Hogerheijde, Michiel R. and van Dishoeck, Ewine F. and Blake, Geoffrey A. and van Langevelde, Huib Jan (1997) Tracing the Envelopes around Embedded Low-Mass Young Stellar Objects with HCO^+ and Millimeter-Continuum Observations. Astrophysical Journal, 489 (1). pp. 293-313. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120913-103501286
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The envelopes and disks around embedded low-mass young stellar objects (YSOs) are investigated through millimeter-continuum and HCO^+ line emission. Nine sources, selected on the basis of their HCO^+ 3-2 emission from an IRAS flux- and color-limited sample of 24 objects, are observed in λ = 3.4 and 2.7 mm continuum emission with the Owens Valley Millimeter Array and in the HCO^+ and H^(13)CO^+ 4-3, 3-2, and 1-0 transitions at the James Clerk Maxwell and IRAM 30 m telescopes. All nine sources are detected at 3.4 and 2.7 mm in the interferometer beam, with total fluxes between 4 and 200 mJy. The visibilities can be fit with an unresolved (<3") point source and, in about half of the sources, with an extended envelope. The point sources, presumably thermal dust emission from circumstellar disks, typically contribute 30%-75% of the continuum flux observed at 1.1 mm in a 19" beam, assuming a spectral slope of 2.5. The fact that at least two-thirds of our sources show point-source emission indicates that circumstellar disks are established early in the embedded phase. The remainder of the 1.1 mm single-dish flux is attributed to an extended envelope, with a mass of 0.001-0.26 M_☉ within a 19" beam. In HCO^+, the J = 1-0 line is seen to trace the surrounding cloud, while the emission from J = 3-2 and 4-3 is concentrated toward the sources. All sources look marginally resolved in these lines, indicative of a power-law brightness distribution. A beam-averaged HCO^+ abundance of (1.2 ± 0.4) × 10^(-8) with respect to H_2 is derived. The 1.1 mm continuum fluxes and HCO^+ line intensities of the envelopes correlate well and are modeled with the simple inside-out collapse model of Shu (1977) and with power-law density distributions of slopes p = 1-3. All models provide satisfactory fits to the observations, indicating that HCO^+ is an excellent tracer of the envelopes. Of the 15 sources of the original sample that were either undetected in HCO^+ 3-2 or too weak to be selected, seven show 1.1 mm single-dish fluxes comparable to our objects. It is proposed that all of the 1.1 mm flux of the former sources should be attributed to compact circumstellar disks. The relative evolutionary phase of a YSO, defined as the current ratio of stellar mass over envelope mass, is traced by the quantity ∫ T_(mb) dV(HCO^+3-2)/L_(bol). Sources that are undetected in HCO^+ are found to have significantly lower values in this tracer than do the objects of our subsample, indicating that the former objects are more evolved. The sources that are weak in HCO^+ 3-2 are indistinguishable from our subsample in this tracer and have intrinsically low masses. It is concluded that HCO^+, especially in its 3-2 and 4-3 transitions, is a sensitive tracer of the early embedded phase of star formation.
|Additional Information:||© 1997 American Astronomical Society. Received 1996 October 18; accepted 1997 June 13. The authors wish to thank Remo Tilanus, Go� ran Sandell, Fred Baas, and Floris van der Tak for carrying out part of the JCMT observations. Lee Mundy is acknowledged for useful discussions concerning the analysis of the continuum observations. The staffs of OVRO, JCMT, and IRAM 30 m are thanked for support during various observing runs. M. R. H. is indebted to the Caltech Divisions of Geological and Planetary Sciences and of Mathematics, Physics, and Astronomy, as well as the Owens Valley Radio Observatory for their hospitality. The Stimuleringsfonds Internationalisering of the Netherlands Organization for Scientific Research (NWO) and the Leids Kerkhoven-Bosscha Fonds provided travel support for M. R. H. Research in Astrochemistry in Leiden is supported by NWO/NFRA through grant 781-76-015. G. A. B. gratefully acknowledges support provided by NASA grants NAGW-2297 and NAGW-1955, and H. J. v. L. support by the European Union under contract CHGECT920011. The referee is thanked for providing insightful comments, which led to a much improved revised manuscript.|
|Subject Keywords:||circumstellar matter; ISM: jets and outflows; radio lines: ISM; stars: formation; stars: pre-main sequence|
|Official Citation:||Tracing the Envelopes around Embedded Low-Mass Young Stellar Objects with HCO+ and Millimeter-Continuum Observations Michiel R. Hogerheijde et al. 1997 ApJ 489 293|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||13 Sep 2012 17:56|
|Last Modified:||26 Dec 2012 16:09|
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