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CALTECH GE 133 - BIPOLAR MOLECULAR OUTFLOWS

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July 24, 1996 9:38 Annual Reviews BACHTEX1 AR12-04Annu. Rev. Astron. Astrophys. 1996. 34:111–54Copyrightc1996 by Annual Reviews Inc. All rights reservedBIPOLAR MOLECULAR OUTFLOWSFROM YOUNG STARS ANDPROTOSTARSRafael BachillerObservatorio Astron´omico Nacional (IGN), Campus Universitario, Apartado1143, E–28800 Alcal´a de Henares (Madrid), SpainKEY WORDS: star formation, interstellar medium, interstellar molecules, jetsABSTRACTA violent outflow of high-velocity gas is one of the first manifestations of theformation of a new star. Such outflows emerge bipolarly from the young objectand involve amounts of energy similar to those involved in accretion processes.The youngest (proto-)stellar low-mass objects known to date (the Class 0 proto-stars) present a particularly efficient outflow activity, indicating that outflow andinfall motions happen simultaneously and are closely linked since the very firststages of the star formation processes.This article reviews the wealth of information being provided by large milli-meter-wave telescopes and interferometers on the small-scale structure of molec-ular outflows, as well as the most recent theories about their origin. The observa-tionsofhighlycollimated COoutflows, extremely highvelocity(EHV) flows,andmolecular “bullets” are examined in detail, since they provide key information onthe origin and propagation of outflows. The peculiar chemistry operating in theassociated shocked molecular regions is discussed, highlighting the recent high-sensitivity observations of low-luminosity sources. The classification schemesand the properties of the driving sources of bipolar outflows are summarized withspecial attention devoted to the recently identified Class 0 protostars. All theseissues are crucial for building a unified theory on the mass-loss phenomena inyoung stars.1110066-4146/96/0915-0111$08.00Annu. Rev. Astro. Astrophys. 1996.34:111-154. Downloaded from arjournals.annualreviews.orgby CALIFORNIA INSTITUTE OF TECHNOLOGY on 10/09/05. For personal use only.July 24, 1996 9:38 Annual Reviews BACHTEX1 AR12-04112 BACHILLER1. INTRODUCTIONThe study of mass-loss phenomena from young stars started in the early 1950swith the discovery by Herbig (1951) and Haro (1952) of small nebulosities withpeculiar emission line spectra. The so-called Herbig-Haro (HH) objects weresoonassociated withstellarwinds(Osterbrock1958) andlater foundtobe duetothe interaction of a highly supersonic stellar wind with the ambient surroundingmaterial (Schwartz 1975). Measurements of proper motions (Cudworth &Herbig 1979) confirmed that the ejection originates from a newly formed star.Moreover, the rapidly moving highly collimated HH jets, discovered in thevisible by Mundt & Fried (1983), also originate from young star positions. Ontheother hand, the presence ofwindsaround youngTTauri starswasrecognizedintheir P Cygniprofiles(Herbig 1962, Kuhi1964)and in centimeterwavelengthcontinuum observations (Cohen et al 1982).Broad lines of CO at millimeter wavelengths generated by high-velocitymolecular gas were discovered toward the Orion A molecular cloud in themid-1970s (Kwan & Scoville 1976, Zuckerman et al 1976). High-velocityCO emission was soon detected toward other objects, and the structure of theoutflowing material was found to be bipolar (Snell et al 1980, Rodr´ıguez et al1980). The first surveys revealed that these bipolar outflows are extraordinar-ily common around young stars (Bally & Lada 1983; Edwards & Snell 1982,1983, 1984). Lada (1985) compiled the first catalog, which contained 68 out-flow sources. Further searches carried out with unbiased selection criteria byusing, for example, the IRAS data base, or the systematic observation of a fullmolecular cloud in CO lines, led to the detection of many more outflows. Fukuiet al (1993) listed 157 outflows confirmed through complete or partial map-ping. Observations since then have increased the number of presently knownmolecular outflows to nearly 200.Outflows from young stars are a ubiquitous and energetic phenomenon; theyhave spectacular observationalmanifestations overa wide range of wavelengthsfrom the ultraviolet to the radio. In general terms, we are now confident thatvirtually all young stellar objects (YSOs) undergo periods of copious massloss. The highest resolution observations available show that the flows emergebipolarly from a stellar or circumstellar region. The fast well-collimated stellarwind sweeps up the ambient molecular gas in its vicinity, forming two cavitiesorientedinoppositedirectionswithrespectto thecentral star. Themolecular gasdisplacedfromthe cavitiesexpandsintheform ofirregularlobesand incompleteshells and constitutes the CO outflow. However, even the most basic questionsabout the outflow phenomenon are still a matter of debate. It is not clear yetwhat physical mechanism produces the outflows, and the underlying stellar orprotostellar wind that should sweep up the fast moving molecular gas is provingto be extremely hard to detect.Annu. Rev. Astro. Astrophys. 1996.34:111-154. Downloaded from arjournals.annualreviews.orgby CALIFORNIA INSTITUTE OF TECHNOLOGY on 10/09/05. For personal use only.July 24, 1996 9:38 Annual Reviews BACHTEX1 AR12-04BIPOLAR MOLECULAR OUTFLOWS 113The new generation of large radiotelescopes and interferometers working atmillimeter and submillimeter wavelengths is providing a wealth of informa-tion on the small-scale structure of bipolar molecular outflows. In addition tothe classical outflows at standard high velocities (SHV, i.e. velocities rangingfrom a few kilometers per second to about 20 km s−1) whose properties weresummarized in the excellent review of Lada (1985), weak CO components thathave extremelyhighvelocities(EHV) havebeen discoveredand mapped towardsome outflows (e.g. Figure 1). The EHV CO components are reminiscent ofthe HH jets observed in the visible and seem to be of a different nature than theSHV components (Bachiller & G´omez-Gonz´alez 1992).The purpose of this article is to review the progress in outflow research sinceLada’s (1985) review, by taking into account the observationscarried out duringthisten-year periodwithmillimeter telescopesofhigh resolutionandsensitivity.Special attention is devoted to the extraordinary outflow activity of “Class 0”sources, possibly the youngest (proto-)stellar low-mass objects known to date.Recent theoretical models for the origin of flows and for the interaction of thewinds with the molecular


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