Far Ultraviolet Spectroscopic ExplorerObservations of Possible Infalling Planetesimals in the 51 Ophiuchi Circumstellar Disk

Abstract

We present the first observations of the circumstellar (CS) disk system 51 Ophiuchi with the Far Ultraviolet Spectroscopic Explorer. We detect several absorption lines arising from the unusual metastable atomic species N I ( 2 D), N I ( 2 P ), and S II ( 2 D). These levels lie 1.8 -3.6 eV above the ground level and have radiative decay lifetimes of 2 days or less, indicating that the lines arise from warm CS gas. The high S/N FUSE spectra, obtained six days apart, also show time-variable absorption features arising from N I, N II, O I ( 1 D), and Fe III, which are redshifted with respect to the stellar velocity. The resolved redshifted absorption extends over many tens of km s −1 (40 for N I, 100 for N II, 65 for O I ( 1 D), and 84 for Fe III). We calculate column densities for all the variable infalling CS gasses, using the apparent optical depth method. The Fe III and N II infalling gasses must be produced through collisional ionization, and the ionization fraction of nitrogen suggests a gas temperature between 20000 and 34000 K. The infalling gas shows a peculiar, non-solar composition, with nitrogen and iron more abundant than carbon. We also set upper limits on the line-of-sight column densities of H 2 and CO. These observations strengthen the connection between 51 Oph and the older debris-disk system β Pictoris, and indicate that there may be infalling planetesimals in the 51 Oph system.

Evidence for a hot dust-free inner disk around 51 Oph

“…Contrary to β Pic, large amount of molecular gas (CO, CO 2 , H 2 O and NO) has been seen around 51 Oph by the Short-Wavelength-Spectrometer on board the Infrared Space Observatory (ISO) (van den Ancker et al 2001). A controversy remains about the quantity of CO. Absorption lines by CO in the Far-Ultraviolet were not detected by FUSE (Roberge et al 2002). Nevertheless, all studies suggest that the circumstellar matter is most likely in the form of a Keplerian disk rather than in a spherical shell.…”

“…The UV and optical part of the SED are well fitted by a Kurucz model and show no excess testifying that the accretion of matter onto the star is low (Waters et al 1988;Malfait et al 1998). However, infalling ionic and atomic gases were observed which prompted Grady et al (1991) to claim that 51 Oph is in the same evolutionary state as β Pic (see also Roberge et al 2002). Contrary to β Pic, large amount of molecular gas (CO, CO 2 , H 2 O and NO) has been seen around 51 Oph by the Short-Wavelength-Spectrometer on board the Infrared Space Observatory (ISO) (van den Ancker et al 2001).…”

High resolution spectroscopy of circumstellar material around A stars

“…In order to search for possible variability they degraded their observations to a resolving power of R = 100 000 (i.e., half the resolution of our spectra presented in this paper), enabling them to compare them to data obtained by Lagrange-Henri et al (1990). The analysis of Crawford et al (1997) does not show evidence for spectral variability in Ca K, while Roberge et al (2002) report the detection of timevariable N, N, O, and Fe lines from FUSE observations in their recent study of HR 6519. Moreover, HR 6519 shows clear evidence for the presence of circumstellar gas from the detection of the Fe line at 4583.84 Å (Dunkin et al 1997) which is not observed in the interstellar medium since it arises from a metastable level 2.8 eV above the ground state.…”

Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks

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