11 research outputs found
Emission from Water Vapor and Absorption from Other Gases at 5-7.5 Microns in Spitzer-IRS Spectra of Protoplanetary Disks
We present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming
region showing emission in Spitzer Space Telescope Infrared Spectrograph (IRS)
5-7.5 micron spectra from water vapor and absorption from other gases in these
stars' protoplanetary disks. Seven stars' spectra show an emission feature at
6.6 microns due to the nu_2 = 1-0 bending mode of water vapor, with the shape
of the spectrum suggesting water vapor temperatures > 500 K, though some of
these spectra also show indications of an absorption band, likely from another
molecule. This water vapor emission contrasts with the absorption from warm
water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other
six of the thirteen stars have spectra showing a strong absorption band,
peaking in strength at 5.6-5.7 microns, which for some is consistent with
gaseous formaldehyde (H2CO) and for others is consistent with gaseous formic
acid (HCOOH). There are indications that some of these six stars may also have
weak water vapor emission. Modeling of these stars' spectra suggests these
gases are present in the inner few AU of their host disks, consistent with
recent studies of infrared spectra showing gas in protoplanetary disks.Comment: 33 pages, 9 figures, to appear in the 20 August, 2014, V791 - 2 issue
of the Astrophysical Journa
Silica in Protoplanetary Disks
Mid-infrared spectra of a few T Tauri stars (TTS) taken with the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope show prominent narrow
emission features indicating silica (crystalline silicon dioxide). Silica is
not a major constituent of the interstellar medium; therefore, any silica
present in the circumstellar protoplanetary disks of TTS must be largely the
result of processing of primitive dust material in the disks surrouding these
stars. We model the silica emission features in our spectra using the opacities
of various polymorphs of silica and their amorphous versions computed from
earth-based laboratory measurements. This modeling indicates that the two
polymorphs of silica, tridymite and cristobalite, which form at successively
higher temperatures and low pressures, are the dominant forms of silica in the
TTS of our sample. These high temperature, low pressure polymorphs of silica
present in protoplanetary disks are consistent with a grain composed mostly of
tridymite named Ada found in the cometary dust samples collected from the
STARDUST mission to Comet 81P/Wild 2. The silica in these protoplanetary disks
may arise from incongruent melting of enstatite or from incongruent melting of
amorphous pyroxene, the latter being analogous to the former. The high
temperatures of 1200K-1300K and rapid cooling required to crystallize tridymite
or cristobalite set constraints on the mechanisms that could have formed the
silica in these protoplanetary disks, suggestive of processing of these grains
during the transient heating events hypothesized to create chondrules.Comment: 47 pages, 9 figures, to appear in the 1 January, 2009 issue of the
Astrophysical Journa
Dust Processing and Grain Growth in Protoplanetary Disks in the Taurus-Auriga Star-Forming Region
Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust
at two temperatures to probe the radial variation in dust composition in the
uppermost layers of protoplanetary disks. Most spectra indicating crystalline
silicates require Mg-rich minerals and silica, but a few suggest otherwise.
Spectra indicating abundant enstatite at higher temperatures also require
crystalline silicates at temperatures lower than those required for spectra
showing high abundance of other crystalline silicates. A few spectra show 10
micron complexes of very small equivalent width. They are fit well using
abundant crystalline silicates but very few large grains, inconsistent with the
expectation that low peak-to-continuum ratio of the 10 micron complex always
indicates grain growth. Most spectra in our sample are fit well without using
the opacities of large crystalline silicate grains. If large grains grow by
agglomeration of submicron grains of all dust types, the amorphous silicate
components of these aggregates must typically be more abundant than the
crystalline silicate components. Crystalline silicate abundances correlate
positively with other such abundances, suggesting that crystalline silicates
are processed directly from amorphous silicates and that neither forsterite,
enstatite, nor silica are intermediate steps when producing either of the other
two. Disks with more dust settling typically have greater crystalline
abundances. Large-grain abundance is somewhat correlated with greater settling
of disks. The lack of strong correlation is interpreted to mean that settling
of large grains is sensitive to individual disk properties. Lower-mass stars
have higher abundances of large grains in their inner regions.Comment: 84 pages, 27 figures, submitted to the Astrophysical Journal on 7
November, 200