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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents



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ID: 448136.0, MPI für Astronomie / Publikationen_mpia
Lack of PAH emission toward low-mass embedded young stellar objects
Authors:Geers, V. C.; van Dishoeck, E. F.; Pontoppidan, K. M.; Lahuis, F.; Crapsi, A.; Dullemond, C. P.; Blake, G. A.
Language:English
Date of Publication (YYYY-MM-DD):2009
Title of Journal:Astronomy and Astrophysics
Journal Abbrev.:A & A
Volume:495
Start Page:837
End Page:846
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:Aims. Polycyclic aromatic hydrocarbons (PAHs) have been detected toward molecular clouds and some young stars with disks, but have not yet been associated with embedded young stars. We present a sensitive mid-infrared spectroscopic survey of PAH features toward a sample of low-mass embedded young stellar objects (YSOs). The aim is to put constraints on the PAH abundance in the embedded phase of star formation using radiative transfer modeling. Methods: VLT-ISAAC L-band spectra for 39 sources and Spitzer IRS spectra for 53 sources are presented. Line intensities are compared to recent surveys of Herbig Ae/Be and T Tauri stars. The radiative transfer codes RADMC and RADICAL are used to model the PAH emission from embedded YSOs consisting of a pre-main-sequence star with a circumstellar disk embedded in an envelope. The dependence of the PAH feature on PAH abundance, stellar radiation field, inclination and the extinction by the surrounding envelope is studied. Results: The 3.3 mum PAH feature is undetected for the majority of the sample (97%), with typical upper limits of 5 × 10-16 W m-2. One source originally classified as class I, <ASTROBJ>IRS 48</ASTROBJ>, shows a strong 3.3 mum feature from a disk. Compact 11.2 mum PAH emission is seen directly towards 1 out of the 53 Spitzer Short-High spectra, for a source that is borderline embedded. For all 12 sources with both VLT and Spitzer spectra, no PAH features are detected in either. In total, PAH features are detected toward at most 1 out of 63 (candidate) embedded protostars (⪉2%), even lower than observed for class II T Tauri stars with disks (11-14%). Models predict the 7.7 mum feature as the best tracer of PAH emission, while the 3.3 mum feature is relatively weak. Assuming typical class I stellar and envelope parameters, the absence of PAH emission is most likely explained by the absence of emitting carriers through a PAH abundance at least an order of magnitude lower than in molecular clouds but similar to that found in disks. Thus, most PAHs likely enter the protoplanetary disks frozen out in icy layers on dust grains and/or in coagulated form. Based on observations obtained at the European Southern Observatory, Paranal, Chile, within the observing program 164.I-0605 (VLT-ISAAC).
Free Keywords:stars: pre-main sequence; stars: planetary systems: protoplanetary disks; stars: circumstellar matter; astrochemistry
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
Identifiers:URL:http://adsabs.harvard.edu/abs/2009A%26A...495..837... [ID No:1]
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