Tielens, A. G. G. M.

Van den Ancker, M. E.

Wesselius, P. R.

English

Abstract: We have studied molecular hydrogen emission in a sample of 21 YSOs using spectra obtained with the Infrared Space Observatory (ISO). H2 emission was detected in 12 sources and can be explained as arising in either a shock, caused by the interaction of an outflow from an embedded YSO with the surrounding molecular cloud, or in a PDR surrounding an exposed young early-type star. The distinction between these two mechanisms can not always be made from the pure rotational H2 lines alone. Other tracers, such as PAH emission or [S I] 25.25 micron emission, are needed to identify the H2 heating mechanism. No deviations from a 3:1 ortho/para ratio of H2 were found. Both shocks and PDRs show a warm and a hot component in H2, which we explain as thermal emission from warm molecular gas (warm component), or UV-pumped infrared fluorescence in the case of PDRs and the re-formation of H2 for shocks (hot component)

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   University of GroningenISO Spectroscopy of H2 in Star Forming RegionsVan den Ancker, M. E.; Wesselius, P. R.; Tielens, A. G. G. M.Published in:The Astrophysical JournalIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Van den Ancker, M. E., Wesselius, P. R., & Tielens, A. G. G. M. (2000). ISO Spectroscopy of H2 in StarForming Regions. The Astrophysical Journal, 592(1).CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 08-06-2022ISO Spectroscopy of H2 in Star FormingRegionsBy M. E. van den Ancker1,2, P. R. Wesselius3AND A. G. G. M. Tielens3,41Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA2University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands3SRON, P.O. Box 800, 9700 AV Groningen, The Netherlands4Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The NetherlandsWe have studied molecular hydrogen emission in a sample of 21 YSOs using spectra obtainedwith the Infrared Space Observatory (ISO). H2 emission was detected in 12 sources and can beexplained as arising in either a shock, caused by the interaction of an outflow from an embeddedYSO with the surrounding molecular cloud, or in a PDR surrounding an exposed young early-type star. The distinction between these two mechanisms can not always be made from thepure rotational H2 lines alone. Other tracers, such as PAH emission or [S i] 25.25 µm emission,are needed to identify the H2 heating mechanism. No deviations from a 3:1 ortho/para ratioof H2 were found. Both shocks and PDRs show a warm and a hot component in H2, whichwe explain as thermal emission from warm molecular gas (warm component), or UV-pumpedinfrared fluorescence in the case of PDRs and the re-formation of H2 for shocks (hot component).1. IntroductionMolecular hydrogen is expected to be ubiquitous in the circumstellar environment ofYoung Stellar Objects (YSOs). It is the main constituent of the molecular cloud fromwhich the young star has formed and is also expected to be the main component of thecircumstellar disk. Most of this material will be at temperatures of 20–30 K and difficultto observe. However, some regions may be heated to temperatures of a few hundred Kand produce observable H2 emission. The intense UV radiation generated by accretionas well as by the central star itself will create a photodissociation region (PDR), of whichthe surface layer is heated by collisions with photoelectrically ejected electrons from grainsurfaces, in any surrounding neutral material. Another possibility to produce warm H2is in shocks caused by the interaction of an outflow with the surrounding molecularcloud. Shocks are usually divided into J- or Jump-shocks, and C- or Continuous-shocks.In J-shocks the molecular material is dissociated in the shock front, where the gas isheated to several times 104 degrees. Behind the shock front, molecular material willre-form, and warm H2 may be observed in the post-shock gas. C-shocks, in contrast, arenot sufficiently powerful to dissociate molecular material, but may produce observableamounts of H2 within the shock front itself.Until recently the study of H2 in star forming regions has mainly concentrated on thestudy of the near-infrared ro-vibrational lines observable from the ground. However, thelaunch of the Infrared Space Observatory (ISO) has opened up the possibility to alsostudy the mid-infrared pure rotational lines of H2, with much lower upper energy levels,and directly detect the thermal emission of warm H2 in a wide variety of sources. Inthese proceedings we report on our study of H2 lines in the ISO spectra of a sample of21 YSOs. We will show that emission of warm H2 is common in the environments ofintermediate- and high-mass YSOs and can be explained by the phenomena of shocksand PDRs outlined above.12 M.E. van den Ancker et al.: H2 in Star Forming RegionsFigure 1. H2 excitation diagrams for programme stars (top four rows) and comparison shocksand PDRs (bottom two rows). Shown are apparent columns of H2 in the pure-rotational (0–0;filled dots), 1–0 (open squares), 2–1 (filled triangles), 2–0 (open stars), 3–2 (open triangles), 3–0(filled squares), 4–3 (open diamonds), and 4–1 transitions (open crosses). Observational errorsare smaller than the size of the plot symbol. The Boltzmann distribution fits are plotted asdashed lines. The solid lines show the sum of both thermal components for each source.M.E. van den Ancker et al.: H2 in Star Forming Regions 3Figure 2. Comparison of observed H2rotational temperatures to theoretical re-lation between continuum fluxes and Trot(PDR models) or summed intensity in allobserved lines and Trot (shocks). Sourceswhich show PAH emission are plotted assquares. Plot symbols are filled for Class Isources. The arrows show the direction ofbeam dilution.2. Observations and AnalysisISO Short Wavelength Spectrometer (SWS; 2.4 – 45 µm) spectra were obtained fora sample of 21 YSOs, mostly of intermediate and high mass. Data were reduced in astandard fashion using calibration files corresponding to OLP version 7.0. In each object,molecular hydrogen line fluxes or upper limits (total flux for line with peak flux 3σ) of0–0 S(0) to S(11), 1–0 Q(1) to Q(6) and 1–0 O(2) to O(7) were determined.Pure rotational (0–0) H2 emission was detected in 12 out of our 21 sources. Ro-vibrational (1–0) H2 emission was detected in 4 sources, all of which were also detectedin the pure rotational lines. A first inspection of our data shows that H2 emission was onlyfound in the vicinity of early-type (< B4) stars or near embedded sources. Qualitativelythis is in agreement with one would expect: the strong UV fluxes of early-type starsare expected to produce extended PDRs, whereas embedded YSOs are expected to drivestrong outflows, causing a shock as the outflow hits the surrounding molecular cloud.The 28.2188 µm 0–0 S(0) line was not detected. This shows directly that we did notdetect the cool quiescent H2 in the molecular cloud. A more qualitative analysis of ourdata can be made by plotting the log of N(J)/g, the apparent column density for a given Jupper level divided by the statistical weight, versus the energy of the upper level. For thestatistical weight we have assumed the high temperature equilibrium relative abundancesof 3:1 for the ortho and para forms of H2. The resulting excitation diagrams are shownin Fig. 1. For comparison we also show excitation diagrams of three sources known tobe dominated by shocks (Orion BN/KL peak 1, IC 443 and RCW 103) and three well-known PDRs (the Orion Bar, NGC 2023 and S140), created using data from literature.In Fig. 1 we also show Boltzmann distribution fits to the low-lying pure rotational lines.The fact that for most sources the the points for ortho and para H2 lie are both well fittedby this nearly straight line proves that our assumption on their relative abundances iscorrect. For a number of sources, the lines at higher energy levels can be seen to deviatestrongly from the Boltzmann fit. In these cases, we have attempted to characterize thisbehaviour, which may reflect the combined effects of UV-pumped infrared fluorescenceand the presence of a very warm, but thin, surface layer in a PDR and may be due to the4 M.E. van den Ancker et al.: H2 in Star Forming Regionseffect of re-formation of H2 in J-shocks, by fitting a second Boltzmann distribution tothe higher energy level populations. The resulting excitation temperatures and derivedmass of molecular hydrogen are also indicated in Fig. 1.Employing predictions of H2 emission from PDR, J-shock and C-shock models byBurton et al. (1992), Hollenbach & McKee (1989) and Kaufman & Neufeld (1996), wedetermined the excitation temperature Trot from the low-lying pure rotational levels as afunction of density n and either incident FUV flux G (in units of the average interstellarFUV field G0) or shock velocity vs in an identical way as was done for the observations.The results of this procedure, plotted against G (PDRs) or the total flux observed in alllines (shocks) are shown in Fig. 2. Note that there is considerable overlap between theTrot predicted by PDR, J-shock and C-shock models. This means that pure rotational H2emission alone cannot distinguish between these mechanisms in all cases and additionalinformation will be needed. However, our ISO spectra provide just such information.Detectable [S i] 25.25 µm emission means that a shock must be present, whereas PAHemission is indicative of the presence of a PDR. The presence or absence of ionic linessuch as [Si ii] 34.82 µm can further distinguish between a C-shock and a J-shock. Theinformation from these lines is used in Fig. 2 to distinguish between likely shocks (circles)and PDRs (squares).In general, the PDR sources fall, within errors, in the parameter space outlined bythe PDR models. Since the J-shocks only predict a very narrow range of Trot, onlyone source, AFGL 2591, is compatible with the observed H2 emission arising in such adissociative shock. All shock sources are compatible with the range in Trot predicted byC-shock models. However, the detection of ionic lines in most of these sources shows thata J-shock component must be present. Most likely real astrophysical shocks are neveras simple as the purely dissociative or non-dissociative shocks in the employed models,but are made up of a combination of the two, with the non-dissociative componentdominating the H2 spectrum.3. ConclusionsWe have shown that pure-rotational emission from warm H2 is readily detectable inthe vicinity of intermediate- and high-mass YSOs and can be used to gain insight in thephysical conditions in the circumstellar material. The main mechanisms that producewarm H2 in these types of environments are shocks and PDRs. No deviations from the3:1 ortho/para ratio of H2 were found for either type of heating mechanism. Both shocksand PDRs show a warm and a hot component in H2. The warm component probes thethermal emission from warm gas. For PDRs the hot component may reflect the combinedeffects of UV-pumped infrared fluorescence and the presence of a thin, very warm surfacelayer. In shocks the hot H2 component may be due to the re-formation of H2 with non-zero formation energy. The warm H2 component in shocks appears to be dominated bythe non-dissociative part of the shock. The evolution of YSOs is expected to be fromshock-dominated to PDR-dominated and H2 may be one of the best tracers of the endof the outflow phase of a young star.REFERENCESBurton, M.G., Hollenbach, D.J., Tielens, A.G.G.M. 1992, ApJ 399, 563Hollenbach, D.J., McKee, C.F. 1989, ApJ 342, 306Hollenbach, D.J., Tielens, A.G.G.M. 1999, Rev. Mod Phys. 71, 173Kaufman, M.J., Neufeld, D.A. 1996, ApJ 456, 611

ISO Spectroscopy of H2 in Star Forming Regions

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ISO Spectroscopy of H2 in Star Forming Regions

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