arXiv subjects

E. F. van Dishoeck

Publications and source records attributed to E. F. van Dishoeck.

366 records · Page 21Linked to original sources

arXiv2000

The Composition and Distribution of Dust Along the Line of Sight Towards the Galactic Center

We discuss the composition of dust and ice along the line of sight to the Galactic Center (GC) based on analysis of mid-infrared spectra (2.4-13 micron) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sagittarius A* and the Quintuplet sources, GCS3 and GCS4. It is evident from the depth of the 3.0 micron H2O and the 4.27 micron CO2 ice features that there is more molecular cloud material along the line of sight toward Sgr A* than GCS3 and 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH4, NH3, and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 micron. Ground-based studies have identified the 3.4 micron feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 micron in the GCS3 spectrum to the C-H stretch in aromatic hydrocarbons. This feature is not detected, and its C-C stretch counterpart appears to be weaker, in the SgrA* spectrum. One of the key questions which now arises is whether aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons.

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arXiv2000

ISO-SWS observations of interstellar solid 13CO2: heated ice and the Galactic 12C/13C abundance ratio

We present observations of the stretching mode of 13CO2 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory. Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to 13CO2 mixed with polar molecules such as H2O, and CH3OH. Several high-mass protostars show an additional narrow substructure at 4.382 um, which we attribute to a polar CO2 ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing. Furthermore, we determine for the first time the Galactic 12C/13C ratio from the solid state. The 12CO2/13CO2 ratio for the local ISM (69+/-15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase (C18O, H2CO) studies. For the few individual objects for which gas phase values are available, the 12C/13C ratios derived from CO2 tend to be higher compared to CO studies (albeit with 2.5 sigma significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO2.

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arXiv1999

ISOCAM spectro-imaging of the H2 rotational lines in the supernova remnant IC443

We report spectro-imaging observations of the bright western ridge of the supernova remnant IC 443 obtained with the ISOCAM circular variable filter (CVF) on board the Infrared Space Observatory (ISO). This ridge corresponds to a location where the interaction between the blast wave of the supernova and ambient molecular gas is amongst the strongest. The CVF data show that the 5 to 14 micron spectrum is dominated by the pure rotational lines of molecular hydrogen (v = 0--0, S(2) to S(8) transitions). At all positions along the ridge, the H2 rotational lines are very strong with typical line fluxes of 10^{-4} to 10^{-3} erg/sec/cm2/sr. We compare the data to a new time-dependent shock model; the rotational line fluxes in IC 443 are reproduced within factors of 2 for evolutionary times between 1,000 and 2,000 years with a shock velocity of 30 km/sec and a pre-shock density of 10^4 /cm3.

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arXiv1998

Envelope structure of deeply embedded young stellar objects in the Serpens Molecular Cloud

Aperture synthesis and single-dish (sub) millimeter molecular lines and continuum observations reveal in great detail the envelope structure of deeply embedded young stellar objects (SMM1, SMM2, SMM3, SMM4) in the densely star-forming Serpens Molecular Cloud. Resolved millimeter continuum emission constrains the density structure to a radial power law with index -2.0 +/- 0.5, and envelope masses of 8.7, 3.0, and 5.3 M_sol for SMM1, SMM3, and SMM4. The core SMM2 does not seem to have a central condensation and may not have formed a star yet. The molecular line observations can be described by the same envelope model, if an additional, small amount of warm (100 K) material is included. This probably corresponds to the inner few hundred AU of the envelope were the temperature is high. In the interferometer beam, the molecular lines reveal the inner regions of the envelopes, as well as interaction of the outflow with the surrounding envelope. Bright HCO+ and HCN emission outlines the cavities, while SiO and SO trace the direct impact of the outflow on ambient gas. Taken together, these observations provide a first comprehensive view of the physical and chemical structure of the envelopes of deeply embedded young stellar objects in a clustered environment on scales between 1000 and 10,000 AU.

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arXiv1997

Sub-arcsecond imaging at 267 GHz of a young binary system: Detection of a <70 AU radius dust disk around T Tau N

The young binary system T Tau was observed with the Owens Valley Millimeter Array at 267 GHz continuum and HCO+ J=3-2 emission at 0.8'' resolution, with the JCMT-CSO single-baseline interferometer in 357 GHz continuum, and with the W. M. Keck Telescope at 4 micron. The 267 GHz emission is unresolved with a flux of 397 +/- 35 mJy, located close to the position of the optical star T Tau N. An upper limit of 100 mJy is obtained toward the infrared companion T Tau S. The 357 GHz continuum emission is unresolved, with a flux of 1.35 +/- 0.68 Jy. HCO+ 3-2 was detected from a 2'' diameter core surrounding T Tau N and S. Both stars are detected ar 4 micron, but there is no evidence of the radio source T Tau R. We propose a model in which T Tau S is intrinsically similar to T Tau N, but is obscured by the outer parts of T Tau N's disk. A fit to the spectral energy distribution (SED) between 21 cm and 1.22 micron is constructed on this basis. Adopting a r^-1 surface density distribution and an exponentially truncated edge, disk masses of 0.04 +/- 0.01 Msol and 6e-5 - 3e-3 Msol are inferred for T Tau N and S, respectively. A 0.005-0.03 Msol circumbinary envelope is also required to fit the millimeter to mid-infrared SED.

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arXiv1995

The Ionization Fraction in Dense Clouds

We present submillimeter observations of various molecular ions toward two dense clouds, NGC 2264 IRS1 and W 3 IRS5, in order to investigate their ionization fraction. Analysis of the line intensity ratios by the way of statistical equilibrium calculations allows determination of the physical parameters: n(H2)~(1-2)e6 cm-3 and T(kin)~50-100 K. Column densities and abundances are also derived. Together, the abundances of the observed ions provide a lower limit to the ionization fraction, which is (2-3)e-9 in both clouds. In order to better constrain the electron abundance, a simple chemical model is built which calculates the steady state abundances of the major positive ions, using the observed abundances wherever available. With reasonable assumptions, good agreement within a factor of two with the observations can be achieved. The calculated electron fraction is x(e)= (1.0-3.3)e-8 in the case of NGC 2264 and x(e)=(0.5-1.1)e-8 for W 3 IRS5. In the first case, the high abundance of N2H+ requires a rather high cosmic ray ionization rate >1e-16s-1, even if all nitrogen is assumed to be in gas phase N2. For W 3 IRS5, ionized metals such as Fe+ and Mg+ could provide 60% of the electrons.

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