A search for near-infrared molecular hydrogen emission in the CTTS LkH$\mathsf{\alpha}$ 264 and the debris disk 49 Ceti

Abstract

We report on the first results of a search for molecular hydrogen emission from protoplanetary disks using CRIRES, ESO's new VLT Adaptive Optics high resolution near-infrared spectrograph. We observed the classical T Tauri star LkHα 264 and the debris disk 49 Cet, and searched for υ = 1−0 S(1) H 2 emission at 2.1218 µm, υ = 1−0 S(0) H 2 emission at 2.2233 µm and υ = 2−1 S(1) H 2 emission at 2.2477 µm. The H 2 line at 2.1218 µm is detected in LkHα 264 confirming the previous observations by Itoh et al. (2003). In addition, our CRIRES spectra reveal the previously observed but not detected H 2 line at 2.2233 µm in LkHα 264. An upper limit of 5.3 ×10 −16 erg s −1 cm −2 on the υ = 2−1 S(1) H 2 line flux in LkHα 264 is derived. The detected lines coincide with the rest velocity of LkHα 264. They have a FWHM of ∼20 km s −1 . This is strongly suggestive of a disk origin for the lines. These observations are the first simultaneous detection of υ = 1−0 S(1) and υ = 1−0 S(0) H 2 emission from a protoplanetary disk. 49 Cet does not exhibit H 2 emission in any of the three observed lines. We derive the mass of optically thin H 2 at T ∼ 1500 K in the inner disk of LkHα 264 and derive stringent limits in the case of 49 Cet at the same temperature. There are a few lunar masses of optically thin hot H 2 in the inner disk (∼0.1 AU) of LkHα 264, and less than a tenth of a lunar mass of hot H 2 in the inner disk of 49 Cet. The measured 1−0 S(0)/1−0 S(1) and 2−1 S(1)/1−0 S(1) line ratios in LkHα 264 indicate that the H 2 emitting gas is at a temperature lower than 1500 K and that the H 2 is most likely thermally excited by UV photons. The υ = 1−0 S(1) H 2 line in LkHα 264 is single peaked and spatially unresolved. Modeling of the shape of the line suggests that the disk should be seen close to face-on (i < 35 • ) and that the line is emitted within a few AU of the LkHα 264 disk. A comparative analysis of the physical properties of classical T Tauri stars in which the H 2 υ = 1−0 S(1) line has been detected and non-detected indicates that the presence of H 2 emission is correlated with the magnitude of the UV excess and the strength of the Hα line. The lack of H 2 emission in the NIR spectra of 49 Cet and the absence of Hα emission suggest that the gas in the inner disk of 49 Cet has dissipated. These results combined with previous detections of 12 CO emission at sub-mm wavelengths indicate that the disk surrounding 49 Cet should have an inner hole. We favor inner disk dissipation by inside-out photoevaporation, or the presence of an unseen low-mass companion as the most likely explanations for the lack of gas in the inner disk of 49 Cet.

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

“…Therefore, in the inner disk, where temperatures are sufficiently high to thermally (i.e., by collisions with dust) excite the H 2 lines, the amount of gas in the optically thin surface layer of the disk is small, quenching the formation of the near-infrared H 2 lines. Because CRIRES observations are typically sensitive from 0.1 to 1 M moon of H 2 gas at T > 1000 K (Carmona et al 2007), our first conclusion is that the lack of near-IR H 2 emission in the 13 sources with non-detections fully agrees with what would be expected from passive Herbig Ae/Be disks with thermally coupled gas and dust. The puzzling new observational result presented in this paper is that H 2 emission is present at radii 5 AU in the disks around the Herbig Ae/Be stars HD 97048 and HD 100546 4 .…”

“…ro-vibrational transitions of H 2 trace thermal emission of hot H 2 at a thousand K, or H 2 gas excited by UV or X-rays (e.g. Weintraub et al 2000;Bary et al 2003Bary et al , 2008Itoh et al 2003;Ramsay Howat & Greaves 2007;Carmona et al 2007Carmona et al , 2008a. Because gas at these temperatures is expected to be located at radii up to a few AU, H 2 near-IR emission has the potential of tracing the gas in the terrestrial planet region of disks if the observed emission is thermal.…”

“…Because gas at these temperatures is expected to be located at radii up to a few AU, H 2 near-IR emission has the potential of tracing the gas in the terrestrial planet region of disks if the observed emission is thermal. H 2 near-IR emission from disks has been mainly studied toward the low-mass T Tauri stars, where it has been detected preferentially in objects exhibiting signatures of a 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 emission (see the discussion sections of Carmona et al 2007, and Bary et al 2008. However, for intermediate mass young-stars, i.e., Herbig Ae/Be stars, there is to date only one reported detection of near-infrared H 2 emission at the velocity of the star, namely in the star HD 97048 (Bary et al 2008).…”

Searching for gas emission lines inSpitzerInfrared Spectrograph (IRS) spectra of young stars in Taurus

“…In the near-IR, the H 2 emission arises from ro-vibrational transitions, probing gas at T ∼ 1000 K (e.g., Weintraub et al 2000;Carmona et al 2007;Bary et al 2008b). After H 2 , CO is the most abundant species found in disks.…”

A search for near-infrared molecular hydrogen emission in the CTTS LkHα264 and the debris disk 49 Ceti

“…This correction on the U − V excess of LkHα 264 affects no conclusion in the original paper, therefore, the conclusions presented in the original paper remain unchanged. References: (1) Carmona et al (2007); (2) Itoh et al (2003). …”