Papers by Lars Kristensen
The Astrophysical Journal, May 27, 2009
We present mid-infrared spectral maps of the NGC 1333 star-forming region, obtained with the infr... more We present mid-infrared spectral maps of the NGC 1333 star-forming region, obtained with the infrared spectrometer on board the Spitzer Space Telescope. Eight pure H2 rotational lines, from S(0) to S(7), are detected and mapped. The H2 emission appears to be associated with the warm gas shocked by the multiple outflows present in the region. A comparison between the observed intensities and the predictions of detailed shock models indicates that the emission arises in both slow (12-24 km s-1) and fast (36-53 km s-1) C-type shocks with an initial ortho-to-para ratio (opr) lsim1. The present H2 opr exhibits a large degree of spatial variations. In the postshocked gas, it is usually about 2, i.e., close to the equilibrium value (~3). However, around at least two outflows, we observe a region with a much lower (~0.5) opr. This region probably corresponds to gas which has been heated up recently by the passage of a shock front, but whose ortho-to-para has not reached equilibrium yet. This, together with the low initial opr needed to reproduce the observed emission, provide strong evidence that H2 is mostly in para form in cold molecular clouds. The H2 lines are found to contribute to 25%-50% of the total outflow luminosity, and thus can be used to ascertain the importance of star formation feedback on the natal cloud. From these lines, we determine the outflow mass loss rate and, indirectly, the stellar infall rate, the outflow momentum and the kinetic energy injected into the cloud over the embedded phase. The latter is found to exceed the binding energy of individual cores, suggesting that outflows could be the main mechanism for core disruption.
Water is a key molecule for determining the physical and chemical structure of star-forming regio... more Water is a key molecule for determining the physical and chemical structure of star-forming regions because of its large abundance variations, both in the gas and in the ice, between warm and cold regions. In this HIFI-led 429 hr Key Program, we are obtaining a comprehensive set of water observations toward a large sample of well-characterized protostars, covering a wide range of masses and luminosities -from the lowest to the highest mass protostars-, as well as evolutionary stages -from the first stages represented by pre-stellar cores to the later stages represented by the pre-main sequence stars surrounded only by their protoplanetary disks. Lines of H2O and its isotopologues, as well as chemically related hydrides, are observed. In addition, selected high-frequency lines of CO isotopes, [O I] and [C II] are obtained with HIFI and PACS, and are complemented by ground-based HDO, CO and dust continuum maps to ensure a self-consistent data set for analysis. Limited mapping information on a few arcmin scale provides information on local variations due to outflows and clustered star formation. Together, the data elucidate the physical processes responsible for the warm gas (passive heating, UV or X-ray-heating, shocks, disks), probe dynamical processes associated with forming stars and planets (outflow, infall, turbulence), reveal the chemical evolution of water and the oxygen-reservoir, and test basic gas-grain chemical interactions. By the time of the COSPAR meeting, about half of our program should have been observed. This talk will present an overview of the main highlights.
ABSTRACT Infrared-bright protostars form a unique class of sources for which both water gas and i... more ABSTRACT Infrared-bright protostars form a unique class of sources for which both water gas and ice can be observed. We introduce a Simplified Water Network (SWaN) to model the data and test it against full gas-grain models as well as Herschel-HIFI spectra of a Class 0 protostar. The combined Herschel-HIFI and Spitzer/VLT data on nine sources are then compared and analysed using SWaN, giving typical ice/gas ratios of 1e3 at the core edge and up to 1e7 deeper into the core. A quantification of the oxygen budget reveals that a large fraction of the oxygen is still unidentified.
The Astrophysical Journal, 2013
ABSTRACT Evaporation of water ice above 100 K in the inner few 100 AU of low-mass embedded protos... more ABSTRACT Evaporation of water ice above 100 K in the inner few 100 AU of low-mass embedded protostars (the so-called hot core) should produce quiescent water vapor abundances of ~10–4 relative to H2. Observational evidence so far points at abundances of only a few 10–6. However, these values are based on spherical models, which are known from interferometric studies to be inaccurate on the relevant spatial scales. Are hot cores really that much drier than expected, or are the low abundances an artifact of the inaccurate physical models? We present deep velocity-resolved Herschel-HIFI spectra of the 312-303 lines of HO and HO (1097 GHz, E u/k = 249 K) in the low-mass Class 0 protostar NGC 1333 IRAS2A. A spherical radiative transfer model with a power-law density profile is unable to reproduce both the HIFI data and existing interferometric data on the HO 313-220 line (203 GHz, E u/k = 204 K). Instead, the HIFI spectra likely show optically thick emission from a hot core with a radius of about 100 AU. The mass of the hot core is estimated from the C18O J = 9-8 and 10-9 lines. We derive a lower limit to the hot water abundance of 2 × 10–5, consistent with the theoretical predictions of ~10–4. The revised HDO/H2O abundance ratio is 1 × 10–3, an order of magnitude lower than previously estimated.
The Astrophysical Journal, 2014
ABSTRACT We have investigated the water abundance in shock-heated molecular gas, making use of $H... more ABSTRACT We have investigated the water abundance in shock-heated molecular gas, making use of $Herschel$ measurements of far-infrared CO and H$_2$O line emissions in combination with $Spitzer$ measurements of mid-IR H$_2$ rotational emissions. We present far-infrared line spectra obtained with $Herschel$'s PACS instrument in range spectroscopy mode towards two positions in the protostellar outflow NGC 2071 and one position each in the supernova remnants W28 and 3C391. These spectra provide unequivocal detections, at one or more positions, of 12 rotational lines of water, 14 rotational lines of CO, 8 rotational lines of OH (4 lambda doublets), and 7 fine-structure transitions of atoms or atomic ions. We first used a simultaneous fit to the CO line fluxes, along with H$_2$ rotational line fluxes measured previously by $Spitzer$, to constrain the temperature and density distribution within the emitting gas; and we then investigated the water abundances implied by the observed H$_2$O line fluxes. The water line fluxes are in acceptable agreement with standard theoretical models for nondissociative shocks that predict the complete vaporization of grain mantles in shocks of velocity $v \sim 25$ km/s, behind which the characteristic gas temperature is $\sim 1300$ K and the H$_2$O/CO ratio is 1.2
ABSTRACT We present far-IR Herschel/PACS spectra of Class O/I YSOs obtained in the Water In Star-... more ABSTRACT We present far-IR Herschel/PACS spectra of Class O/I YSOs obtained in the Water In Star-forming regions with Herschel (WISH) key program. Class 0 and Class I are the earliest stages of evolution of young stellar objects, where collimated outflows from the protostar carve out cavities in dense envelopes. Our PACS survey of CO, H2O, OH and [OI] line emission from 16 YSOs uniquely trace the warm (200-1000 K) gas. Such observations reveal how shocks and UV photons heat the cavity walls and deposit energy into the envelope.
Astronomy & Astrophysics, 2011
OH is an important molecule in the H2O chemistry and the cooling budget of star-forming regions. ... more OH is an important molecule in the H2O chemistry and the cooling budget of star-forming regions. The goal of the Herschel key program `Water in Star-forming regions with Herschel' (WISH) is to study H2O and related species during protostellar evolution. Our aim in this letter is to assess the origin of the OH emission from star-forming regions and constrain the properties of the emitting gas. High-resolution observations of the OH 2Pi1/2 J = 3/2-1/2 triplet at 1837.8 GHz (163.1 micron) towards the high-mass star-forming region W3 IRS 5 with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel reveal the first hyperfine velocity-resolved OH far-infrared spectrum of a star-forming region. The line profile of the OH emission shows two components: a narrow component (FWHM approx. 4-5 km/s) with partially resolved hyperfine structure resides on top of a broad (FWHM approx. 30 km/s) component. The narrow emission agrees well with results from radiative transfer calculations of a spherical envelope model for W3 IRS 5 with a constant OH abundance of approx. 8e-9. Comparison with H2O yields OH/H2O abundance ratios of around 1e-3 for T > 100 K and around unity for T < 100K, consistent with the current picture of the dense cloud chemistry with freeze-out and photodesorption. The broad component is attributed to outflow emission. An abundance ratio of OH/H2O > 0.028 in the outflow is derived from comparison with results of water line modeling. This ratio can be explained by a fast J-type shock or a slower UV-irradiated C-type shock.
Astronomy & Astrophysics, 2010
Observations of Serpens have been performed at the JCMT using Harp-B. Maps over a 4.5'x5.4' regio... more Observations of Serpens have been performed at the JCMT using Harp-B. Maps over a 4.5'x5.4' region were made in a frequency window around 338 GHz, covering the 7-6 transitions of methanol. Emission is extended over each source, following the column density of H2 but showing up also particularly strongly around outflows. The rotational temperature is low, 15-20 K, and does not vary with position within each source. The abundance is typically 10^-9 - 10^-8 with respect to H2 in the outer envelope, whereas "jumps" by factors of up to 10^2 -10^3 inside the region where the dust temperature exceeds 100 K are not excluded. A factor of up to ~ 10^3 enhancement is seen in outflow gas. In one object, SMM4, the ice abundance has been measured to be ~ 3x10^-5 with respect to H2 in the outer envelope, i.e., a factor of 10^3 larger than the gas-phase abundance. Comparison with C18O J=3-2 emission shows that strong CO depletion leads to a high gas-phase abundance of CH3OH not just for the Serpens sources, but for a larger sample of protostars. The observations illustrate the large-scale, low-level desorption of CH3OH from dust grains, extending out to and beyond 7500 AU from each source, a scenario which is consistent with non-thermal (photo-)desorption from the ice. The observations also illustrate the usefulness of CH3OH as a tracer of energetic input in the form of outflows, where methanol is sputtered from the grain surfaces. Finally, the observations provide further evidence of CH3OH formation through CO hydrogenation proceeding on grain surfaces in low-mass envelopes.
Astronomy & Astrophysics, 2010
DK Cha is an intermediate-mass star in transition from an embedded configuration to a star plus d... more DK Cha is an intermediate-mass star in transition from an embedded configuration to a star plus disk stage. We aim to study the composition and energetics of the circumstellar material during this pivotal stage. Using the Range Scan mode of PACS on the Herschel Space Observatory, we obtained a spectrum of DK Cha from 55 to 210 micron as part of the DIGIT Key Program. Almost 50 molecular and atomic lines were detected, many more than the 7 lines detected in ISO-LWS. Nearly the entire ladder of CO from J=14-13 to 38-37 (E_u/k = 4080 K), water from levels as excited as E_u/k = 843 K, and OH lines up to E_u/k = 290 K were detected. The continuum emission in our PACS SED scan matches the flux expected from a model consisting of a star, a surrounding disk of 0.03 Solar mass, and an envelope of a similar mass, supporting the suggestion that the object is emerging from its main accretion stage. Molecular, atomic, and ionic emission lines in the far-infrared reveal the outflow's influence on the envelope. The inferred hot gas can be photon-heated, but some emission could be due to C-shocks in the walls of the outflow cavity.
Astronomy & Astrophysics, 2011
Context. Candidate high-mass star-forming regions can be identified through the occurrence of 6.7... more Context. Candidate high-mass star-forming regions can be identified through the occurrence of 6.7 GHz methanol masers. In these sources the methanol abundance of the gas must be enhanced, because the masers require a considerable methanol path length. The place and time of origin of this enhancement is not well known. Similarly, it is debated in which of the physical components of the high-mass star-forming region the masers are located. Aims: The aim of this study is to investigate the distribution and excitation of the methanol gas around Cep A and to describe the physical conditions of the region. In addition the large-scale abundance distribution is determined to understand the morphology and kinematics of star-forming regions in which methanol masers occur. Methods: The spatial distribution of methanol is studied by mapping the line emission, as well as the column density and excitation temperature, which are estimated using rotation diagrams. For a limited number of positions the parameters are checked with non-LTE models. Furthermore, the distribution of the methanol abundance is derived in comparison with archival dust continuum maps. Results: Methanol is detected over a 0.3 × 0.15 pc area centred on the Cep A HW2 source and shows an outflow signature. Most of the gas can be characterized by a moderately warm rotation temperature (30-60 K). At the central position two velocity components are detected with different excitation characteristics, the first related to the large-scale outflow. The second component, uniquely detected at the central location, is probably associated with the maser emission on much smaller scales of 2″. A detailed analysis reveals that the highest densities and temperatures occur for these inner components. In the inner region the dust and gas are shown to have different physical parameters. Conclusions: Abundances of methanol in the range 10-9 - 10-7 are inferred, with the abundance peaking at the maser position. The geometry of the large-scale methanol is in accordance with previous determinations of the Cep A geometry, in particular those from methanol masers. The dynamical and chemical time-scales are consistent with a scenario where the methanol originates in a single driving source associated with the HW2 object and the masers in its equatorial region.
Astronomy & Astrophysics, 2010
Observations of Serpens have been performed at the JCMT using Harp-B. Maps over a 4.5'x5.4' regio... more Observations of Serpens have been performed at the JCMT using Harp-B. Maps over a 4.5'x5.4' region were made in a frequency window around 338 GHz, covering the 7-6 transitions of methanol. Emission is extended over each source, following the column density of H2 but showing up also particularly strongly around outflows. The rotational temperature is low, 15-20 K, and does not vary with position within each source. The abundance is typically 10^-9 - 10^-8 with respect to H2 in the outer envelope, whereas "jumps" by factors of up to 10^2 -10^3 inside the region where the dust temperature exceeds 100 K are not excluded. A factor of up to ~ 10^3 enhancement is seen in outflow gas. In one object, SMM4, the ice abundance has been measured to be ~ 3x10^-5 with respect to H2 in the outer envelope, i.e., a factor of 10^3 larger than the gas-phase abundance. Comparison with C18O J=3-2 emission shows that strong CO depletion leads to a high gas-phase abundance of CH3OH not just for the Serpens sources, but for a larger sample of protostars. The observations illustrate the large-scale, low-level desorption of CH3OH from dust grains, extending out to and beyond 7500 AU from each source, a scenario which is consistent with non-thermal (photo-)desorption from the ice. The observations also illustrate the usefulness of CH3OH as a tracer of energetic input in the form of outflows, where methanol is sputtered from the grain surfaces. Finally, the observations provide further evidence of CH3OH formation through CO hydrogenation proceeding on grain surfaces in low-mass envelopes.
The aim of this study is to investigate the distribution and excitation of the methanol gas aroun... more The aim of this study is to investigate the distribution and excitation of the methanol gas around Cep A and to describe the physical conditions of the region. In addition the large scale abundance distribution is determined in order to understand the morphology and kinematics of star forming regions in which methanol masers occur. The spatial distribution of the methanol is studied by mapping line emission, as well as the column density and excitation temperature, which are estimated using rotation diagrams. For a limited number of positions the parameters are checked with non-LTE models. Furthermore, the distribution of the methanol abundance is derived in comparison with archival dust continuum maps. Methanol is detected over a 0.3x0.15 pc area centred on the Cep A HW2 source, showing an outflow signature. Most of the gas can be characterized by a moderately warm rotation temperature (30-60K). At the central position two velocity components are detected with different excitation characteristics, the first related to the large-scale outflow. The second component, uniquely detected at the central location, is probably associated with the maser emission on much smaller scales of 2". Detailed analysis reveals that the highest densities and temperatures occur for these inner components. In the inner region the dust and gas are shown to have different physical parameters. Abundances of methanol in the range 10E-9 - 10E-7 are inferred, with the abundance peaking at the maser position. The geometry of the large-scale methanol is in accordance with previous determinations of the Cep A geometry, in particular those from methanol masers. The dynamical and chemical time-scales are consistent with the methanol originating from a single driving source associated with the HW2 object and the masers in its equatorial region.
Astronomy & Astrophysics, 2009
AIMS Our aim is to characterize the size, mass, density and temperature profiles of the protostel... more AIMS Our aim is to characterize the size, mass, density and temperature profiles of the protostellar envelope of HH~46 IRS 1 and its surrounding cloud material as well as the effect the outflow has on its environment.METHODS The CHAMP+ and LABOCA arrays on the APEX telescope, combined with lower frequency line receivers, are used to obtain a large continuum map and smaller heterodyne maps in various isotopologues of CO and HCO+. The high-J lines of CO (6--5 and 7--6) and its isotopologues together with [C I] 2--1, observed with CHAMP+, are used to probe the warm molecular gas in the inner few hundred AU and in the outflowing gas. The data are interpreted with continuum and line radiative transfer models. RESULTS Broad outflow wings are seen in CO low- and high-J lines at several positions, constraining the gas temperatures to a constant value of ~100 K along the red outflow axis and to ~60 K for the blue outflow. The derived outflow mass is of order 0.4--0.8 M_sol, significantly higher than previously found. The bulk of the strong high-J CO line emission has a surprisingly narrow width, however, even at outflow positions. These lines cannot be fit by a passively heated model of the HH 46 IRS envelope. We propose that it originates from photon heating of the outflow cavity walls by ultraviolet photons originating in outflow shocks and the accretion disk boundary layers. At the position of the bow shock itself, the UV photons are energetic enough to dissociate CO. The envelope mass of ~5 M_sol is strongly concentrated towards HH 46 IRS with a density power law of -1.8.
Astronomy & Astrophysics, 2010
Aims: We study the complex H2 outflows in the inner 300 AU of the young triple star system T Taur... more Aims: We study the complex H2 outflows in the inner 300 AU of the young triple star system T Tauri, with the goal of understanding the origin, excitation and evolution of the circumstellar matter. Methods: Using high spatial resolution, integral-field spectroscopy in the J and K photometric bands from SINFONI/VLT, we trace the spatial distribution of 12 H2 ro-vibrational emission lines, as well as one forbidden Fe II line. The ratio of line strengths provides a two-dimensional view of both the variable extinction and excitation temperature in this region, while the line-center velocities, coupled with previously published imagery, allow an assessment of the 3D space velocities and evolution of the outflows. Results: Several spatially distinct flows - some with a bow shock structure - appear within 1.5 arcsec of the stars. Data taken two years apart clearly show the evolution of these flows. Some structures move and evolve, while others are stationary in the plane of the sky. The two-dimensional extinction map shows that the extinction between T Tau N and T Tau S is very high. In addition to being clumpy the extincting material forms part of a filament that extends to the east of the stars. In areas with strong line emission, the v = 1-0 S(1)/v = 2-1 S(1) line ratio ranges from 8 to 20, indicating that all of the observed H2 is shock excited. The outflows in the immediate vicinity of T Tau S span ~270° and are all blue-shifted, suggesting that they are produced by more than one star. We propose that T Tau N drives the east-west outflow, while T Tau Sa and T Tau Sb are the sources of the southeast-northwest and a previously undetected southwest outflow, respectively. There is a large spatial overlap between the [Fe II] line emission and previously measured UV fluorescent H2 emission, showing that both may be produced in J-shocks. Based on observations collected at the European Southern Observatory, Paranal, Chile under the programme 60.A-9041(A).
Astronomy & Astrophysics, 2010
Aims: We employ archival Spitzer slit-scan observations of the HH211 outflow in order to investig... more Aims: We employ archival Spitzer slit-scan observations of the HH211 outflow in order to investigate its warm gas content, assess the jet mass flux in the form of H2 and probe for the existence of an embedded atomic jet. Methods: Detected molecular and atomic lines are interpreted by means of emission line diagnostics and an existing grid of molecular shock models. The physical properties of the warm gas are compared against other molecular jet tracers and to the results of a similar study towards the L1448-C outflow. Results: We have detected and mapped the v=0-0 S(0) - S(7) H2 lines and fine-structure lines of S, Fe+, and Si+. H2 is detected down to 5" from the source and is characterized by a "cool" T~300K and a "warm" T~1000 K component, with an extinction Av ~ 8 mag. The amount of cool H2 towards the jet agrees with that estimated from CO assuming fully molecular gas. The warm component is well fitted by C-type shocks with a low beam filling factor ~ 0.01-0.04 and a mass-flux similar to the cool H2. The fine-structure line emission arises from dense gas with ionization fraction ~0.5 - 5 x 10e-3, suggestive of dissociative shocks. Line ratios to sulfur indicate that iron and silicon are depleted compared to solar abundances by a factor ~10-50. Conclusions: Spitzer spectral mapping observations reveal for the first time a cool H$_2$ component towards the CO jet of HH211 consistent with the CO material being fully molecular and warm at ~ 300 K. The maps also reveal for the first time the existence of an embedded atomic jet in the HH211 outflow that can be traced down to the central source position. Its significant iron and silicon depletion excludes an origin from within the dust sublimation zone around the protostar. The momentum-flux seems insufficient to entrain the CO jet, although current uncertainties on jet speed and shock conditions are too large for a definite conclusion.
Astronomy & Astrophysics, 2010
... 2004; Kristensen et al. 2007; Schultz et al. 1999). We model one of these bow shocks, which h... more ... 2004; Kristensen et al. 2007; Schultz et al. 1999). We model one of these bow shocks, which has been observed with the ESO-VLT (Gustafsson 2006). The chosen bow shock has previously been modelled by Kristensen et al. ... Open with DEXTER. 2.1 Shock geometry. ...
Astronomy & Astrophysics, 2007
Aims:We seek to study excitation mechanisms in the inner region of the Orion Molecular Cloud by c... more Aims:We seek to study excitation mechanisms in the inner region of the Orion Molecular Cloud by comparing observations of ortho- and para-lines of H2 with theoretical models of slow shocks and photodissociation regions. Methods: K-band observations of H2 obtained with the Canada-France-Hawaii 3.6 m telescope using the PUEO adaptive optics system are reported. Data were centered on the Becklin-Neugebauer object northwest of the Trapezium stars. Narrow-band filters were used to isolate emission from the v=1-0 S(1) ortho- and v=1-0 S(0) para-lines at a spatial resolution of 0.45 arcsec (~200 AU). We are able to combine their intensity to obtain the column densities of rovibrationally excited ortho and para H2 levels of the molecular gas at high spatial resolution. Results: The resulting line ratios show variations between 2 and the statistical equilibrium value of 6. We find 4 different classes of emission, characterised by the ratio of the v=1-0 S(1) and S(0) line brightness and the absolute line brightness. Shock models are used to estimate the physical properties of pre-shock density and shock velocity for these 4 classes. We find that the pre-shock density is in the range of 10^5-107 cm-3 and shock velocities lie between 10 and 40 km s-1. Studies of individual objects, using additional constraints of shock velocity and width, allow quite precise physical conditions to be specified in three prominent bow shocks, one with a shock speed of 18±2 km s-1 and pre-shock density 1±0.5×106 cm-3 (3σ) and two with shock speeds of ~36±2 km s-1 and pre-shock densities of 7.5±2.5×104 cm-3. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. Appendix A is only available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, 2008
Aims:We present a new method for reproducing high spatial resolution observations of bow shocks b... more Aims:We present a new method for reproducing high spatial resolution observations of bow shocks by using 1D plane parallel shock models. As an example we analyse one bow shock located in the Orion Molecular Cloud (OMC1). Methods: We use high spatial resolution near-infrared observations of H2 rovibrational emission to constrain shock models. These observations have been made at the ESO-VLT using a combination of the NACO adaptive optics system and infrared camera array and the Fabry-Perot interferometer. Three rovibrational H2 lines have been observed: v= 1-0 S(1) at 2.12 μm, v= 1-0 S(0) at 2.23 μm and v= 2-1 S(1) at 2.25 μm. The spatial resolution is 0.15 arcsec ~ 70 AU. We analyse a single bow shock located in our field, featuring a very well defined morphology and high brightness. Results: One dimensional shock models are combined to estimate the physical properties of pre-shock density, shock velocity and transverse magnetic field strength along the bow shock. We find that the pre-shock density is constant at ~5 × 105 cm-3 and shock velocities lie between ~35 km s-1 in the wings of the shock and ~50 km s-1 at the apex. We also find that the transverse magnetic field is stronger at the apex and weaker further down the wings varying between ~2 and 4 mGauss. Predictions of shock velocity and magnetic field strength agree with previous independent observations. Based on observations obtained at the European Southern Observatories, VLT, Chile.
Astronomy & Astrophysics, 2003
Observations are reported of IR emission in H 2 , around 2 µm in the K-band, obtained with the ES... more Observations are reported of IR emission in H 2 , around 2 µm in the K-band, obtained with the ESO 3.6 m telescope using the ADONIS adaptive optics system. Data cover a region of the Orion Molecular Cloud north of the Trapezium stars and SW of the Becklin-Neugebauer object. Excellent seeing yielded diffraction limited images in the v = 2−1 S(1) line at 2.247 µm. Excitation temperature images were created by combining these data with similar data for H 2 emission in the v = 1−0 S(1) line reported earlier . Shock models are used to estimate densities in emitting clumps of material. In local zones with high excitation temperatures, post-shock densities are found to be as high as several times 10 8 cm −3 , an order of magnitude denser than our previous estimates. We propose that the nature of these zones is dictated by the combined activity of shocks, which create dense structures, and the powerful radiation field of θ 1 C Ori which photoevaporates the boundaries of these structures.
In order to compare recent observations of shocks in the interstellar medium with models, we have... more In order to compare recent observations of shocks in the interstellar medium with models, we have run a large grid of 25 000 shock models. These models are very sophisticated and includes a very detailed chemistry. We use the results from this grid to reproduce observations of a bow shock located in the Orion Molecular Cloud. In doing that we are constraining physical conditions in an active star forming region.
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Papers by Lars Kristensen