W. M. Goss

National Radio Astronomy Observatory, Charlottesville, Virginia, United States

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Publications (696)2746.19 Total impact

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    ABSTRACT: We present images of C110α and H110α radio recombination line (RRL) emission at 4.8 GHz and images of H166α, C166α, and X166α RRL emission at 1.4 GHz, observed toward the star-forming region NGC 2024. The 1.4 GHz image with angular resolution ~70'' is obtained using Very Large Array (VLA) data. The 4.8 GHz image with angular resolution ~17'' is obtained by combining VLA and Green Bank Telescope data in order to add the short and zero spacing data in the uv plane. These images reveal that the spatial distributions of C110α line emission is confined to the southern rim of the H II region close to the ionization front whereas the C166α line emission is extended in the north-south direction across the H II region. The LSR velocity of the C110α line is 10.3 km s–1 similar to that of lines observed from molecular material located at the far side of the H II region. This similarity suggests that the photodissociation region (PDR) responsible for C110α line emission is at the far side of the H II region. The LSR velocity of C166α is 8.8 km s–1. This velocity is comparable with the velocity of molecular absorption lines observed from the foreground gas, suggesting that the PDR is at the near side of the H II region. Non-LTE models for carbon line-forming regions are presented. Typical properties of the foreground PDR are T PDR ~ 100 K, cm–3, n H ~ 1.7 × 104 cm–3, and path length l ~ 0.06 pc, and those of the far side PDR are T PDR ~ 200 K, 50 cm–3, n H ~ 1.7 × 105 cm–3, and l ~ 0.03 pc. Our modeling indicates that the far side PDR is located within the H II region. We estimate the magnetic field strength in the foreground PDR to be 60 μG and that in the far side PDR to be 220 μG. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024. The H166α spectrum shows narrow (1.7 km s–1) and broad (33 km s–1) line features. The narrow line has spatial distribution and central velocity (~9 km s–1) similar to that of the foreground carbon line emission, suggesting that they are associated. Modeling the narrow H166α emission provides physical properties T PDR ~ 50 K, cm–3, and l ~ 0.01 pc and implies an ionization fraction of ~10–4. The broad H166α line originates from the H II region. The X166α line has a different spatial distribution compared to other RRLs observed toward NGC 2024 and is probably associated with cold dust clouds. Based on the expected low depletion of sulfur in such clouds and the –8.1 km s–1 velocity separation between the X166α and C166α lines, we interpret that the X166α transition arises from sulfur.
    The Astrophysical Journal 09/2014; 793(2):83. · 6.73 Impact Factor
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    ABSTRACT: We present images of C110$\alpha$ and H110$\alpha$ radio recombination line (RRL) emission at 4.8 GHz and images of H166$\alpha$, C166$\alpha$ and X166$\alpha$ RRL emission at 1.4 GHz, observed toward the starforming region NGC 2024. The 1.4 GHz image with angular resolution $\sim$ 70\arcsec\ is obtained using VLA data. The 4.8 GHz image with angular resolution $\sim$ 17\arcsec\ is obtained by combining VLA and GBT data. The similarity of the LSR velocity (10.3 \kms\) of the C110$\alpha$ line to that of lines observed from molecular material located at the far side of the \HII\ region suggests that the photo dissociation region (PDR) responsible for C110$\alpha$ line emission is at the far side. The LSR velocity of C166$\alpha$ is 8.8 \kms. This velocity is comparable with the velocity of molecular absorption lines observed from the foreground gas, suggesting that the PDR is at the near side of the \HII\ region. Non-LTE models for carbon line forming regions are presented. Typical properties of the foreground PDR are $T_{PDR} \sim 100$ K, $n_e^{PDR} \sim 5$ \cmthree, $n_H \sim 1.7 \times 10^4$ \cmthree, path length $l \sim 0.06$ pc and those of the far side PDR are $T_{PDR} \sim$ 200 K, $n_e^{PDR} \sim$ 50 \cmthree, $n_H \sim 1.7 \times 10^5$ \cmthree, $l \sim$ 0.03 pc. Our modeling indicates that the far side PDR is located within the \HII\ region. We estimate magnetic field strength in the foreground PDR to be 60 $\mu$G and that in the far side PDR to be 220 $\mu$G. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024.
    07/2014;
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    ABSTRACT: We use MERLIN, VLA and VLBA observations of Galactic \HI absorption towards 3C~138 to estimate the structure function of the \HI opacity fluctuations at AU scales. Using Monte Carlo simulations, we show that there is likely to be a significant bias in the estimated structure function at signal-to-noise ratios characteristic of our observations, if the structure function is constructed in the manner most commonly used in the literature. We develop a new estimator that is free from this bias and use it to estimate the true underlying structure function slope on length scales ranging $5$ to $40$~AU. From a power law fit to the structure function, we derive a slope of $0.81^{+0.14}_{-0.13}$, i.e. similar to the value observed at parsec scales. The estimated upper limit for the amplitude of the structure function is also consistent with the measurements carried out at parsec scales. Our measurements are hence consistent with the \HI opacity fluctuation in the Galaxy being characterized by a power law structure function over length scales that span six orders of magnitude. This result implies that the dissipation scale has to be smaller than a few AU if the fluctuations are produced by turbulence. This inferred smaller dissipation scale implies that the dissipation occurs either in (i) regions with densities $\gtrsim 10^3 $cm$^-3$ (i.e. similar to that inferred for "tiny scale" atomic clouds or (ii) regions with a mix of ionized and atomic gas (i.e. the observed structure in the atomic gas has a magneto-hydrodynamic origin).
    05/2014;
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    ABSTRACT: We use MERLIN, VLA and VLBA observations of Galactic \HI absorption towards 3C~138 to estimate the structure function of the \HI opacity fluctuations at AU scales. Using Monte Carlo simulations, we show that there is likely to be a significant bias in the estimated structure function at signal-to-noise ratios characteristic of our observations, if the structure function is constructed in the manner most commonly used in the literature. We develop a new estimator that is free from this bias and use it to estimate the true underlying structure function slope on length scales ranging $5$ to $40$~AU. From a power law fit to the structure function, we derive a slope of $0.81^{+0.14}_{-0.13}$, i.e. similar to the value observed at parsec scales. The estimated upper limit for the amplitude of the structure function is also consistent with the measurements carried out at parsec scales. Our measurements are hence consistent with the \HI opacity fluctuation in the Galaxy being characterized by a power law structure function over length scales that span six orders of magnitude. This result implies that the dissipation scale has to be smaller than a few AU if the fluctuations are produced by turbulence. This inferred smaller dissipation scale implies that the dissipation occurs either in (i) regions with densities $\gtrsim 10^3 $cm$^-3$ (i.e. similar to that inferred for "tiny scale" atomic clouds or (ii) regions with a mix of ionized and atomic gas (i.e. the observed structure in the atomic gas has a magneto-hydrodynamic origin).
    Monthly Notices of the Royal Astronomical Society 04/2014; 442(1). · 5.52 Impact Factor
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    ABSTRACT: We present new Very Large Array 6cm H2CO observations toward four extragalactic radio continuum sources (B0212+735, 3C111, NRAO150, BL Lac) to explore the structure of foreground Galactic clouds as revealed by absorption variability. This project adds a new epoch in the monitoring observations of the sources reported by Marscher and collaborators in the mid 1990's. Our new observations confirm the monotonic increase in H$_2$CO absorption strength toward NRAO150. We do not detect significant variability of our 2009 spectra with respect to the 1994 spectra of 3C111, B0212+735 and BL Lac; however we find significant variability of the 3C111 2009 spectrum with respect to archive observations conducted in 1991 and 1992. Our analysis supports that changes in absorption lines could be caused by chemical and/or geometrical gradients in the foreground clouds, and not necessarily by small scale (~10 AU) high density molecular clumps within the clouds.
    The Astrophysical Journal 02/2014; 784(2). · 6.73 Impact Factor
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    ABSTRACT: Accretion flows onto massive stars must transfer mass so quickly that they are themselves gravitationally unstable, forming dense clumps and filaments. These density perturbations interact with young massive stars, emitting ionizing radiation, alternately exposing and confining their HII regions. As a result, the HII regions are predicted to flicker in flux density over periods of decades to centuries rather than increasing monotonically in size as predicted by simple Spitzer solutions. We have recently observed the Sgr B2 region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and BnA) at a resolution of 0.25''. These observations were made to compare in detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr B2 contains 41 UC HII regions, 6 of which are hypercompact. The new observations of Sgr B2 allow comparison of relative peak flux densites for the HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the most source-rich massive star forming regions in the Milky Way. The new 1.3 cm continuum images indicate that four of the 41 UC HII regions exhibit significant changes in their peak flux density, with one source (K3) dropping in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing in peak flux density. The results are consistent with statistical predictions from simulations of high mass star formation, suggesting that they offer a solution to the lifetime problem for ultracompact HII regions.
    12/2013; 781(2).
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    Mark Morris, Jun-Hui Zhao, W. M. Goss
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    ABSTRACT: Deep imaging of the Sgr A complex at 6 cm wavelength with the B and C configurations of the Karl G. Jansky VLA has revealed a new population of faint radio filaments. Like their brighter counterparts that have been observed throughout the Galactic Center on larger scales, these filaments can extend up to ~10 parsecs, and in most cases are strikingly uniform in brightness and curvature. Comparison with a survey of Paschen-alpha emission reveals that some of the filaments are emitting thermally, but most of these structures are nonthermal: local magnetic flux tubes illuminated by synchrotron emission. The new image reveals considerable filamentary substructure in previously known nonthermal filaments (NTFs). Unlike NTFs previously observed on larger scales, which tend to show a predominant orientation roughly perpendicular to the Galactic plane, the NTFs in the vicinity of the Sgr A complex are relatively randomly oriented. Two well-known radio sources to the south of Sgr A - sources E and F - consist of numerous quasi-parallel filaments that now appear to be particularly bright portions of a much larger, strongly curved, continuous, nonthermal radio structure that we refer to as the "Southern Curl". It is therefore unlikely that sources E and F are HII regions or pulsar wind nebulae. The Southern Curl has a smaller counterpart on the opposite side of the Galactic Center - the Northern Curl - that, except for its smaller scale and smaller distance from the center, is roughly point-reflection symmetric with respect to the Southern Curl. The curl features indicate that some field lines are strongly distorted, presumably by mass flows. The point symmetry about the center then suggests that the flows originate near the center and are somewhat collimated.
    Proceedings of the International Astronomical Union 12/2013; 9(S303).
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    ABSTRACT: Based on our deep image of Sgr A using broadband data observed with the Jansky VLA at 6 cm, we present a new perspective of the radio bright zone at the Galactic center. We further show the radio detection of the X-ray Cannonball, a candidate neutron star associated with the Galactic center SNR Sgr A East. The radio image is compared with the Chandra X-ray image to show the detailed structure of the radio counterparts of the bipolar X-ray lobes. The bipolar lobes are likely produced by the winds from the activities within Sgr A West, which could be collimated by the inertia of gas in the CND, or by the momentum driving of Sgr A*; and the poloidal magnetic fields likely play an important role in the collimation. The less-collimated SE lobe, in comparison to the NW one, is perhaps due to the fact that the Sgr A East SN might have locally reconfigured the magnetic field toward negative galactic latitudes. In agreement with the X-ray observations, the time-scale of ~ $1\times10^4$ yr estimated for the outermost radio ring appears to be comparable to the inferred age of the Sgr A East SNR.
    Proceedings of the International Astronomical Union 11/2013; 9(S303).
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    ABSTRACT: We report the Very Large Array (VLA) detection of the radio counterpart of the X-ray object referred to as the "Cannonball," which has been proposed to be the remnant neutron star resulting from the creation of the Galactic center supernova remnant, Sagittarius A East. The radio object was detected both in our new VLA image from observations in 2012 at 5.5 GHz and in archival VLA images from observations in 1987 at 4.75 GHz and in the period from 1990 to 2002 at 8.31 GHz. The radio morphology of this object is characterized as a compact, partially resolved point source located at the northern tip of a radio "tongue" similar to the X-ray structure observed by Chandra. Behind the Cannonball, a radio counterpart to the X-ray plume is observed. This object consists of a broad radio plume with a size of 30 ×15 , followed by a linear tail having a length of 30 . The compact head and broad plume sources appear to have relatively flat spectra (∝ ν α) with mean values of α = −0.44 ± 0.08 and −0.10 ± 0.02, respectively, and the linear tail shows a steep spectrum with the mean value of −1.94 ± 0.05. The total radio luminosity integrated from these components is ∼8 × 10 33 erg s −1 , while the emission from the head and tongue amounts for only ∼1.5 × 10 31 erg s −1 . Based on the images obtained from the two epochs' observations at 5 GHz, we infer the proper motion of the object: μ α = 0.001 ± 0.003 arcsec yr −1 and μ δ = 0.013 ± 0.003 arcsec yr −1 . With an implied velocity of 500 km s −1 , a plausible model can be constructed in which a runaway neutron star surrounded by a pulsar wind nebula was created in the event that produced Sgr A East. The inferred age of this object, assuming that its origin coincides with the center of Sgr A East, is approximately 9000 yr.
    The Astrophysical Journal 11/2013; 777:146. · 6.73 Impact Factor
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    ABSTRACT: Our analysis of a VLBA 12-hour synthesis observation of the OH masers in a well-known star-forming region W49N has yielded valuable data that enables us to probe distributions of magnetic fields in both the maser columns and the intervening interstellar medium (ISM). The data consisting of detailed high angular-resolution images (with beam-width ~20 milli-arc-seconds) of several dozen OH maser sources or "spots", at 1612, 1665 and 1667 MHz, reveal anisotropic scatter broadening, with typical sizes of a few tens of milli-arc-seconds and axial ratios between 1.5 to 3. Such anisotropies have been reported earlier by Desai, Gwinn & Diamond (1994) and interpreted as induced by the local magnetic field parallel to the Galactic plane. However, we find a) the apparent angular sizes on the average a factor of ~2.5 less than those reported by Desai et al. (1994), indicating significantly less scattering than inferred earlier, and b) a significant deviation in the average orientation of the scatter-broadened images (by ~10 degrees) from that implied by the magnetic field in the Galactic plane. More intriguingly, for a few Zeeman pairs in our set, significant differences (up to 6 sigma) are apparent in the scatter broadened images for the two hands of circular polarization, even when apparent velocity separation is less than 0.1 km/s. This may possibly be the first example of a Faraday rotation contribution to the diffractive effects in the ISM. Using the Zeeman pairs, we also study the distribution of magnetic field in the W49N complex, finding no significant trend in the spatial structure function. In this paper, we present the details of our observations and analysis leading to these findings, discuss implications of our results for the intervening anisotropic magneto-ionic medium, and suggest the possible implications for the structure of magnetic fields within this star-forming region.
    The Astrophysical Journal 07/2013; 775(1). · 6.73 Impact Factor
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    ABSTRACT: We present a comprehensive view of the W51B HII region complex and the W51C supernova remnant (SNR) using new radio observations from the VLA, VLBA, MERLIN, JCMT, and CSO along with archival data from Spitzer, ROSAT, ASCA, and Chandra. Our VLA data include the first 400 cm (74 MHz) continuum image of W51 at high resolution (88 arcsec). The 400 cm image shows non-thermal emission surrounding the G49.2-0.3 HII region, and a compact source of non-thermal emission (W51B_NT) coincident with the previously-identified OH (1720 MHz) maser spots, non-thermal 21 and 90 cm emission, and a hard X-ray source. W51B_NT falls within the region of high likelihood for the position of TeV gamma-ray emission. Using the VLBA three OH (1720 MHz) maser spots are detected in the vicinity of W51B_NT with sizes of 60 to 300 AU and Zeeman effect magnetic field strengths of 1.5 to 2.2 mG. The multiwavelength data demonstrate that the northern end of the W51B HII region complex has been partly enveloped by the advancing W51C SNR and this interaction explains the presence of W51B_NT and the OH masers. This interaction also appears in the thermal molecular gas which partially encircles W51B_NT and exhibits narrow pre-shock (DeltaV 5 km/s) and broad post-shock (DeltaV 20 km/s) velocity components. RADEX radiative transfer modeling of these two components yield physical conditions consistent with the passage of a non-dissociative C-type shock. Confirmation of the W51B/W51C interaction provides additional evidence in favor of this region being one of the best candidates for hadronic particle acceleration known thus far.
    The Astrophysical Journal 05/2013; 771(2). · 6.73 Impact Factor
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    ABSTRACT: We present HI 21cm observations of the Orion Nebula, obtained with the Karl G. Jansky Very Large Array, at an angular resolution of 7.2"x5.7" and a velocity resolution of 0.77 km/s. Our data reveal HI absorption towards the radio continuum of the HII region, and HI emission arising from the Orion Bar photon-dominated region (PDR) and from the Orion-KL outflow. In the Orion Bar PDR, the HI signal peaks in the same layer as the H2 near-infrared vibrational line emission, in agreement with models of the photodissociation of H2. The gas temperature in this region is approximately 540K, and the HI abundance in the interclump gas in the PDR is 5-10% of the available hydrogen nuclei. Most of the gas in this region therefore remains molecular. Mechanical feedback on the Veil manifests itself through the interaction of ionized flow systems in the Orion Nebula, in particular the Herbig-Haro object HH202, with the Veil. These interactions give rise to prominent blueward velocity shifts of the gas in the Veil. The unambiguous evidence for interaction of this flow system with the Veil shows that the distance between the Veil and the Trapezium stars needs to be revised downwards to about 0.4pc. The depth of the ionized cavity is about 0.7pc, which is much smaller than the depth and the lateral extent of the Veil. Our results reaffirm the blister model for the M42 HII region, while also revealing its relation to the neutral environment on a larger scale.
    The Astrophysical Journal 11/2012; 762(2). · 6.73 Impact Factor
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    ABSTRACT: We present a study of short time-scale variability of OH masers within a contiguous 15-hour Very Long Baseline Array observation of the high-mass star-forming region, W3 (OH). With an angular resolution of ~7 mas and a velocity resolution of 53 m s-1, we isolate emission from masers in the field into individual Gaussian-shaped components, each a few milliarcseconds in size. We compute dynamic spectra for individual maser features with a time resolution of 1 minute by fitting for the flux density of all sources in the field simultaneously in the uv-domain. We isolate intrinsic maser variability from interstellar scintillation and instrumental effects. We find fluctuations in the maser line shape on time scales of 5 to 20 minutes, corresponding to maser column lengths of 0.5 to 2.0 Astronomical Units.
    Proceedings of the International Astronomical Union 07/2012; 8(S287):465-469.
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    ABSTRACT: Our analysis of a VLBA 12-hour synthesis observations of the OH masers in W49N has provided detailed high angular-resolution images of the maser sources, at 1612, 1665 and 1667 MHz. The images, of several dozens of spots, reveal anisotropic scatter broadening; with typical sizes of a few tens of milli-arc-seconds and axial ratios between 1.5 to 3. The image position angles oriented perpendicular to the galactic plane are interpreted in terms of elongation of electron-density irregularities parallel to the galactic plane, due to a similarly aligned local magnetic field. However, we find the apparent angular sizes on the average a factor of 2.5 less than those reported by Desai et al., indicating significantly less scattering than inferred earlier. The average position angle of the scattered broadened images is also seen to deviate significantly (by about 10 degrees) from that implied by the magnetic field in the Galactic plane. More intriguingly, for a few Zeeman pairs in our set, we find significant differences in the scatter broadened images for the two hands of polarization, even when apparent velocity separation is less than 0.1 km/s. Here we present the details of our observations and analysis, and discuss the interesting implications of our results for the intervening anisotropic magneto-ionic medium, as well as a comparison with the expectations based on earlier work.
    Proceedings of the International Astronomical Union 07/2012;
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    ABSTRACT: The structure function of opacity fluctuations is a useful statistical tool to study tiny scale structures of neutral hydrogen. Here we present high resolution observation of HI absorption towards 3C 138, and estimate the structure function of opacity fluctuations from the combined VLA, MERLIN and VLBA data. The angular scales probed in this work are ~ 10-200 milliarcsec (about 5-100 AU). The structure function in this range is found to be well represented by a power law S_tau(x) ~ x^{beta} with index beta ~ 0.33 +/- 0.07 corresponding to a power spectrum P_tau(U) ~ U^{-2.33}. This is slightly shallower than the earlier reported power law index of ~ 2.5-3.0 at ~ 1000 AU to few pc scales. The amplitude of the derived structure function is a factor of ~ 20-60 times higher than the extrapolated amplitude from observation of Cas A at larger scales. On the other hand, extrapolating the AU scale structure function for 3C 138 predicts the observed structure function for Cas A at the pc scale correctly. These results clearly establish that the atomic gas has significantly more structures in AU scales than expected from earlier pc scale observations. Some plausible reasons are identified and discussed here to explain these results. The observational evidence of a shallower slope and the presence of rich small scale structures may have implications for the current understanding of the interstellar turbulence.
    The Astrophysical Journal 03/2012; 749(2). · 6.73 Impact Factor
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    ABSTRACT: We present high angular resolution continuum observations of the high-mass protostar NGC 7538 S with BIMA and CARMA at 3 and 1.4 mm, Very Large Array (VLA) observations at 1.3, 2, 3.5, and 6 cm, and archive Infrared Array Camera (IRAC) observations from the Spitzer Space Observatory, which detect the star at 4.5, 5.8, and 8 μm. The star looks rather unremarkable in the mid-IR. The excellent positional agreement of the IRAC source with the VLA free-free emission, the OH, CH3OH, H2O masers, and the dust continuum confirms that this is the most luminous object in the NGC 7538 S core. The continuum emission at millimeter wavelengths is dominated by dust emission from the dense cold cloud core surrounding the protostar. Including all array configurations, the emission is dominated by an elliptical source with a size of ~8'' × 3''. If we filter out the extended emission we find three compact millimeter sources inside the elliptical core. The strongest one, SA, coincides with the VLA/IRAC source and resolves into a double source at 1.4 mm, where we have subarcsecond resolution. The measured spectral index, α, between 3 and 1.4 mm is ~2.3, and steeper at longer wavelengths, suggesting a low dust emissivity or that the dust is optically thick. We argue that the dust in these accretion disks is optically thick and estimate a mass of an accretion disk or infalling envelope surrounding SA to be ~60 M ☉.
    The Astrophysical Journal 02/2012; 746(2):187. · 6.73 Impact Factor
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    ABSTRACT: We present high angular resolution continuum observations of the high-mass protostar NGC 7538S with BIMA and CARMA at 3 and 1.4 mm, VLA observations at 1.3, 2, 3.5 and 6 cm, and archive IRAC observations from the Spitzer Space Observatory, which detect the star at 4.5, 5.8, and 8 $\mu$m. The star looks rather unremarkable in the mid-IR. The excellent positional agreement of the IRAC source with the VLA free-free emission, the OH, CH$_3$OH, H$_2$O masers, and the dust continuum confirms that this is the most luminous object in the NGC 7538S core. The continuum emission at millimeter wavelengths is dominated by dust emission from the dense cold cloud core surrounding the protostar. Including all array configurations, the emission is dominated by an elliptical source with a size of ~ 8" x 3". If we filter out the extended emission we find three compact mm-sources inside the elliptical core. The strongest one, $S_A$, coincides with the VLA/IRAC source and resolves into a double source at 1.4 mm, where we have sub-arcsecond resolution. The measured spectral index, $\alpha$, between 3 and 1.4 mm is ~ 2.3, and steeper at longer wavelengths, suggesting a low dust emissivity or that the dust is optically thick. We argue that the dust in these accretion disks is optically thick and estimate a mass of an accretion disk or infalling envelope surrounding S$_A$ to be ~ 60 solar masses.
    01/2012;
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    ABSTRACT: The interstellar medium is known to have significant structures over a wide range of scales. These structures are generally interpreted as the signature of turbulence in the ISM. Here we present the results from high resolution observation of HI absorption towards 3C138 and the estimated structure function of the tiny scale opacity fluctuations from the combined VLA, MERLIN and VLBA data. The structure function is well represented by a power law with power law index of 0.33 over 5 - 100 AU. The amplitude of the structure function suggests significantly higher opacity fluctuations at these scales than the expected value from the extrapolation of observations at larger scales. This indication of the presence of rich tiny scale structures may be used to constrain models of turbulent ISM.
    01/2012;
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    ABSTRACT: I will present results showing the state of the art observations of the ionized gas in young star-forming regions in galaxies with extremely high star formation rates. Measuring the density, mass, and kinematics of the ionized gas in these regions tells us about the state of the interstellar medium in young star-forming regions and about the massive stars themselves. Unfortunately, the state of the ionized gas is difficult to determine in the optical due to the high extinction from the remnants of the surrounding natal cocoon of dust and gas. The ionized gas in these obscured regions can be more accurately traced using radio recombination lines (RRLs). RRLs have been difficult to detect in galaxies other than the brightest dozen nearby starburst galaxies because of the limited sensitivity and bandwidth of the previous generation of radio telescopes. RRL observations of fainter and more distant galaxies are now possible with the recent enormous improvements in radio telescope instrumentation. Our RRL observations showcase the power of these revitalized tracers of the ionized gas in young star-forming galaxies.
    01/2012;
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    ABSTRACT: Obtaining pulsar parallaxes via relative astrometry (also known as differential astrometry) yields distances and transverse velocities that can be used to probe properties of the pulsar population and the interstellar medium. Large programs are essential to obtain the sample sizes necessary for these population studies, but they must be efficiently conducted to avoid requiring an infeasible amount of observing time. This paper describes the PSRPI astrometric program, including the use of new features in the DiFX software correlator to efficiently locate calibrator sources, selection and observing strategies for a sample of 60 pulsars, initial results, and likely science outcomes. Potential applications of high-precision relative astrometry to measure source structure evolution in defining sources of the International Celestial Referent Frame are also discussed.
    10/2011;

Publication Stats

7k Citations
2,746.19 Total Impact Points

Institutions

  • 1983–2014
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
    • European Southern Observatory
      Arching, Bavaria, Germany
  • 2013
    • University of Tasmania
      Hobart Town, Tasmania, Australia
  • 2012
    • The University of Edinburgh
      • Institute for Astronomy (IfA)
      Edinburgh, Scotland, United Kingdom
  • 2010
    • University of Iowa
      • Department of Physics and Astronomy
      Iowa City, IA, United States
  • 2009
    • University of Bonn
      • Argelander-Institute of Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 2007–2008
    • New Mexico Institute of Mining and Technology
      • Department of Physics
      Socorro, New Mexico, United States
  • 2004–2005
    • Cornell University
      • Department of Astronomy
      Ithaca, New York, United States
  • 2003–2005
    • University of New Mexico
      • Department of Physics & Astronomy
      Albuquerque, New Mexico, United States
  • 1983–2005
    • CSU Mentor
      Long Beach, California, United States
  • 2002
    • Agnes Scott College
      Louisiana, United States
  • 1975–2002
    • Raman Research Institute
      Bengalūru, Karnātaka, India
  • 1977–1998
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
  • 1996
    • The University of Manchester
      Manchester, England, United Kingdom
  • 1994
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, IL, United States
  • 1993
    • Northwestern University
      Evanston, Illinois, United States
    • Middlebury College
      Middlebury, Indiana, United States
  • 1983–1993
    • University of Kentucky
      • Department of Physics & Astronomy
      Lexington, KY, United States
  • 1992
    • Università degli Studi di Palermo
      Palermo, Sicily, Italy
  • 1988
    • Princeton University
      • Department of Physics
      Princeton, New Jersey, United States
  • 1974–1985
    • University of Groningen
      Groningen, Groningen, Netherlands
  • 1984
    • University of Illinois, Urbana-Champaign
      • Department of Astronomy
      Urbana, Illinois, United States
    • University of Houston – Victoria
      Houston, Texas, United States
    • Tata Institute of Fundamental Research
      Mumbai, Mahārāshtra, India
  • 1974–1982
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 1976
    • The Commonwealth Scientific and Industrial Research Organisation
      Canberra, Australian Capital Territory, Australia