arXiv subjects

Michiel R. Hogerheijde

Publications and source records attributed to Michiel R. Hogerheijde.

58 records · Page 4Linked to original sources

arXiv1999

Detection of H2 pure rotational line emission from the GG~Tau binary system

We present the first detection of the low-lying pure rotational emission lines of H2 from circumstellar disks around T~Tauri stars, using the Short Wavelength Spectrometer on the Infrared Space Observatory. These lines provide a direct measure of the total amount of warm molecular gas in disks. The J=2->0 S(0) line at 28.218 mum and the J=3->1 S(1) line at 17.035 mum have been observed toward the double binary system GG Tau. Together with limits on the J=5->3 S(3) and J=7->5 S(5) lines, the data suggest the presence of gas at T_kin=110+-10 K with a mass of (3.6+-2.0)x10^-3 M_sol (3sigma). This amounts to ~3% of the total gas + dust mass of the circumbinary disk as imaged by millimeter interferometry, but is larger than the estimated mass of the circumstellar disk(s). Possible origins for the warm gas seen in H2 are discussed in terms of photon and wind-shock heating mechanisms of the circumbinary material, and comparisons with model calculations are made.

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arXiv1999

Models and Observations of the Chemistry near Young Stellar Objects

The study of the chemical evolution of gas and dust from pre-stellar dense cores to circumstellar disks around young stars forms an essential part of understanding star- and planet formation. Throughout the collapse- and protostellar phases, simple and complex molecules are formed, many of which deplete onto cold grains and are eventually incorporated into the icy planetesimals of new solar systems. Tracing this chemical evolution provides a wealth of information, not only about the chemical processing in primitive solar nebulae, but also about physical processes which occur in the immediate surroundings of young stellar objects (YSOs). Here we review the chemical processes which occur in the protostellar environment, and models and observations of the chemical structure of the various stages of star formation. We briefly discuss the way in which molecular abundances are derived from observations, and conclude with two examples: the low- to intermediate mass YSOs in Serpens, and the massive YSOs in W3.

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arXiv1998

Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201

ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations are presented of the reflection nebula Ced 201, which is a photon-dominated region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+ data, the carbon budget and physical structure of the reflection nebula are constrained. The obtained data set is the first one to contain all important cooling lines of a cool PDR, and allows a comparison to be made with classical PDRs. To this effect one- and three-dimensional PDR models are presented which incorporate the physical characteristics of the source, and are aimed at understanding the dominant heating processes of the cloud. The contribution of very small grains to the photo-electric heating rate is estimated from these models and used to constrain the total abundance of PAHs and small grains. Observations of the pure rotational H_2 lines with ISO, in particular the S(3) line, indicate the presence of a small amount of very warm, approximately 330 K, molecular gas. This gas cannot be accommodated by the presented models.

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arXiv1997

Tracing the envelopes around embedded low-mass young stellar objects with HCO+ and millimeter-continuum observations

Interferometer observations of millimeter-continuum (OVRO) and single-dish observations of HCO+ and H13CO+ J=1-0, 3-2, and 4-3 (JCMT, IRAM 30m) are presented of nine embedded low-mass young stellar objects (YSOs) in Taurus. All nine objects are detected at 3.4 and 2.7 mm, with fluxes of 4-200 mJy, and consist of unresolved (<3 arcsec) point sources, plus, toward about half of the objects, an extended envelope. The point sources likely are circumstellar disks, showing that these are established early in the embedded phase. Literature values of 1.1 mm continuum emission are used to trace the envelopes, carrying 0.001-0.26 M(sol). In HCO+, the 1-0 lines trace the surrounding clouds, while the 3-2 and 4-3 are concentrated toward the sources with intensities well correlated with the envelope flux. An HCO+/H2 abundance of 1.2e-8 is derived. The HCO+ line strengths and envelope fluxes can be fit simultaneously with the simple collapse model of Shu (1977), and related density power laws with slopes p=1-3. As an indicator of the relative evolutionary phase of a YSO, the ratio of HCO+ 3-2 line intensity over bolometric luminosity is proposed, which is roughly proportional to the current ratio of envelope over stellar mass. It is concluded that HCO+ 3-2 and 4-3 are excellent tracers of the early embedded phase of star formation.

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