Papers by Farhad YusefZadeh
Proceedings of 7th INTEGRAL Workshop — PoS(Integral08), 2009
Proceedings of the International Astronomical Union, 2007
Compact OH(1720 MHz) masers have proven to be excellent signposts for the interaction of supernov... more Compact OH(1720 MHz) masers have proven to be excellent signposts for the interaction of supernova remnants with adjacent molecular clouds. Less appreciated has been the weak, extended OH(1720 MHz) emission which accompanies strong compact maser sources. Recent single-dish and interferometric observations reveal the majority of maser-emitting supernova remnants(SNRs) have accompanying regions of extended maser emission. Enhanced OH abundance created by the passing shock is observed both as maser emission and absorption against the strong background of the remnant. Modeling the observed OH profiles gives an estimate of the physical conditions in which weak, extended maser emission arises. I will discuss how we can realize the utility of this extended maser emission, particularly the potential to measure the strength of the post-shock magnetic field via Zeeman splitting over these large-scales.
We present a synthesis of a number of recent observations in the near--IR H2 and [Fe II] lines, O... more We present a synthesis of a number of recent observations in the near--IR H2 and [Fe II] lines, OH (1720 MHz) maser line and various radio continuum measurements using the NICMOS of the HST, UNSWIRF on the AAT and the VLA. These observations suggest that the outer edge of the CND is collisionally excited whereas the inner edge is likely
OH(1720 MHz) and methanol masers are now recognized to be excellent probes of the interactions of... more OH(1720 MHz) and methanol masers are now recognized to be excellent probes of the interactions of supernova remnants with molecular clouds and tracers of massive star formation, respectively. To better understand the nature of star formation activity in the central region of the Galaxy, we have used these two classes of masers combined with the IRAC and MIPS data to study prominent sites of ongoing star formation in the nuclear disk. The nuclear disk is characterized by massive GMCs with elevated gas temperatures, compared to their dust temperatures. We note an association between methanol masers and a class of mid-infrared "green sources". These highly embedded YSOs show enhanced 4.5µm emission due to excited molecular lines. The distribution of methanol masers and supernova remnants suggest a low efficiency of star formation (with the exception of Sgr B2), which we believe is due to an enhanced flux of cosmic ray electrons impacting molecular clouds in the nuclear disk. We also highlight the importance of cosmic rays in their ability to heat molecular clouds, and thus increase the gas temperature.
The Journal of Physical Chemistry A, 2013
We present 74 MHz radio continuum observations of the Galactic center region. These measurements ... more We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free-free absorption along the line of sight. We focus on one cloud, G0.13-0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (α 74MHz 327MHz = 1.3 ± 0.3)
The Astrophysical Journal, 2011
We present a study of signatures of ongoing star formation in a sample of protostellar objects wi... more We present a study of signatures of ongoing star formation in a sample of protostellar objects with enhanced 4.5 µm emission ('green' sources) near the Galactic center. To understand how star formation in the Galactic center region compares to that of the Galactic disk, we used the Expanded Very Large Array to observe radiatively excited Class II 6.7 GHz CH 3 OH masers and collisionally excited Class I 44 GHz CH 3 OH masers, both tracers of high-mass star formation, toward a sample of 34 Galactic center and foreground 'green' sources. We find that 33±15% of Galactic center sources are coincident with 6.7 GHz masers, and that 44±17% of foreground sources are coincident with 6.7 GHz masers. For 44 GHz masers, we find correlation rates of 27±13% and 25±13% for Galactic center green sources and foreground green sources, respectively. Based on these CH 3 OH maser detection rates, as well as correlations of green sources with other tracers of star formation, such as 24 µm emission and infrared dark clouds (IRDCs), we find no significant difference between the green sources in the Galactic center and those foreground to it. This suggests that once the star formation process has begun, the environmental differences between the Galactic center region and the Galactic disk have little effect on its observational signatures. We do find, however, some evidence that may support a recent episode of star formation in the Galactic center region.
The Astrophysical Journal, 2009
We have identified two moderately bright, rapidly variable transients in new and archival X-ray d... more We have identified two moderately bright, rapidly variable transients in new and archival X-ray data near the Galactic center. Both objects show strong, flaring variability on timescales of tens to thousands of seconds, evidence of N H variability, and hard spectra. XMMU J174445.5−295044 is seen at 2-10 keV fluxes of 3 × 10 −11 to <10 −12 erg cm −2 s −1 , with N H at or above 5 × 10 22 cm −2 , by XMM-Newton, Chandra, and Suzaku. A likely Two Micron All Sky Survey (2MASS) counterpart with K S = 10.2 shows colors indicative of a late-type star. CXOU J174042.0−280724 is a likely counterpart to the fast hard transient IGR J17407−2808. Chandra observations find F X (2-10 keV) ∼ 10 −12 erg cm −2 s −1 , with large N H variations (from 2 × 10 22 to >2 × 10 23 cm −2). No 2MASS counterpart is visible, to K S > 13. XMMU J174445.5−295044 seems likely to be a new symbiotic star or symbiotic X-ray binary, while CXOU J174042.0−280724 is more mysterious, likely an unusual low-mass X-ray binary.
The Astrophysical Journal, 1999
A survey of the inner 8 • × 1 • of the Galactic plane toward the Galactic center has been carried... more A survey of the inner 8 • × 1 • of the Galactic plane toward the Galactic center has been carried out at the 1720 MHz transition of OH molecule using the VLA in its D configuration with a resolution of ≈70 ′′ × 45 ′′. The detection of compact 1720 MHz OH masers associated with three supernova remnants G357.7+0.3, G1.13-0.1 (Sgr D) and G1.4-0.1 as well as new extended maser line emission from G357.7+0.3 and G357.7-0.1 (the Tornado Nebula) were then followed up by A-array observations with spectral and spatial resolutions of 0.3 kms −1 and ≈3 ′′ × 2 ′′ , respectively. The 1720 MHz OH maser line emission is considered to be a powerful shock diagnostic and is collisionally pumped by H 2 molecules at the site where C-type supernova shocks drive into adjacent molecular clouds. The new observations show clear evidence of extended features coincident with compact and bright masers, the best example of which is a coherent feature over a scale of about 20 pc surrounding the shell of the SNR G357.7+0.3. We argue that this remarkable feature is an OH maser and is physically associated with the remnant. This implies that the ambient molecular cloud is uniform in its density and temperature with restricted range of pumping conditions and survives the passage of a large-scale shock front.
The Astrophysical Journal, 1999
High-resolution VLA observations of 1720 MHz OH maser emission from Sgr A East and the circumnucl... more High-resolution VLA observations of 1720 MHz OH maser emission from Sgr A East and the circumnuclear disk with spatial and spectral resolutions of 2: 00 5 1: 00 3 and 0.27 kms 1 are reported. This follow-up observational study focuses on the recent discovery of a number of such OH maser features and their intense circularly polarized maser lines detected toward these Galactic center sources. The 1720 MHz maser line of OH arises from collisionally excited gas behind a C-type shock and is an important diagnostic of the interaction process that may occur between molecular clouds and associated X-ray emitting shell-type supernova remnants. The present observations have conrmed that the observed Stokes V signal is due to Zeeman splitting and that the OH masers are angularly broadened by the scattering medium toward the Galactic center. The scale length of the magnetic eld uctuations in the scattering medium toward the Galactic center is estimated to begreater than 0.1-0.2 pcusing the correlation of the position angles of the scatter-broadened maser spots. In addition, the kinematics of the maser spots associated with Sgr A East are used to place a 5 pcdisplacement b e t w een this extended radio structure and the Galactic center.
The Astrophysical Journal, 2003
The unusual class of magnetized nonthermal radio filaments (NTF), threads and streaks with their ... more The unusual class of magnetized nonthermal radio filaments (NTF), threads and streaks with their unique physical characteristics are found only within the inner couple of degrees of the Galactic center. Also, a number of young, mass-losing and rare stellar clusters are recognized to lie in the Galactic center region. The latter characteristic of the Galactic center region is used to explain the origin of the nonthermal radio filaments. We consider a mechanism in which the collective winds of massive WR and OB stars within a dense stellar environment produce shock waves that can accelerate particles to relativistic energies. This mechanism is an extension of a model originally proposed by Rosner and Bodo (1996), who suggested that energetic nonthermal particles are produced in a terminal shock of mass-losing stars. The large-scale distribution of the magnetic field in the context of this model is argued to have neither poloidal geometry nor pervasive throughout the Galactic center region.
The Astrophysical Journal, 2007
The inner couple of hundred parsecs of our Galaxy are characterized by a significant amount of sy... more The inner couple of hundred parsecs of our Galaxy are characterized by a significant amount of synchrotronemitting gas. Many of the best studied sources in this region exhibit a mixture of 6.4 keV Fe K emission, molecular line emission, and nonthermal radio continuum radiation. The spatial correlation between fluorescent Fe K line emission at 6.4 keV and molecular line emission from Galactic center molecular clouds has been explained as reflected X-rays from a past outburst of Sgr A Ã. Here we present a multiwavelength study of this region and find a correlation between the nonthermal radio filaments and the X-ray features. This correlation, when combined with the distribution of molecular gas, suggests against the irradiation model. Instead, we account for this distribution in terms of the impact of the relativistic particles from local (nonthermal filaments) and extended sources with diffuse neutral gas producing both nonthermal bremsstrahlung X-ray continuum emission and diffuse 6.4 keV line emission. The production rate of Fe K photons associated with the injection of electrons into a cloud as a function of column density is calculated. The required energy density of low-energy cosmic rays associated with the synchrotron-emitting radio filaments or extended features is estimated to be in the range between 20 and $10 3 eV cm À3 for Sgr C, Sgr B1, Sgr B2, and ''the 45 and À30 km s À1 '' clouds. We also generalize this idea to explain the cosmic-ray heating of molecular gas, the interstellar cosmic-ray ionization, the pervasive production of the diffuse K line, and TeVemission from the Galactic center molecular clouds. In particular, we suggest that inverse Compton scattering of the submillimeter radiation from dust by relativistic electrons may contribute substantially to the large-scale diffuse TeV emission observed toward the central regions of the Galaxy.
The Astrophysical Journal, 2007
Understanding the processes occurring in the nuclear disk of our Galaxy is interesting in its own... more Understanding the processes occurring in the nuclear disk of our Galaxy is interesting in its own right, as part of the Milky Way Galaxy, but also because it is the closest galactic nucleus. It has been more than two decades since it was recognized that the general phenomenon of higher gas temperature in the inner few hundred parsecs by comparison with local clouds in the disk of the Galaxy. This is one of the least understood characteristics of giant molecular clouds having a much higher gas temperature than dust temperature in the inner few degrees of the Galactic center. We propose that an enhanced flux of cosmic-ray electrons, as evidenced recently by a number of studies, are responsible for directly heating the gas clouds in the nuclear disk, elevating the temperature of molecular gas (∼ 75K) above the dust temperature (∼ 20K). In addition we report the detection of nonthermal radio emission from Sgr B2-F based on low-frequency GMRT and VLA observations. The higher ionization fraction and thermal energy due to the impact of nonthermal electrons in star forming sites have important implications in slowing down star formation in the nuclear disk of our galaxy and nuclei of galaxies.
The Astrophysical Journal, 2010
A group of four compact HII regions associated with the well-known 50 km s −1 molecular cloud is ... more A group of four compact HII regions associated with the well-known 50 km s −1 molecular cloud is the closest site of ongoing star formation to the dynamical center of the Galaxy, at a projected distance of ∼6 pc. We present a study of ionized gas based on the [NeII] (12.8 µm) line, as well as multi-frequency radio continuum, HST Paα and Spitzer IRAC observations of the most compact member of the HII group, Sgr A East HII D. The radio continuum image at 6cm shows that this source breaks up into two equally bright ionized features, D1 and D2. The SED of the D source is consistent with it being due to a 25±3 M ⊙ star with a luminosity of 8 ± 3 × 10 4 L ⊙. The inferred mass, effective temperature of the UV source and the ionization rate are compatible with a young O9-B0 star. The ionized features D1 and D2 are considered to be ionized by UV radiation collimated by an accretion disk. We consider that the central massive star photoevaporates its circumstellar disk on a timescale of 3×10 4 years giving a mass flux ∼ 3 × 10 −5 M ⊙ yr −1 and producing the ionized material in D1 and D2 expanding in an inhomogeneous medium. The ionized gas kinematics, as traced by the [Ne II] emission, is difficult to interpret, but it could be explained by the interaction of a bipolar jet with surrounding gas along with what appears
The Astrophysical Journal, 1982
Ntil-1C9uT RESOLUTICN UitSERVATICNS Uk THE 12.26 MICRON EMISSION F&UM H2 IN ThE CHICN MOLECULAS C... more Ntil-1C9uT RESOLUTICN UitSERVATICNS Uk THE 12.26 MICRON EMISSION F&UM H2 IN ThE CHICN MOLECULAS CLOUL (Caiiforaia Uuiv.) 14 P HC A02 /NF A01 Uacias CSCL OJA G3/89 .39303
The Astrophysical Journal, 2008
We present the results of GBT observations of all four ground-state hydroxyl(OH) transitions towa... more We present the results of GBT observations of all four ground-state hydroxyl(OH) transitions toward 15 supernova remnants(SNRs) which show OH(1720 MHz) maser emission. This species of maser is well established as an excellent tracer of an ongoing interaction between the SNR and dense molecular material. For the majority of these objects we detect significantly higher flux densities with a single dish than has been reported with interferometric observations. We infer that spatially extended, low level maser emission is a common phenomenon that traces the large-scale interaction in maser-emitting SNRs. Additionally we use a collisional pumping model to fit the physical conditions under which OH is excited behind the SNR shock front. We find the observed OH gas associated with the SNR interaction having columns N OH ≤ 1.5×10 17 cm −2 , temperatures of 20-125 K, and densities ∼10 5 cm −3. Subject headings: ISM: supernova remnants and clouds-masers and shock waves-radio lines: ISM
The Astrophysical Journal, 1998
Sub-arcsecond radio continuum observations of the Galactic center region at λ6 and 2cm reveal a 0... more Sub-arcsecond radio continuum observations of the Galactic center region at λ6 and 2cm reveal a 0.5 ′′ diameter source with a shell-like morphology. This source is linearly polarized at a level of 16% at λ6cm and has a steep nonthermal spectrum with spectral index 1.6 between λ6 and 2 cm. The distance to this source is not known but the large rotation measure value of 3000 rad m −2 suggests that G359.87+0.18 is likely to be located in the inner Galaxy or at an extragalactic distance. We discuss possible interpretations of this object as a recent supernova, a very young supernova remnant, a nova remnant, or an extragalactic source. All possibilities are highly problematic.
The Astrophysical Journal, 2008
A ring of dense molecular gas extending 2-7 pc orbits the supermassive black hole Sgr A* at the c... more A ring of dense molecular gas extending 2-7 pc orbits the supermassive black hole Sgr A* at the center of our Galaxy. Using the Green Bank Telescope, we detected water maser lines and both narrow (0.35 km s −1) and broad (30-50 km s −1) methanol emission from the molecular ring. Two of the strongest methanol lines at 44 GHz are confirmed as masers by interferometric observations. These class I methanol masers are collisionally excited and are signatures of early phases of massive star formation in the disk of the Galaxy, suggesting that star formation in the molecular ring is in its early phase. Close inspection of the kinematics of the associated molecular clumps in the HCN (J=1-0) line reveals broad redshifted wings indicative of disturbance by protostellar outflows from young (few ×10 4 yr), massive stars embedded in the clumps. The thermal methanol profile has a similar shape, with a narrow maser line superimposed on a broad, redshifted wing. Additional evidence for the presence of young massive protostars is provided by shocked molecular hydrogen and a number of striking ionized and molecular linear filaments in the vicinity of methanol sources suggestive of 0.5-pc scale protostellar jets. Given that the circumnuclear molecular ring is kinematically unsettled and thus is likely be the result of a recent capture, the presence of both methanol emission and broad, red-shifted HCN emission suggests that star formation in the circumnuclear ring is in its infancy.
The Astrophysical Journal, 2003
Radio surveys of supernova remnants (SNRs) in the Galaxy have uncovered 19 SNRs accompanied by OH... more Radio surveys of supernova remnants (SNRs) in the Galaxy have uncovered 19 SNRs accompanied by OH maser emission at 1720 MHz. This unusual class of maser sources is suggested to be produced behind a shock front from the expansion of a supernova remnant running into a molecular cloud. An important ingredient of this model is that X-ray emission from the remnant enhances the production of OH molecule. The role of X-ray emission from maser emitting (ME) SNRs is investigated by comparing the X-ray induced ionization rate with theory. One aspect of this model is verified: there is a strong association between maser emitting and mixed-morphology (MM) or thermal composite SNRs-center-filled thermal X-ray emission surrounded by shell-like radio morphology. We also present ROSAT and ASCA observations of two maser emitting SNRs: G21.8-0.6 (Kes 69) and G357.7-0.1 (Tornado).
The Astrophysical Journal, 2000
The Trifid nebula (M20) is a well-known prominent optical HII region trisected by obscuring dust ... more The Trifid nebula (M20) is a well-known prominent optical HII region trisected by obscuring dust lanes. Radio continuum VLA observations of this Nebula show free-free emission at λ3.6 and 6cm from three stellar sources lying close to the O7V star at the center of the nebula. We argue that neutral material associated with these stars are photoionized externally by the UV radiation from the hot central star. We also report the discovery of a barrel-shaped SNR G7.06-0.12 at the northwest rim of the nebula and two shell-like features G6.67-0.42 and G6.83-0.21 adjacent to W28 and M20. We discuss the nature of these features and their possible relationship to the pulsar PSR 1801-2306 and W28 OH (1720 MHz) masers.
The Astrophysical Journal, 2009
OH(1720 MHz) masers are excellent signposts of interaction between supernova remnants(SNRs) and m... more OH(1720 MHz) masers are excellent signposts of interaction between supernova remnants(SNRs) and molecular clouds. Using the GBT and VLA we have surveyed 75 SNRs and 6 candidates for masers. Four SNRs are detected with OH masers: G5.4-1.2, G5.7-0.0, G8.7-0.1 and G9.7-0.0. Two SNRs, G5.7-0.0 and G8.7-0.1, have TeV γ-ray counterparts which may indicate a local cosmic ray enhancement. It has been noted that maser-emitting SNRs are preferentially distributed in the Molecular Ring and Nuclear Disk. We use the present and existing surveys to demonstrate that masers are strongly confined to within |l| ≤ 50 • at a rate of 15% of the total SNR population. All new detections are within 10 • Galactic longitude emphasizing this trend. Additionally, a substantial number of SNR masers have peak fluxes at or below the detection threshold of existing surveys. This calls into question whether maser surveys of Galactic SNRs can be considered complete and how many maser-emitting remnants remain to be detected in the Galaxy.
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Papers by Farhad YusefZadeh