762 research outputs found
Polarisation observations of VY Canis Majoris H_2O 5_(32)â4_(41) 620.701 GHz maser emission with HIFI
Context. Water vapour maser emission from evolved oxygen-rich stars remains poorly understood. Additional observations, including polarisation studies and simultaneous observation of different maser transitions may ultimately lead to greater insight.
Aims. We have aimed to elucidate the nature and structure of the VY CMa water vapour masers in part by observationally testing a theoretical prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz maser components of ortho H_2O.
Methods. In its high-resolution mode (HRS) the Herschel Heterodyne Instrument for the Far Infrared (HIFI) offers a frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of 0.06 kmâs^(-1), which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground based observations of the 22.235 GHz maser with the Max-Planck-Institut fĂŒr Radioastronomie 100-m telescope at Effelsberg, provided a ratio of 620.701 GHz to 22.235 GHz emission.
Results. We report the first astronomical detection to date of H_2O maser emission at 620.701 GHz. In VY CMa both the 620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser peaks are superposed on what appears to be a broad emission component, jointly ejected from the star. We observed the 620.701 GHz emission at two epochs 21 days apart, both to measure the potential direction of linearly polarised maser components and to obtain a measure of the longevity of these components. Although we do not detect significant polarisation levels in the core of the line, they rise up to approximately 6% in its wings
High-Resolution 4.7 Micron Keck/NIRSPEC Spectroscopy of the CO Emission from the Disks Surrounding Herbig Ae Stars
We explore the high-resolution (λ/Îλ = 25,000; Îv = 12 km s^(-1)) M-band (4.7-5.1 ÎŒm) spectra of several disk-dominated Herbig Ae (HAe) systems: AB Aur, MWC 758, MWC 480, HD 163296, and VV Ser. All five objects show ^(12)CO v = 1-0 emission lines up to J = 42, but there is little or no evidence of moderate-J, v = 2-1 transitions despite their similar excitation energies. AB Aur shows ^(13)CO emission as well. The line/continuum ratios and intensity profiles are well correlated with inclination, and they trace collisionally driven emission from the inner disk (R_(th) ⟠0.5-1 AU) as well as resonance fluorescence to much larger radii (R_(hÎœ) ⟠50-100 AU for J ⟠10). The temperature, density, and radiation field profiles required to fit the CO emission are in good agreement with models of HAe disks derived from their spectral energy distributions. High-resolution and high dynamic range infrared spectroscopy of CO, and future observations of less abundant species, thus provide direct access to the physicochemical properties and surface structure of disks in regions where planet formation likely occurs
Methane Abundance Variations toward the Massive Protostar NGC 7538 : IRS9
Absorption and emission lines originating from the nu3 C-H stretching
manifold of gas phase CH4 were discovered in the high resolution (R=25,000)
infrared L band spectrum along the line of sight toward NGC 7538 : IRS9. These
observations provide a diagnostic of the complex dynamics and chemistry in a
massive star forming region. The line shapes resemble P Cygni profiles with the
absorption and emission components shifted by ~7 km/s with respect to the
systemic velocity. Similar velocity components were observed in CO at 4.7 um,
but in contrast to CH4, the CO shows deep absorption due to a high velocity
outflow as well as absorption at the systemic velocity due to the cold outer
envelope. It is concluded that the gas phase CH4 abundance varies by an order
of magnitude in this line of sight: it is low in the envelope and the outflow
(X[CH4]<0.4e-6), and at least a factor of 10 larger in the central core. The
discovery of solid CH4 in independent ground and space based data sets shows
that methane is nearly entirely frozen onto grains in the envelope. It thus
appears that CH4 is formed by grain surface reactions, evaporates into the gas
phase in the warm inner regions of protostellar cores and is efficiently
destroyed in shocks related to outflows.Comment: Scheduled for publication in ApJ 615, 01 Nov. 2004. 11 page
CO Rovibrational Emission as a Probe of Inner Disk Structure
We present an analysis of CO emission lines from a sample of T Tauri, Herbig Ae/Be, and transitional disks with known inclinations in order to study the structure of inner disk molecular gas. We calculate CO inner radii by fitting line profiles with a simple parameterized model. We find that, for optically thick disks, CO inner radii are strongly correlated with the total system luminosity (stellar plus accretion) and consistent with the dust sublimation radius. Transitional disk inner radii show the same trend with luminosity, but are systematically larger. Using rotation diagram fits, we derive, for classical T Tauri disks, emitting areas consistent with a ring of width ~0.15 AU located at the CO inner radius; emitting areas for transitional disks are systematically smaller. We also measure lower rotational temperatures for transitional disks, and disks around Herbig Ae/Be stars, than for those around T Tauri stars. Finally, we find that rotational temperatures are similar to, or slightly lower than, the expected temperature of blackbody grains located at the CO inner radius, in contrast to expectations of thermal decoupling between gas and dust
High-Resolution 4.7 Micron Keck/NIRSPEC Spectra of Protostars. II. Detection of the ^(13)CO Isotope in Icy Grain Mantles
The high-resolution (R = 25,000) infrared M-band spectrum of the massive protostar NGC 7538 IRS 9 shows a narrow absorption feature at 4.779 ÎŒm (2092.3 cm^(-1)) that we attribute to the vibrational stretching mode of the ^(13)CO isotope in pure CO icy grain mantles. This is the first detection of ^(13)CO in icy grain mantles in the interstellar medium. The ^(13)CO band is a factor of 2.3 narrower than the apolar component of the ^(12)CO band. With this in mind, we discuss the mechanisms that broaden solid-state absorption bands. It is shown that ellipsoidally shaped pure CO grains fit the bands of both isotopes at the same time. Slightly worse but still reasonable fits are also obtained by CO embedded in N_2-rich ices and thermally processed O_2-rich ices. In addition, we report new insights into the nature and evolution of interstellar CO ices by comparing the very high resolution multicomponent solid ^(12)CO spectrum of NGC 7538 IRS 9 with that of the previously studied low-mass source L1489 IRS. The narrow absorption of apolar CO ices is present in both spectra but much stronger in NGC 7538 IRS 9. It is superposed on a smooth broad absorption feature well fitted by a combination of CO_2 and H_2O-rich laboratory CO ices. The abundances of the latter two ices, scaled to the total H_2O ice column, are the same in both sources. We thus suggest that thermal processing manifests itself as evaporation of apolar ices only and not the formation of CO_2 or polar ices. Finally, the decomposition of the ^(12)CO band is used to derive the ^(12)CO/^(13)CO abundance ratio in apolar ices. A ratio of ^(12)CO/^(13)CO = 71 ± 15 (3 Ï) is deduced, in good agreement with gas-phase CO studies (~77) and the solid ^(12)CO_2/^(13)CO_2 ratio of 80 ± 11 found in the same line of sight. The implications for the chemical path along which CO_2 is formed are discussed
The Environment and Nature of the Class I Protostar Elias 29: Molecular Gas Observations and the Location of Ices
A (sub-)millimeter line and continuum study of the Class I protostar Elias 29 in the Ï Ophiuchi molecular cloud is presented whose goals are to understand the nature of this source and to locate the ices that are abundantly present along this line of sight. Within 15"-60" beams, several different components contribute to the line emission. Two different foreground clouds are detected, an envelope/disk system and a dense ridge of HCO^+-rich material. The latter two components are spatially separated in millimeter interferometer maps. We analyze the envelope/disk system by using inside-out collapse and flared disk models. The disk is in a relatively face-on orientation (<60°), which explains many of the remarkable observational features of Elias 29, such as its flat spectral energy distribution, its brightness in the near-infrared, the extended components found in speckle interferometry observations, and its high-velocity molecular outflow. It cannot account for the ices seen along the line of sight, however. A small fraction of the ices is present in a (remnant) envelope of mass 0.12-0.33 M_â, but most of the ices (~70%) are present in cool (T < 40 K) quiescent foreground clouds. This explains the observed absence of thermally processed ices (crystallized H_2O) toward Elias 29. Nevertheless, the temperatures could be sufficiently high to account for the low abundance of apolar (CO, N_2, O_2) ices. This work shows that it is crucial to obtain spectrally and spatially resolved information from single-dish and interferometric molecular gas observations in order to determine the nature of protostars and to interpret Infrared Space Observatory and future Space Infrared Telescope Facility observations of ices and silicates along a pencil beam
Molecular Gas in the Inner 1 AU of the TW Hya and GM Aur Transitional Disks
We report the detection of CO rovibrational emission from the transitional disks around the T Tauri stars TW Hya and GM Aur. Transitional disks are characterized by significant mid- to far-infrared (IR) dust emission combined with a relative deficit in the near-IR, indicating the presence of an optically thick outer disk but a reduced surface density of small dust grains in the inner disk. Kinematic fits to the resolved CO emission lines demonstrate that they arise from within the tenuous inner disk. Excitation diagram analyses yield rotational temperatures also consistent with small emission radii as well as densities implying dynamically significant amounts of gas in the inner disk and a gas-to-small dust grain ratio in excess of that in dense clouds. Nevertheless, gas densities are not high enough to maintain current accretion rates for more than a few hundred years without replenishment, and transfer of gas from the outer to inner disk is therefore likely required
Spitzer Spectroscopy of Ices: From Molecular Cores to Planet-Forming Disks
Icy grain mantles are a major reservoir of the molecular inventory of dense clouds and circumstellar envelopes and disks. The ice abundances and in particular the dependence of abundances on the astrophysical environment are still poorly characterized. Numerous physical and chemical processes may modify the ices in the evolutionary sequence from dense cores to planet-forming disks. Using Spitzer/IRS and ground-based thermal infrared spectrometers we determine the ice inventory toward low mass protostars and address questions on evolutionary scenarios. The initial results from our Spitzer Legacy program âFrom Molecular Cores to Planet-Forming Disksâ (c2d) indicate that ice abundances relative to H_2O commonly vary by factors of 2â5 in different sight-lines. For some species (CO) outgassing is likely responsible, but for others (CH_3OH, CO_2, NH_3) different factors, such as cloud history, must be involved
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