arXiv subjects

T. L. Wilson

Publications and source records attributed to T. L. Wilson.

At least 55 records · Page 3Linked to original sources

arXiv2005

Interstellar 12C/13C ratios through CH+ ll 3957,4232 absorption in local clouds: incomplete mixing in the ISM

The 12C/13C isotope ratio is a tracer of stellar yields and the efficiency of mixing in the ISM. 12CH+/13CH+ is not affected by interstellar chemistry, and is the most secure way of measuring 12C/13C in the diffuse ISM. R= 12C/13C is 90 in the solar system. Previous measurements of 12CH+ ll3957.7,4232.3 and 13CH+ ll3958.2,4232.0 absorption toward nearby stars indicate some variations in 12C/13C, with values ranging from 40 to 90 suggesting inefficient mixing. Except for the cloud toward zeta Oph, these R values are strongly affected by noise. With UVES on the VLT we have improved on the previous interstellar 12C/13C measurements. The weighted 12C/13C ratio in the local ISM is 78.27 +- 1.83, while the weighted dispersion of our measurements is 12.7, giving a 6.9 sigma scatter. Thus we report on a 6.9 sigma detection of 16.2% root-mean-square variations in the carbon isotopic ratio on scales of ~100 pc: R= 74.7 +- 2.3 in the zetaOph cloud, while R = 88.6 +- 3.0 toward HD152235 in the Lupus clouds, R = 62.2 +- 5.3 towards HD110432 in the Coalsack, and R = 98.9 +- 10.1 toward HD170740. The observed variations in 13C/12C are the first significant detection of chemical heterogeneity in the local ISM.

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arXiv2005

Hubble Space Telescope NICMOS Imaging of W3 IRS 5: A Trapezium in the Making?

We present Hubble Space Telescope NICMOS imaging of W3 IRS 5, a binary high-mass protostar. In addition to the two protostars, NICMOS images taken in the F222M and F160W filters show three new 2.22 micron sources with very red colors; these sources fall within a region 5600 AU in diameter, and are coincident with a 100 solar mass dense molecular clump. Two additional point sources are found within 0.4'' (800 AU) of one of the high-mass protostars; these may be stellar companions or unresolved emission knots from an outflow. We propose that these sources constitute a nascent Trapezium system in the center of the W3 IRS 5 cluster containing as many as five proto OB stars. This would be the first identification of a Trapezium still deeply embedded in its natal gas.

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arXiv2004

An Improved Estimate of the Mass of Dust in Cassiopeia A

Recent observations of sub-millimeter continuum emission toward supernova remnants (SNR) have raised the question of whether such emission is caused by dust within the SNR and thus produced by the supernova itself or along the line-of-sight. The importance of the present work is to establish evidence for the production of large amounts of dust in supernovae. The best tests can be made for young supernovae in our galaxy. Cassiopeia A is the best candidate for a measurement.

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arXiv2004

ISO observations of the Galactic center Interstellar Medium: neutral gas and dust

The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100-200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines and the full continuum spectra of the dust between 40 and 190 microns. A warm dust component with a temperature between 27 and 42 K is needed to fit the spectra. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 10$^3$ cm^-3 and an incident far-ultraviolet field 10$^3$ times higher than the local interstellar radiation field. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10-30% of the total H2 column density with a temperature of ~ 150 K. We discuss other possible heating mechanisms (short version).

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arXiv2003

High-Resolution Continuum Imaging at 1.3 and 0.7 cm of the W3 IRS 5 Region

High-resolution images of the hypercompact HII regions (HCHII) in W3 IRS 5 taken with the Very Large Array (VLA) at 1.3 and 0.7 cm are presented. Four HCHII regions were detected with sufficient signal-to-noise ratios to allow the determination of relevant parameters such as source position, size and flux density. The sources are slightly extended in our ~0.2 arcsecond beams; the deconvolved radii are less than 240 AU. A comparison of our data with VLA images taken at epoch 1989.1 shows proper motions for sources IRS 5a and IRS 5f. Between 1989.1 and 2002.5, we find a proper motion of 210 mas at a position angle of 12 deg for IRS 5f and a proper motion of 190 mas at a position angle of 50 deg for IRS 5a. At the assumed distance to W3 IRS 5, 1.83 +/- 0.14 kpc, these offsets translate to proper motions of ~135 km/s and ~122 km/s$ respectively. These sources are either shock ionized gas in an outflow or ionized gas ejected from high mass stars. We find no change in the positions of IRS 5d1/d2 and IRS 5b; and we show through a comparison with archival NICMOS 2.2 micron images that these two radio sources coincide with the infrared double constituting W3 IRS 5. These sources contain B or perhaps O stars. The flux densities of the four sources have changed compared to the epoch 1989.1 results. In our epoch 2002.5 data, none of the spectral indicies obtained from flux densities at 1.3 and 0.7 cm are consistent with optically thin free-free emission; IRS 5d1/d2 shows the largest increase in flux density from 1.3 cm to 0.7 cm. This may be an indication of free-free optical depth within an ionized wind, a photoevaporating disk, or an accretion flow. It is less likely that this increase is caused by dust emission at 0.7 cm.

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arXiv2003

A 13CO and C18O Survey of the Molecular Gas Around Young Stellar Clusters Within 1kpc of the Sun

As the first step of a multi-wavelength investigation into the relationship between young stellar clusters and their environment we present fully-sampled maps in the J=1--0 lines of 13CO and C18O and the J=2--1 line of C18O for a selected group of thirty young stellar groups and clusters within 1kpc of the Sun. This is the first systematic survey of these regions to date. The clusters range in size from several stars to a few hundred stars. Thirty fields ranging in size from 8'x 8' to 60'x 30' were mapped with 47'' resolution simultaneously in the J=1-0 lines with Five College Radio Observatory. Seventeen sources were mapped over fields ranging in size from 3'x 3' to 13'x 13' in the J=2--1 line with 35'' resolution with the Heinrich Hertz Telescope. We compare the cloud properties derived from each of the three tracers in order to better understand systematic uncertainties in determing masses and linewidths. Cloud masses are determined independently using the 13CO and C18O transitions, these masses range from 30 to 4000 M_sun. Finally, we present a simple morphological classification scheme which may serve as a rough indicator of cloud evolution.

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arXiv2003

Warm molecular gas, dust and ionized gas in the 500 central pc of the Galaxy

We present infrared and millimeter observations of molecular gas, dust and ionized gas towards a sample of clouds distributed along the 500 central pc of the Galaxy. The clouds were selected to investigate the physical state, in particular the high gas temperatures, of the Galactic center region (GCr) clouds located far from far-infrared of thermal radio continuum sources. We have found that there is ionized gas associated with the molecular gas. The ionizing radiation is hard (~35000 K) but diluted due to the inhomogeneity of the medium. We estimate that ~30 % of the warm molecular gas observed in the GCr clouds is heated by ultra-violet radiation in photo-dissociation regions.

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arXiv2002

VLBI observations at 147 GHz: first detection of transatlantic fringes in bright AGN

At 147 GHz (2mm wavelength), we detected three prominent AGN (NRAO150, 3C279, 1633+382) with Very Long Baseline Interferometry (VLBI) with an angular resolution of only 18 micro-arcseconds on the baseline between two antennas in Arizona (10m HHT and 12m KittPeak) and the IRAM 30m antenna on Pico Veleta in Spain. This is a new world record in radio interferometry and astronomical imaging and opens fascinating future possibilities to directly image and study the innermost regions in Quasars and other Active Galactic Nuclei.

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arXiv2002

CO Isotopes in Planetary Nebulae

Standard stellar evolution theory is inconsistent with the observed isotopic carbon ratio, 12C/13C, in evolved stars. This theory is also inconsistent with the 3He/H abundance ratios observed in Galactic HII regions, when combined with chemical evolution theory. These discrepancies have been attributed to an extra, non-standard mixing which further processes material during the RGB and should lower both the 12C/13C and 3He/H abundance ratios for stars with masses < 2 solar masses. Measurements of isotopic ratios in planetary nebulae probe material which escapes the star to be further processed by future generations of stars. We have measured the carbon isotopic abundance ratio, 12C/13C, in 11 planetary nebulae (PNe) by observing the J=2-->1 and J=3-->2 millimeter transitions of 12CO and 13CO in molecular clouds associated with the PNe. A large velocity gradient (LVG) model has been used to determine the physical conditions for each PNe where both transitions have been detected. We detect both 12CO and 13CO in 9 PNe. If 12CO/13CO = 12C/13C, the range of 12C/13C is 2.2--31. Our results support theories which include some form of extra mixing.

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arXiv2001

The Impact of Massive Stars on the Formation of Young Stellar Clusters

Massive OB stars play an important role in the evolution of molecular clouds and star forming regions. The OB stars both photo--ionize molecular gas as well as sweep up and compress interstellar gas through winds, ionization fronts, and supernovae. In this contribution, we examine the effect massive stars have on the formation of young stellar clusters. We first discuss the processes by which OB stars destroy cluster--forming molecular cores, and hence terminate star formation. We overview observational evidence that OB stars forming in young stellar clusters destroy their parental cores on a timescale of 0.1 Myr, and we discuss some of the implications of this result. We then summarize extensive observations of the NGC 281 molecular cloud complex, and present evidence that two types of triggered star formation are occurring in this complex. Our goal is to underscore the impact massive stars have on cluster formation over distances ranging from 0.1 pc to 300 pc.

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arXiv2001

Molecular Gas Around Young Stellar Clusters

We have begun a survey of the molecular gas surrounding 31 young clusters in order to investigate the link between environment and the resulting cluster. We present here a preliminary comparison of two clusters in our sample: GGD12-15 and Mon R2. Since both clusters are located in the MonR2 molecular cloud at a distance of 830 pc, observational biases due to differing sensitivities and angular resolutions are avoided.

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arXiv2001

Submillimeter CO Line Emission from Orion

Images of an 8 square minute region around the Orion KL source have been made in the J=7-6 (806 GHz) and J=4-3 (461 GHz) lines of CO with angular resolutions of 13" and 18". These data were taken employing on-the-fly mapping and position switching techniques. Our J=7-6 data set is the largest image of Orion with the highest sensitivity and resolution obtained so far in this line. Most of the extended emission arises from a Photon Dominated Region (PDR), but 8% is associated with the Orion ridge. For the prominent Orion KL outflow, we produced ratios of the integrated intensities of our J=7-6 and 4-3 data to the J=2-1 line of CO. Large Velocity Gradient (LVG) models fit the outflow ratios better than PDR models. The LVG models give H_2 densities of ~10^5 per ccm. The CO outflow is probably heated by shocks. In the Orion S outflow, the CO line intensities are lower than for Orion KL. The 4-3/2-1 line ratio is 1.3 for the blue shifted wing and 0.8 for the red shifted wing. Emission in the jet feature extending 2' to the SW of Orion S was detected in the J=4-3 but not the J=7-6 line; the average 4-3/2-1 line ratio is ~1. The line ratios in the Orion S outflow and jet features are consistent with both PDR and LVG models. Comparisons of the intensities of the J=7-6 and J=4-3 lines from the Orion Bar with PDR models show that the ratios exceed predictions by a factor of 2. Either clumping or additional heating by mechanisms such as shocks, may be the cause of this discrepancy.

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arXiv2001

A High Density Thin layer confining the HII region M42. HHT measurements

We present HHT observations in the N=3-2 rotational transition of the CN radical toward selected positions of the Trapezium region and of the molecular Ridge in the Orion molecular cloud. Two of the positions in the Ridge were also observed in the N=2-1 line of CN and 13CN. The N=3-2 CN lines have been combined with observations of the N=2-1 and N=1-0 transitions of CN, and of the N=2-1 of 13CN to estimate the physical conditions and CN abundances in the molecular gas. We analyze in detail the excitation of the CN lines and find that the hyperfine ratios of the N=3-2 line are always close to the Local Thermodynamic Equilibrium (LTE) optically thin values even in the case of optically thick emission. This is due to different excitation temperatures for the different hyperfine lines. From the line intensity ratios between the different CN transitions we derive H_2 densities of ~10^5cm^-3 for the molecular Ridge and of ~3x10^6cm^-3 for the Trapezium region. The CN column densities are one order of magnitude larger in the Ridge than in the Trapezium region, but the CN to H_2 ratio is similar both in the Trapezium and in the Ridge. The combination of the low CN column densities, high H_2 densities and relatively high CN abundances toward the Trapezium region requires that the CN emission arises from a thin layer with a depth along the line of sight of only \~5x10^15cm. This high density thin layer of molecular gas seems to be related with material that confines the rear side of the HII region Orion A. However the molecular layer is not moving as expected from the expansion of the HII region, but it is ``static'' with respect to the gas in the molecular cloud. We discuss the implication of a high density ``static'' layer in the evolution of an HII region.

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arXiv2000

Warm H2 in the Galactic center region

We present ISO observations of several H2 pure-rotational lines (from S(0) to S(5)) towards a sample of 16 molecular clouds distributed along the central ~ 500 pc of the Galaxy. We also present C18O and 13CO J=1->0 and J=2->1 observations of these sources made with the IRAM-30m telescope. With the CO data we derive H2 densities of 10e(3.5-4.0) cm-3 and H2 column densities of a few 10e22 cm-2. We have corrected the H2 data for ~ 30 magnitudes of visual extinction using a self-consistent method. In every source, we find that the H2 emission exhibits a large temperature gradient. The S(0) and S(1) lines trace temperatures (T) of ~150 K while the S(4) and S(5) lines indicate temperatures of ~ 600K. The warm H2 column density is typically ~1-2 x 10e22 cm-2, and is predominantly gas with T=150 K. This is the first direct estimate of the total column density of the warm molecular gas in the Galactic center region. These warm H2 column densities represent a fraction of ~ 30 % of the gas traced by the CO isotopes emission. The cooling by H2 in the warm component is comparable to that by CO. Comparing our H2 and CO data with available ammonia NH3 observations from literature one obtains relatively high NH3 abundances of a few 10e(-7) in both the warm and the cold gas. A single shock or Photo-Dissociation Region (PDR) cannot explain all the observed H2 lines. Alternatives for the heating mechanisms are discussed.

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arXiv2000

Dense gas in nearby galaxies XIII. CO submillimeter line emission from the starburst galaxy M82

12CO 1-0,2-1,4-3,7-6, and 13CO 1-0, 2-1, and 3-2 line was mapped with angular resolutions of 13" - 22" toward the nuclear region of starburst galaxy M82. The difference of lobe spacings in submillimeter (~15") and millimeter (~26") lines indicates the presence of a `low' and a `high' CO excitation component. An LVG excitation analysis of the submillimeter lines leads to inconsistencies, since area and volume filling factors are almost the same, resulting in cloud sizes along the lines-of-sight that match the entire size of the M82 starburst region. Nevertheless, LVG column densities agree with estimates derived from the dust emission in the far infrared and at submillimeter wavelengths. Accounting for high UV fluxes and variations in kinetic temperature and assuming that the observed emission arises from photon dominated regions (PDRs) resolves the problems related to an LVG treatment of the radiative transfer. 12CO/13CO line intensity ratios > 10 indicate that the bulk of the CO emission arises in UV-illuminated diffuse cloud fragments of small column density and sub-parsec cloud sizes with area filling factors >> 1. Thus CO arises from quite a different gas component than the classical high density tracers. The dominance of such a diffuse molecular interclump medium also explains observed high [CI}/CO line intensity ratios. PDR models do not allow a determination of the relative abundances of 12CO to 13CO. Ignoring magnetic fields, the CO gas appears to be close to the density limit for tidal disruption. A warm diffuse ISM not only dominates the CO emission in the starburst region of M82 but is also ubiquitous in the central region of our Galaxy, where tidal stress, cloud-cloud collisions, shocks, high gas pressure, and high stellar densities may all contribute to the formation of a highly fragmented molecular debris.

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arXiv2000

Non-equilibrium H2 ortho-to-para ratio in two molecular clouds of the Galactic Center

We present ISO observations of the S(0), S(1), S(2), and S(3) rotational lines of molecular hydrogen from two molecular clouds near the Galactic Center (GC). We have also measured continuum dust emission at infrared wavelengths with ISO and the rotational radio lines J=1-0 of 13CO and C18O and J=2-1 of C18O with the IRAM-30m telescope. Using the dust continuum spectra and the CO lines we derive a total visual extinction of 15-20 magnitudes toward these GC clouds. After correcting the H2 data for extinction, the gas temperatures are \~250 K and the column densities of warm gas are ~2e21 cm-2. This is the first direct measure of the H2 column densities of the warm component; with this, we estimate an NH3 abundance in the warm gas of ~ 2e-7. The column density of warm gas is, at least, a factor of 100 larger than the corresponding column densities derived from the warm dust. The observed ortho-to-para ratio (OTPR) is ~ 1, clearly below the local thermodynamical equilibrium (LTE) OTPR for gas at 250 K of ~3. Low velocity shocks (~ 10 km/s) are the most likely explanation for the column densities of warm gas and dust and the non-LTE H2 OTPR.

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arXiv1999

Jet driven molecular outflows in Orion

We present high sensitivity and high angular resolution images of the high velocity (vLSR>30kms^-1) CO emission in the J=1-0 and J=2-1 lines of the Orion KL region. These results reveal the morphology of the high-velocity CO emission at the most extreme velocities. High velocity emission have been only detected in two regions: BN/KL (IRc2/I) and Orion-S. The Orion-S region contains a very young (dynamical age of 10^3years), very fast (~110kms^-1) and very compact (<0.16pc) bipolar outflow. From the morphology of the high-velocity gas we estimate that the position of the powering source must be ~20'' north of FIR4. For the IRc2/I molecular outflow the morphology of the moderate velocity (<60kms^-1) gas shows a weak bipolarity around IRc2/I. The gas at the most extreme velocities does not show any bipolarity around IRc2/I, if any, it is found ~30'' north from these sources. The blue and redshifted gas at moderate velocities shows similar spatial distribution with a systematic trend for the size of the high-velocity gas to decrease as the terminal radial velocity increases. The size-velocity relationship is fitted with a simple velocity law which considers a highly collimated jet and entrained material outside the jet moving in the radial direction. We also find that most of the CO outflowing at moderate velocities is located at the head of the jet. Our results and the spatial distribution and kinematics of the shock tracers in this outflow can be explained if the IRc2/I outflow is driven by a precessing jet oriented along the line of sight.

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arXiv1999

Jets and high-velocity bullets in the Orion A outflows. Is the IRc2 outflow powered by a variable jet?

We present high sensitivity maps of the High Velocity (HV) CO emission toward the molecular outflows around IRc2 and Orion-S in the Orion A molecular cloud. The maps reveal the presence of HV bullets in both outflows with velocities between 40-80kms-1 from the ambient gas velocity. The blue and redshifted CO HV bullets associated with the IRc2 outflow are distributed in thin (12"-20", 0.02-0.04pc) elliptical ring-like structures with a size of ~10"x50" (0.02x0.1pc). The CO emission at the most extreme blue and redshifted velocities (EHV) peaks 20" north of source I, just inside the rings of the HV bullets. The low velocity H2O masers and the H2* bullets around IRc2 are located at the inner edges of the ring of CO HV bullets and surrounding the EHV CO emission. Furthermore, the high velocity H2O masers are very well correlated with the EHV CO emission. This morphology is consistent with a model of a jet driven molecular outflow oriented close to the line of sight. In the Orion-S outflow, the morphology of the CO HV bullets shows a bipolar structure in the southeast-northwest direction, and the H2O masers are found only at low velocities in the region between the exciting source and the CO HV bullets. The morphology of the CO HV bullets, the radial velocities and the spatial distribution of the H2O masers in both outflows, as well as the H2* features around IRc2, are consistent with a model in which these outflows are driven by a jet variable in direction.In this scenario, the large traverse velocity measured for the H2O masers in the IRc2 outflow, ~18kms-1, supports the evolutionary connection between the jet and the shell-like outflows.

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