A survey for near-infrared H₂emission in Herbig Ae/Be stars: emission from the outer disks of HD 97048 and HD 100546

Abstract

We report on a sensitive search for H 2 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 15 Herbig Ae/Be stars employing CRIRES, the ESO-VLT near-infrared high-resolution spectrograph, at R ∼ 90 000. We report the detection of the H 2 1-0 S(1) line toward HD 100546 and HD 97048. In the other 13 targets, the line is not detected. The H 2 1-0 S(0) and 2-1 S(1) lines are undetected in all sources. These observations are the first detection of near-IR H 2 emission in HD 100546. The H 2 1-0 S(1) lines observed in HD 100546 and HD 97048 are observed at a velocity consistent with the rest velocity of both stars, suggesting that they are produced in the circumstellar disk. In HD 97048 the emission is spatially resolved and it is observed to extend at least up to 200 AU from the star. We report an increase of one order of magnitude in the H 2 1-0 S(1) line flux with respect to previous measurements taken in 2003 for this star, which suggests line variability. In HD 100546 the emission is tentatively spatially resolved and may extend at least up to 50 AU from the star. Modeling of the H 2 1-0 S(1) line profiles and their spatial extent with flat Keplerian disks shows that most of the emission is produced at a radius larger than 5 AU. Upper limits to the H 2 1-0 S(0)/1-0 S(1) and H 2 2-1 S(1)/1-0 S(1) line ratios in HD 97048 are consistent with H 2 gas at T > 2000 K and suggest that the emission observed may be produced by X-ray excitation. The upper limits for the line ratios for HD 100546 are inconclusive. Because the H 2 emission is located at large radii, for both sources a thermal emission scenario (i.e., gas heated by collisions with dust) is implausible. We argue that the observation of H 2 emission at large radii may be indicative of an extended disk atmosphere at radii >5 AU. This may be explained by a hydrostatic disk in which gas and dust are thermally decoupled or by a disk wind caused by photoevaporation.

Resolving the inner regions of the HD 97048 circumstellar disk with VLT/NACO polarimetric differential imaging

“…The lack of emission closer to the star was taken as a possible sign of photo-dissociation of CO as [OI], and hence gas, seems to exist also in the inner 10 AU. Interestingly, Carmona et al (2011) found that the H 2 1-0 S(1) ro-vibrational line at 2.12 μm was not only clearly detected but extended from ∼5-10 AU out to >200 AU. Photo-dissociation in the inner disk regions and heating by high-energy UV and X-ray photons was suggested as a possible origin for the observed emission.…”

“…Single-dish observations of the mm continuum emission suggest a circumstellar mass of ∼0.2 M (Henning et al 1998) but part of this mass is probably residing in an envelope, also seen in extended MIR emission (Prusti et al 1994;Siebenmorgen et al 2000), surrounding the star-disk system. Several gas emission lines ([OI], CO and H 2 ) have been detected and were, at least partly, spectrally and spatially resolved (Acke & van den Ancker 2006;van der Plas et al 2009;Martin-Zaïdi et al 2007Carmona et al 2011).…”

DIGIT survey of far-infrared lines from protoplanetary disks

“…Interestingly these are the only two Herbig AeBe sources where ro-vibrational H 2 emission has been detected so far (Carmona et al 2011). The velocity profile of the line suggests extended H 2 emission to more that 50 AU (radius) from the star (Carmona et al 2011).…”

Origin of the wide-angle hot H2in DG Tauri

“…The associated magnetic fields would have A&A 564, A11 (2014) strong consequences for the formation and migration of planets (Fromang et al 2005;Bai & Stone 2013). The H 2 emission from warm (T = 1000-3000 K) gas was recently detected within 100 AU of more evolved T Tauri stars (TTSs) and Herbig Ae/Be stars, both in the near-infrared and far-ultraviolet (FUV) domains (e.g., Valenti et al 2000;Ardila et al 2002;Bary et al 2003;Carmona et al 2011). When studied at high spectral resolution, these lines split into two populations: most show narrow profiles centered on the stellar velocity, suggesting an origin in a warm disc atmosphere (quiescent H 2 ), but 30% of them display profiles blueshifted by 10-30 km s −1 , sometimes with blue wings extending up to -100 km s −1 , suggesting formation in an outflow (Takami et al 2004;Herczeg et al 2006;Greene et al 2010;Carmona et al 2011;France et al 2012).…”

“…The H 2 emission from warm (T = 1000-3000 K) gas was recently detected within 100 AU of more evolved T Tauri stars (TTSs) and Herbig Ae/Be stars, both in the near-infrared and far-ultraviolet (FUV) domains (e.g., Valenti et al 2000;Ardila et al 2002;Bary et al 2003;Carmona et al 2011). When studied at high spectral resolution, these lines split into two populations: most show narrow profiles centered on the stellar velocity, suggesting an origin in a warm disc atmosphere (quiescent H 2 ), but 30% of them display profiles blueshifted by 10-30 km s −1 , sometimes with blue wings extending up to -100 km s −1 , suggesting formation in an outflow (Takami et al 2004;Herczeg et al 2006;Greene et al 2010;Carmona et al 2011;France et al 2012). In particular, long-slit spectra of DG Tau provide evidence of a warm H 2 outflow at -20 km s −1 , shifted by 30 AU along the blue jet direction and with an estimated width 80 AU (Takami et al 2004;Schneider et al 2013b).…”