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A CRIRES Search for Near-IR H2 Emission from Herbig Ae/Be Stars

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A CRIRES Search for Near-IR H2 Emission from Herbig Ae/Be Stars UvA-DARE (Digital Academic Repository) Warm gas in protoplanetary disks van der Plas, G. Publication date 2010 Link to publication Citation for published version (APA): van der Plas, G. (2010). Warm gas in protoplanetary disks. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:29 Sep 2021 A CRIRES search for near-IR H2 5 emission from Herbig Ae/Be stars A. Carmona, G. van der Plas, M.E. van den Ancker, M. Audard, L.B.F.M Waters, B. Acke, D. Fedele, E. Pantin To be submitted. Abstract We report on a sensitive search for H2 1-0 S(1), 1-0 S(0) and 2-1 S(1) ro-vibrational emission at 2.12, 2.22 and 2.25 μm in a sample of 14 Herbig Ae/Be stars, employing CRIRES, the ESO-VLT near-infrared high-resolution spectrograph, at R∼90,000. We report the detection of the H2 1-0 S(1) line towards the Herbig Ae/Be stars HD 100546 and HD 97048. In the other 12 targets, the line is absent. H2 1-0 S(0) and 2-1 S(1) emission is absent in all the sources. These observations are the first detection of near-IR H2 emission in HD 100546, and double the number of Herbig Ae/Be stars with near-IR H2 emission to two. The H2 1-0 S(1) lines observed in HD 100546 and HD 97048 are spatially resolved and are at the rest velocity of the stars, suggesting that they may be produced in the circumstellar disk. In both sources, the H2 1-0 S(1) line is single peaked. The non-detections of near-IR H2 emission can be explained as a natural consequence of a classical passive disk structure in which the gas and dust are thermally coupled (i.e. the amount of optically thin hot H2 gas in the inner disk is too low to produce detectable levels of H2 emission). In the case of HD 100546 and HD 97048, modeling of the profile and spatial extend of the H2 1-0 S(1) line with a flat keplerian disk indicates that the emission is produced at a radius larger than 10 AU. Comparison of the H2 1-0 S(1) line profile with both the CO ro-vibrational emission at 4.7 μm 97 5. A CRIRES search for near-IR H2 emission from Herbig Ae/Be stars and the 6300 A˚ [OI] profile in both sources, shows that the H2 line is narrower. In the context of emission from a Keplerian disk, this indicates that the H2 emission is produced further out in the disk than the CO or [OI] emission. As the H2 emission is located at large radii, for both sources a thermal emission scenario is implausible. We argue that the observation of H2 emission at large radii may be indicative of the presence of an extended disk atmosphere at radii >10 AU. This may be possibly induced by a hydrostatic disk in which gas and dust are thermally decoupled, UV / X-ray heating, or photo-evaporation. 5.1 Introduction Circumstellar disks around pre-main sequence stars are the birth-places of planets. The characterization of their physical properties is of paramount importance. At the time when giant planets are in formation, protoplanetary disks are gas rich. However, observationally, there are relatively few constrains of the gas content in the disk, in particular for the inner disk (R<20 AU), the region where planets are expected to form. To study the gas content in the disk, molecular and atomic line emission is used. As the disk has a radial gradient of temperature, different transitions of diverse gas tracers probe different radii in the disk (see reviews by Najita et al. 2007 and Carmona 2010). Such emission lines are produced in the surface layers of the disk above the continuum forming surface. By far the main constituent of the gas in protoplanetary disks is molecu- lar hydrogen (H2). However, given its physical nature (H2 is an homonuclear molecule that lacks a dipole moment), H2 transitions are very weak. In con- sequence, in contrast to other gas tracers (most notably CO), H2 emission is harder to detect. With the advent of space-born observatories and ground- based high-resolution infrared spectrographs, it has been possible to start the search and study of H2 emission from disks around young stars from the UV to the mid-IR. In the specific case of the near-infrared ro-vibrational transi- tions, H2 traces thermal emission of hot (T ≥ 1000 K) or UV or X-ray excited H2 (e.g. Weintraub et al. 2000; Bary et al. 2003, 2008; Itoh et al. 2003; Ramsay Howat & Greaves 2007; Carmona et al. 2007, 2008a). As gas at these temperatures is expected to be located at a radii up to a few AU, H2 near-IR emission has the potential of tracing the gas in the terrestrial planet region of disks, if the emission observed is thermal. H2 near-IR emission from disks has been mainly studied towards the low- mass T Tauri stars, where it has been detected preferentially in objects ex- hibiting signatures of high accretion rate (i.e. large Hα equivalent widths and U − V excess) and in a few weak-line T Tauri stars with bright X-ray emis- 98 5.2 Observations and Data Reduction sion (see the discussion sections of Carmona et al. 2007, 2008a, and Bary et al. 2008). However, in the case of intermediate mass young-stars, i.e., Her- big Ae/Be stars, to date, there is only one reported detection of quiescent near-infrared H2 emission, namely in the star HD 97048 (Bary et al. 2008). As Herbig Ae/Be stars are bright in the near-IR, the detection of the faint H2 lines is challenging, in particular at low spectral resolution. In this chap- ter, we present the results of a search for H2 1-0 S(1), 1-0 S(0) and 2-1 S(1) emission towards a sample of Herbig Ae/Be stars employing the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES), the ESO-VLT near- infrared high-resolution spectrograph, at the resolution R ∼ 90,000. The chapter is organized as follows. In Section 5.2, we describe our CRIRES observations and the data reduction techniques used. In Section 5.3, we present our results, constrain the excitation mechanism of the detected lines based on the line ratios, analyze the lines detected observed in the context of a flat disk model, and compare the H2 line profile observed with the line profiles of other gas tracers. Finally, in Section 5.4, we discuss the relatively large number of non-detections found and explore diverse scenarios to explain the detections of near-IR H2 emission in HD 100546 and HD 97048. 5.2 Observations and Data Reduction We observed a sample of 14 nearby (d< 250 pc) Herbig Ae/Be stars of diverse spectral types and disk geometries based on the Spectral Energy Distribution (SED) classification introduced by Meeus et al. (2001) and SED modeling by Dullemond et al. (2001) (i.e. group I: flared disks, group II: self-shadowed disks). We summarize the physical properties of the stars of our program in Table 5.1. The sources were observed with CRIRES mounted at the ESO-VLT UT1 (Antu) atop Cerro Paranal, Chile (K¨aufl et al. 2004). The observations are standard long-slit spectroscopy observations. We employed a slit of width 0.2”, with the slit rotated along the parallactic angle, resulting in a spectral resolution R of 90,000 (or 3.2 km s−1). We used Adaptive Optics (MACAO - Multi-Applications Curvature Adaptive Optics) to optimize the signal-to- noise ratio and the spatial resolution. The typical spatial resolution achieved was 220 mas (i.e., FWHM in the continuum). In order to correct for sky and telescope emission, we used the standard nodding technique along the slit employing a nod throw of 10”. To correct for telluric absorption and flux-calibrate the spectrum, we observed spectrophotometric standard stars immediately before or after the science observations with the same instrument 99 5. A CRIRES search for near-IR H2 emission from Herbig Ae/Be stars spectropolarimetry of the (2002) argue that HD 58647 may not be a Herbig AeBe star but a classical Be star. Baines et al. (2006) based on a ”self-shadowed” disks. SED classification by Meeus et al. (2001). Group I sources are with ”flared” disks and Group II sources are Table 5.1: HD 190073HD 179218 20:03:02.51R CrA 19:11:11.25 +05:44:16.7HD 169142 +15:47:15.651 Oph 18:24:29.78HD A2IVpe 150193 19:01:53.65HD -29:46:49.4 144432 A0IVe -36:57:07.6 16:40:17.92HD 17:31:24.95 141569 16:06:57.96HD -23:53:45.2 135344B -23:57:45.5 +3.0 A5Ve 15:49:57.75HD -27:43:09.8 A5IIev 15:15:48.95 101412 +15.4HD -03:55:16.4 100546 -37:08:56.1 A1Ve 11:39:44.46HD A9/ A0Ve 97048 F0Ve 11:33:25.44 II HD -60:10:27.7 95881 A0Ve -3.0 -70:11:41.2 -3.0 I 11:08:03.32 A0Ve 11:01:57.62 -77:39:17.5 +2.0 B9.5Ve -6.0 -11.4 -71:30:48.3 B9Vne II A1/A2III/IV I A0pshe e -2.0 -1.0 II II II -3.0 +15.8 +36 hole hole +16.8 I/II I II I HD 58647 Star Coordinates, spectral types, radial velocities and SED spectral group b See van der Plas et al.
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