and bubbles) in the Perseus molecular cloud using the COM-PLETE survey large-scale 12 CO(1-0) and 13 CO(1-0) maps. The twelve shells reported here are spread throughout most of the Perseus cloud and have circular or arc-like morphologies with a range in radius of about 0.1 to 3 pc. Most of them have not been detected before most likely as maps of the region lacked the coverage and resolution needed to distinguish them. The majority of the shells are coincident with infrared nebulosity of similar shape and have a candidate powering source near the center. We suggest they are formed by the interaction of spherical or very wide-angle winds powered by young stars inside or near the Perseus molecular cloud -a cloud that is commonly considered to be mostly forming low-mass stars. Two of the twelve shells are powered by high-mass stars close to the cloud, while the others appear to be powered by low or intermediate mass stars in the cloud. We argue that winds with a mass loss rate of about 10 −8 to 10 −6 M yr −1 are required to produce the observed shells. Our estimates indicate that the energy input rate from these stellar winds is similar to the turbulence dissipation rate. We conclude that in Perseus the total energy input from both collimated protostellar outflows and powerful spherical winds from young stars is sufficient to maintain the turbulence in the molecular cloud. Large scale molecular line and IR continuum maps of a sample of clouds will help determine the frequency of this phenomenon in other star forming regions.

Most of the observed features can be interpreted in the framework of non-axisymmetric magnetospheric accretion. We consider two possible models. In the first the asymmetry is induced by orbital motion of an invisible, low mass secondary, which also influences the gasflows around the star, the second considers rotational modulation of a single star with an inclined or asymmetric magnetosphere.

We report the discovery of a double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We analyze the system spectroscopically and photometrically to empirically determine precise, distance-independent masses, radii, effective temperatures, and luminosities for both components. The measured masses for the primary and secondary, accurate to ∼ 1%, are 1.01 M ⊙ and 0.73 M ⊙ , respectively; thus the primary is a definitive pre-main-sequence solar analog, and the secondary is the lowestmass star yet discovered among pre-main-sequence eclipsing binary systems. We use these fundamental measurements to test the predictions of pre-main-sequence stellar evolutionary tracks. None of the models we examined correctly predict the masses of the two components simultaneously, and we implicate differences between the theoretical and empirical effective temperature scales for this failing. All of the models predict the observed slope of the mass-radius relationship reasonably well, though the observations tend to favor models with low convection efficiencies. Indeed, considering our newly determined mass measurements together with other dynamical mass measurements of pre-main-sequence stars in the literature, as well as measurements of Li abundances in these stars, we show that the data strongly favor evolutionary models with inefficient convection in the stellar interior, even though such models cannot reproduce the properties of the present-day Sun.

In this paper we present Spitzer Infrared Spectrograph spectroscopy for 14 intermediate-mass young stellar objects. We use Spitzer spectroscopy to investigate the physical properties of these sources and their environments. Our sample can be divided into two types of objects: young isolated, embedded objects with spectra that are dominated by ice and silicate absorption bands, and more evolved objects that are dominated by extended emission from polycyclic aromatic hydrocarbons (PAHs) and pure H2 rotational lines. We are able to constrain the illuminating FUV fields by classifying the PAH bands below 9micron. For most of the sources we are able to detect several atomic fine structure lines. In particular, the [NeII] line appearing in two regions could originate from unresolved photodissociation regions (PDRs) or J-shocks. We relate the identified spectral features to observations obtained from NIR through submillimeter imaging. The spatial extent of several H2 and PAH bands is matched with morphologies identified in previous Spitzer/IRAC observations. This also allows us to distinguish between the different H2 excitation mechanisms. In addition, we calculate the optical extinction from the silicate bands and use this to constrain the spectral energy distribution fit, allowing us to estimate the masses of these YSOs.

Aims. An important question about Be stars is whether Be stars are born as Be stars or whether they become Be stars during their evolution. It is necessary to observe young clusters to answer this question. Methods. To this end, observations of stars in NGC 6611 and the star-formation region of Eagle Nebula have been carried out with the ESO-WFI in slitless spectroscopic mode and at the VLT-GIRAFFE (R≃ 6400-17000). The targets for the GIRAFFE observations were pre-selected from the literature and our catalogue of emission-line stars based on the WFI study. GIRAFFE observations allowed us to study accurately the population of the early-type stars with and without emission lines. For this study, we determined the fundamental parameters of OBA stars thanks to the GIRFIT code. We also studied the status of the objects (main sequence or pre-main sequence stars) by using IR data, membership probabilities, and location in HR diagrams.

We present Hubble Space Telescope (HST) Near-Infrared Camera and Multi-object Spectrometer (NICMOS) observations of the reflection nebulosity associated with the T Tauri star HH 30. The images show the scattered light pattern characteristic of a highly inclined, optically thick disk with a prominent dustlane whose width decreases -2with increasing wavelength. The reflected nebulosity exhibits a lateral asymmetry in the upper lobe on the opposite side to that reported in previously published Wide Field Planetary Camera 2 (WFPC2) images. The radiation transfer model which most closely reproduces the data has a flared accretion disk with dust grains larger than standard interstellar medium grains by a factor of approximately 2.1. A single hotspot on the stellar surface provides the necessary asymmetry to fit the images and is consistent with previous modeling of the light curve and images. Photometric analysis results in an estimated extinction of A V 80; however, since the photometry measures only scattered light rather than direct stellar flux, this a lower limit. The radiative transfer models require an extinction of A V = 7, 900.

We have completed an optical spectroscopic survey of an unbiased, extinction-limited sample of candidate young stars covering 1.3 deg2 of the rho Ophiuchi star-forming region. While infrared, X-ray, and optical surveys of the cloud have identified many young stellar objects (YSOs), these surveys are biased toward particular stages of stellar evolution and are not optimal for studies of the disk

Aims. We want to investigate whether brown dwarfs (BDs) form like stars or are ejected embryos. We study the presence of disks around BDs in the Taurus cloud, and discuss implications for substellar formation models. Methods. We use photometric measurements from the visible to the far infrared to determine the spectral energy distributions (SEDs) of Taurus BDs. Results. We use Spitzer color indices, Hα as an accretion indicator, and models fit to the SEDs in order to estimate physical parameters of the disks around these BDs. We study the spatial distribution of BDs with and without disks across the Taurus aggregates, and we find that BDs with and without disks are not distributed regularly across the Taurus cloud. Conclusions. We find that 48% ± 14% of Taurus BDs have a circumstellar disk signature, a ratio similar to recent results from previous authors in other regions. We fit the SEDs and find that none of the disks around BDs in Taurus can be fitted convincingly with a flaring index β = 0, indicating that heating by the central object is efficient and that the disks we observe retain a significant amount of gas. We find that BDs with disks are proportionally more numerous in the northern Taurus filament, possibly the youngest filament. We do not find such a clear segregation for classical T Tauri stars (CTTS) and weak-lined T Tauri stars (WTTS), suggesting that, in addition to the effects of evolution, any segregation effects could be related to the mass of the object. A by-product of our study is to propose a recalibration of the Barrado y Navascués & Martín (2003) accretion limit in the substellar domain. The global shape of the limit fits our data points if it is raised by a factor 1.25-1.30.

We present the photometric catalogs for the star-forming cluster NGC 602 in the wing of the Small Magellanic Cloud covering a range of wavelengths from optical (HST/ACS F555W, F814W and SMARTS/ANDICAM V, I) to infrared (Spitzer /IRAC 3.6, 4.5, 5.8, and 8 µm and MIPS 24 µm). Combining this with IRSF (InfraRed Survey Facility) near-infrared photometry (J, H, K s ), we compare the young main sequence (MS) and pre-main sequence (PMS) populations prominent in the optical with the current young stellar object (YSO) populations revealed by the infrared (IR). We analyze the MS and PMS population with isochrones in color-magnitude diagrams to derive ages and masses.

The Monitor project is a photometric monitoring survey of nine young (1-200 Myr) clusters in the solar neighbourhood to search for eclipses by very low mass stars and brown dwarfs and for planetary transits in the light curves of cluster members. It began in the autumn of 2004 and uses several 2 to 4 m telescopes worldwide. We aim to calibrate the relation between age, mass, radius and where possible luminosity, from the K-dwarf to the planet regime, in an age range where constraints on evolutionary models are currently very scarce. Any detection of an exoplanet in one of our youngest targets ( 10 Myr) would also provide important constraints on planet formation and migration timescales and their relation to proto-planetary disc lifetimes. Finally, we will use the light curves of cluster members to study rotation and flaring in low-mass pre-main sequence stars.