arXiv subjects

T. L. Wilson

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

At least 37 records · Page 2Linked to original sources

arXiv2012

CH3OCH3 in Orion-KL: a striking similarity with HCOOCH3

We used several data sets from the Plateau de Bure Interferometer to map the dimethyl ether emission in Orion-KL with different arcsec spatial resolutions and different energy levels to compare with our previous methyl formate maps. Our data show remarkable similarity between the dimethyl ether (CH3OCH3) and the methyl formate (HCOOCH3) distributions even on a small scale (1.8"x0.8" or about 500 AU). This long suspected similarity, seen from both observational and theoretical arguments, is demonstrated with unprecedented confidence, with a correlation coefficient of maps of 0.8. A common precursor is the simplest explanation of our correlation. Comparisons with previous laboratory work and chemical models suggest the major role of grain surface chemistry and a recent release, probably with little processing, of mantle molecules by shocks. In this case the CH3O radical produced from methanol ice would be the common precursor (whereas ethanol, C2H5OH, is produced from the radical CH2OH). The alternative gas phase scheme, where protonated methanol CH3OH2+ is the common precursor to produce methyl formate and dimethyl ether through reactions with HCOOH and CH3OH, is also compatible with our data. Our observations cannot yet definitely allow a choice between the different chemical processes, but the tight correlation between the distributions of HCOOCH3 and CH3OCH3 strongly contrasts with the different behavior we observe for the distributions of ethanol and formic acid. This provides a very significant constraint on models.

astro-ph.GA↗

arXiv2012

Herschel/PACS Spectroscopic Survey of Protostars in Orion: The Origin of Far-Infrared CO Emission

We present far-IR (57-196 mu) spectra of 21 protostars in the Orion molecular clouds, obtained with the Photodetector Array Camera and Spectrometer (PACS) onboard the Herschel Space observatory, as part of the Herschel Orion Protostar Survey (HOPS) program. We analyzed the CO emission lines (J_up = 14-46) in the PACS spectra, extracted within a projected distance of <= 2000 AU centered on the protostar. The total luminosity of the CO lines observed with PACS (L(CO)) is found to increase with increasing L_bol. The CO rotational temperature implied by the line ratios increases with J, and at least 3-4 rotational temperature components are required to fit the observed rotational diagram. The rotational temperature components are remarkably invariant between protostars and show no dependence on L_bol, T_bol or envelope density, implying that if the emitting gas is in LTE, the CO emission must arise in multiple temperature components that remain independent of L_bol over two orders of magnitudes. The observed CO emission can also be modeled as arising from a single temperature gas component or from a medium with a power-law temperature distribution; both of these require sub-thermally excited molecular gas at low densities (n(H_2) <= 10^6 cm^-3) and high temperatures (T >= 2000 K). Our results suggest that the contribution from PDRs along the envelope cavity walls is unlikely to be the dominant component of the CO emission observed with PACS. Instead, the "universality" of the rotational temperatures and the observed correlation between L(CO) and L_bol can most easily be explained if the observed CO emission originates in shock-heated, hot (T >= 2000 K), sub-thermally excited (n(H_2) <= 10^6 cm^-3) molecular gas. Post-shock gas at these densities is more likely to be found within the outflow cavities along the molecular outflow or along the cavity walls at radii >= several 100-1000 AU.

astro-ph.GA↗

arXiv2012

Sense and sensitivity: How ALMA receivers work

In previous articles, we described how electromagnetic waves emitted from objects in the sky are collected by the ALMA antennas (Anatomy of ALMA), and how they are combined in order to produce images. Before these images can be processed, they are picked up by the antennas and concentrated by the large main mirror and a smaller secondary mirror in the so called focal point of each antenna. In order to process the data they must be first converted to electromagnetic waves of a lower frequency and amplified. This is the role of the ALMA receivers. In principle they work like a normal AM receiver, but at much higher frequencies. Here we describe how they work and what makes them special.

astro-ph.IM↗

arXiv2012

Multiwavelength Observations of V2775 Ori, an Outbursting Protostar in L 1641: Exploring the Edge of the FU Orionis Regime

Individual outbursting young stars are important laboratories for studying the physics of episodic accretion and the extent to which this phenomenon can explain the luminosity distribution of protostars. We present new and archival data for V2775 Ori (HOPS 223), a protostar in the L 1641 region of the Orion molecular clouds that was discovered by Caratti o Garatti et al. (2011) to have recently undergone an order-of-magnitude increase in luminosity. Our near-infrared spectra of the source have strong blueshifted He I 10830 absorption, strong H2O and CO absorption, and no H I emission, all typical of FU Orionis sources. With data from IRTF, 2MASS, HST, Spitzer, WISE, Herschel, and APEX that span from 1 to 70 microns pre-outburst and from 1 to 870 microns post-outburst, we estimate that the outburst began between 2005 April and 2007 March. We also model the pre- and post-outburst spectral energy distributions of the source, finding it to be in the late stages of accreting its envelope with a disk-to-star accretion rate that increased from about 2x10^-6 M_sun/yr to about 10^-5 M_sun/yr during the outburst. The post-outburst luminosity at the epoch of the FU Orionis-like near-IR spectra is 28 L_sun, making V2775 Ori the least luminous documented FU Orionis outburster with a protostellar envelope. The existence of low-luminosity outbursts supports the notion that a range of episiodic accretion phenomena can partially explain the observed spread in protostellar luminosities.

astro-ph.SR↗

arXiv2011

The Structure of NGC 1976 in the Radio Range, v2

High angular resolution radio continuum images of NGC 1976 (M42, Orion A) at frequency=330 MHz (wavelength=91 cm), 1.5 GHz (20 cm) and 10.6 GHz (2.8 cm), have been aligned, placed on a common grid, smoothed to common resolutions of 80" (=0.16 pc at 420 pc) and 90" (=0.18 pc) and compared on a position-by-position basis. The results are not consistent with a single value of Te. The best fit to the continuum data is a multi-layer model based on radio recombination line (RRL) data with a monotonic variation, from Te=8500K in the higher intensity, more compact region at the rear of NGC 1976 to Te =6000K in the low intensity, extended region in the foreground. An estimate of temperature fluctuations toward the peak from this model yields t2=0.003. This is a factor of 10 lower than fluctuation values from optical collisionally excited line data.

astro-ph.GA↗

arXiv2011

Techniques of Radio Astronomy

This chapter provides an overview of the techniques of radio astronomy. This study began in 1931 with Jansky's discovery of emission from the cosmos, but the period of rapid progress began fifteen years later. From then to the present, the wavelength range expanded from a few meters to the sub-millimeters, the angular resolution increased from degrees to finer than milli arc seconds and the receiver sensitivities have improved by large factors. Today, the technique of aperture synthesis produces images comparable to or exceeding those obtained with the best optical facilities. In addition to technical advances, the scientific discoveries made in the radio range have contributed much to opening new visions of our universe. There are numerous national radio facilities spread over the world. In the near future, a new era of truly global radio observatories will begin. This chapter contains a short history of the development of the field, details of calibration procedures, coherent/heterodyne and incoherent/bolometer receiver systems, observing methods for single apertures and interferometers, and an overview of aperture synthesis.

astro-ph.IM↗

arXiv2011

The Structure of NGC 1976 in the Radio Range

High angular resolution radio continuum images of NGC 1976 (M42, Orion A) at frequency=330 MHz (wavelength=91 cm), 1.5 GHz (20 cm) and 10.6 GHz (2.8 cm), have been aligned, placed on a common grid, smoothed to common resolutions of 80" (=0.16 pc at 420 pc) and 90" (=0.18 pc) and compared on a position-by-position basis. The results are not consistent with a single value of Te. Rather, there is a significant variation, from Te =6000K in the low intensity, extended region to Te=8500K in the higher intensity, more compact region. The best fit to the data is a multi-layer model obtained from radio recombination line (RRL) data. An estimate of temperature fluctuations from the model yields t2=0.003. This is a factor of 10 lower than fluctuation values from optical O++ line data.

physics.space-ph↗

arXiv2011

High-resolution EVLA image of dimethyl ether (CH$_{3}$)$_{2}$O in Orion--KL

We report the first sub-arc second (0.65$\arcsec$ $\times$ 0.51$\arcsec$) image of the dimethyl ether molecule, (CH$_{3}$)$_{2}$O, toward the Orion Kleinmann-Low nebula (Orion--KL). The observations were carried at 43.4 GHz with the Expanded Very Large Array (EVLA). The distribution of the lower energy transition 6$_{1,5} - 6_{0,6}$, EE (E$\rm_{u}$ = 21 K) mapped in this study is in excellent agreement with the published dimethyl ether emission maps imaged with a lower resolution. The main emission peaks are observed toward the Compact Ridge and Hot Core southwest components, at the northern parts of the Compact Ridge and in an intermediate position between the Compact Ridge and the Hot Core. A notable result is that the distribution of dimethyl ether is very similar to that of another important larger O-bearing species, the methyl formate (HCOOCH$_{3}$), imaged at lower resolution. Our study shows that higher spectral resolution (WIDAR correlator) and increased spectral coverage provided by the EVLA offer new possibilities for imaging complex molecular species. The sensitivity improvement and the other EVLA improvements make this instrument well suited for high sensitivity, high angular resolution, molecular line imaging.

astro-ph.GA↗

arXiv2010

The sub-mm J=6-5 line of 13CO in Orion

We present a fully sampled map covering the Orion Hot Core and dense molecular ridge, in the sub-millimeter J=6-5 rotational transition of 13CO, at 0.45 mm with a resolution of 13 arcsec and 0.5 km s^-1. The map covers 3 arc min by 2arc min . The profile centered on the Hot Core peaks at 8.5 km s^-1 and has a peak intensity of 40 K, corrected antenna temperature. It shows line wings from 30 km s^-1 to -20 km s^-1. The map of intensity, integrated from 0 to +18 km s^-1, shows a prominent maximum <5 arcsec from the center of the Orion Hot Core. The FWHP is 3 arcsec, larger than the regions containing complex molecules. Single dish measurements of lines from the J=2-1 or J=1-0 transitions of CO isotopes show no such distinct maximum. Correcting for optical depth 1.5 in the J=6-5 line of 13CO, and assuming that the level populations are thermalized at 150 K, the beam averaged column density between 0 to +18 km s^-1 is N(13CO )=6.8 10^17 cm^-2 and N(CO)=5.2 10^19 cm-2. When combined with published dust emission data, the CO/ H2 number ratio is 2 {\cdot} 10^-5, a factor of ~5 lower than the canonical value, 10^-4. For the Orion South and Orion Ridge region, the column density of CO is <25% of that found for the Hot Core but CO/H2 ratios are similar. Models of Photodissociation Regions, PDRs, predict that CO lines from PDRs are only marginally optically thick. Thus our map traces warm and dense molecular gas rather than PDRs.

astro-ph.SR↗

arXiv2010

A survey of the Galactic center region in HCO+, H13CO+ and SiO

Aims: A large scale survey of the Galactic center region in he 3 mm rotational transitions of SiO, HCO+ and H13CO+ (beamsize ~ 3. 6 arcmin) was conducted to provide an estimate of cloud conditions, heating mechanisms, chemistry and other properties. Methods: Using the NANTEN 4m telescope from Nagoya University, a region between -5.75<l<5.6 (degree) and -0.68<b<1.3 (degree) was mapped in the J=1-0 lines of HCO+ and H13CO+ and in the J=2-1 line of SiO with a spacing of 3.75 arcmin (HCO+) and 1.875 arcmin (SiO and H13CO+). Results: Velocity channel maps, longitude-velocity maps and latitude-velocity maps are presented. We identify 51 molecular clouds; 33 of them belong to the Galactic center, and 18 to disk gas. We derive an average of the luminosity ratio of SiO(J=2-1)/CO(J=1-0) in clouds belonging to the Galactic center of 4.9x10^{-3} and for disk clouds of 3.4x 10^{-3}. The luminosity ratio of HCO^+(J=1-0)/CO(J=1\to0) in the Galactic Center is 3.5x 10^{-2}, and for disk clouds is 1.5x 10^{-2}. We can distinguish clearly between regions where the SiO or HCO+ are dominating.

astro-ph.GA↗

arXiv2009

Introduction to Millimeter/Sub-Millimeter Astronomy

This is an introduction to the basic elements needed for the measurements and interpretation of data in the millimeter and sub-mm wavelength range. A more complete version will be published in the proceedings of the Saas Fee Winter School 2008.

astro-ph.GA↗

arXiv2009

Detection of Star Formation in the Unusually Cold Giant Molecular Cloud G216

The giant molecular cloud G216-2.5, also known as Maddalena's cloud or the Maddalena-Thaddeus cloud, is distinguished by an unusual combination of high gas mass (1-6 x 10^5) solar masses, low kinetic temperatures (10 K), and the lack of bright far infrared emission. Although star formation has been detected in neighboring satellite clouds, little evidence for star formation has been found in the main body of this cloud. Using a combination of mid-infrared observations with the IRAC and MIPS instruments onboard the Spitzer space telescope, and near-IR images taken with the Flamingos camera on the KPNO 2.1-meter, we identify a population of 41 young stars with disks and 33 protostars in the center of the cloud. Most of the young stellar objects are coincident with a filamentary structure of dense gas detected in CS (2-1). These observations show that the main body of G216 is actively forming stars, although at a low stellar density comparable to that found in the Taurus cloud.

astro-ph.GA↗

arXiv2007

Interstellar 12C/13C from CH+ absorption lines: Results from an extended survey

The 12C/13C isotope ratio in the interstellar medium (ISM), and its evolution with time, is an important tracer of stellar yields. Spatial variations of this ratio can be used to study mixing in the ISM. We want to determine this ratio and its spatial variations in the local ISM from CH+ absorption lines in the optical towards early-type stars. The aim is to determine the average value for the local ISM and study possible spatial variations. We observed a large number of early-type stars with Feros to extend the sample of suitable target stars for CH+ isotope studies. The best suited targets were observed with Uves with higher signal-to-noise ratio and spectral resolution to determine the isotope ratio from the interstellar CH+ lines. This study significantly expands the number of 13CH+ detections. We find an average ratio of = 76.27 +- 1.94 or, for f = 1/R, = (120.46 +- 3.02) 10^{-4}. The scatter in f is 6.3 sigma( ). This findings strengthens the case for chemical inhomogeneity in the local ISM, with important implications for the mixing in the ISM. Given the large scatter, the present-day value in the ISM is not significantly larger than the solar value, which corresponds to the local value 4.5 Gyr ago.

astro-ph↗

arXiv2007

3-He in the Milky Way Interstellar Medium: Ionization Structure

The cosmic abundance of the 3-He isotope has important implications for many fields of astrophysics. We are using the 8.665 GHz hyperfine transition of 3-He+ to determine the 3-He/H abundance in Milky Way HII regions and planetary nebulae. This is one in a series of papers in which we discuss issues involved in deriving accurate 3-He/H abundance ratios from the available measurements. Here we describe the ionization correction we use to convert the 3-He+/H+ abundance, y3+, to the 3-He/H abundance, y3. In principle the nebular ionization structure can significantly influence the y3 derived for individual sources. We find that in general there is insufficient information available to make a detailed ionization correction. Here we make a simple correction and assess its validity. The correction is based on radio recombination line measurements of H+ and 4-He+, together with simple core-halo source models. We use these models to establish criteria that allow us to identify sources that can be accurately corrected for ionization and those that cannot. We argue that this effect cannot be very large for most of the sources in our observational sample. For a wide range of models of nebular ionization structure we find that the ionization correction factor varies from 1 to 1.8. Although large corrections are possible, there would have to be a conspiracy between the density and ionization structure for us to underestimate the ionization correction by a substantial amount.

astro-ph↗

arXiv2006

A Molecular Gas Study of Low Luminosity Radio Galaxies

We discuss CO spectral line data of a volume-limited sample of 23 nearby (z<0.03) low luminosity radio galaxies, selected from the B2 catalogue. We investigate whether the CO properties of our sample are correlated with the properties of the host galaxy, and in particular with the dust component. We find strong evidences for a physical link between the dust disks probed by HST in the galaxy cores and the molecular gas probed by the CO spectral lines, which in two cases display a double-horn shape, consistent with ordered rotation. On the other hand, from a preliminary comparison with other samples of radio sources we find no significant differences in molecular gas properties between FRI and FRII radio sources. In order to confirm the suggestion that the CO is dynamically associated with the core dust disks, the most suitable sources of our sample will be proposed for interferometric imaging at PdBI.

astro-ph↗

arXiv2006

Report by the ESA-ESO Working Group on "The Herschel-ALMA Synergies"

The Herschel Satellite and the Atacama Large Millimeter Array (ALMA) are two very large sub-mm and far infrared (FIR) astronomy projects that are expected to come into operation in this decade. This report contains descriptions of these instruments, emphasising the overlaps in wavelength range and additional complementarities. A short rationale for studying sub-mm and far infrared astronomy is given. Following this, brief presentations of Herschel and ALMA are presented, with references to more detailed documents and use cases. Emphasis is placed on the synergies between these facilities, and the challenges of comparing data produced using both. Specific examples of projects are given for a number of areas of astronomical research where these facilities will lead to dramatic improvements. This report is addressed to an audience of non-specialist astronomers who may be interested in extending their areas of research by making use of Herschel and ALMA instruments.

astro-ph↗

arXiv2006

The detection of the (J,K)=(18,18) line of NH3

The first astronomical detection of the metastable ($J,K$) = (18,18) line of NH3 is reported. With 3130 K above the ground state, this is the NH3 line with by far the highest energy detected in interstellar space. It is observed in absorption toward the galactic center star forming region Sgr B2. There is a clear detection toward Sgr B2(M) and a likely one toward SgrB2(N). An upper limit for emission is determined for Orion-KL. If we combine the (18,18) line results from Sgr B2(M) with the previously measured (12,12) absorption line, we find a rotation temperature of >1300 K for the absorbing cloud. This is at least a factor of two higher than previously derived values from less highly excited ammonia lines, giving a lower limit to the kinetic temperature. There is a hot low density gas component in the envelope of SgrB2. It is possible that the (18,18) line arises in this region. The radial velocity of the low density, hot envelope is the same as that of the dense hot cores, so the (18,18) line could also arise in the dense hot cores where non-metastable (J>K) absorption lines from energy levels of up to 1350 K above the ground state have been observed. A discussion of scenarios is presented.

astro-ph↗

arXiv2006

A Search for Molecular Gas in Low Luminosity Radio Galaxies

We discuss CO spectral line data of a volume-limited sample of 23 nearby (z<0.03) low luminosity radi galaxies, selected from the B2 catalogue. Most of such objects (16/23) have HST imaging. Our aim is to establish the distribution of molecular gas masses in low luminosity radio galaxies, in comparison with other radio source samples, confirm the suggestion that the CO is in ordered rotation, determine its relation to the dust disks observed in these objects with HST and establish targets for future interferometric imaging.

astro-ph↗