49 research outputs found
Spitzer Infrared Spectrograph survey of young stars in the Chamaeleon I star-forming region
We present 5 to 36 micron mid-infrared spectra of 82 young stars in the ~2
Myr old Chamaeleon I star-forming region, obtained with the Spitzer Infrared
Spectrograph (IRS). We have classified these objects into various evolutionary
classes based on their spectral energy distributions and the spectral features
seen in the IRS spectra. We have analyzed the mid-IR spectra of Class II
objects in Chamaeleon I in detail, in order to study the vertical and radial
structure of the protoplanetary disks surrounding these stars. We find evidence
for substantial dust settling in most protoplanetary disks in Chamaeleon I. We
have identified several disks with altered radial structures in Chamaeleon I,
among them transitional disk candidates which have holes or gaps in their
disks. Analysis of the silicate emission features in the IRS spectra of Class
II objects in Chamaeleon I shows that the dust grains in these disks have
undergone significant processing (grain growth and crystallization). However,
disks with radial holes/gaps appear to have relatively unprocessed grains. We
further find the crystalline dust content in the inner (< 1-2 AU) and the
intermediate (< 10 AU) regions of the protoplanetary disks to be tightly
correlated. We also investigate the effects of accretion and stellar
multiplicity on the disk structure and dust properties. Finally, we compare the
observed properties of protoplanetary disks in Cha I with those in slightly
younger Taurus and Ophiuchus regions and discuss the effects of disk evolution
in the first 1-2 Myr.Comment: 80 pages, 32 Figures, Accepted for publication in the Astrophysical
Journal Supplement Serie
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
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
Spitzer IRS Spectroscopy of IRAS-Discovered Debris Disks
We have obtained Spitzer Space Telescope IRS 5.5 - 35 micron spectra of 59
main sequence stars that possess IRAS 60 micron excess. The spectra of five
objects possess spectral features that are well-modeled using micron-sized
grains and silicates with crystalline mass fractions 0% - 80%, consistent with
T-Tauri and Herbig AeBe stars. With the exception of eta Crv, these objects are
young with ages <50 Myr. The spectra for the majority of objects are
featureless, suggesting that the emitting grains probably have radii a > 10
micron. We have modeled the excess continua using a continuous disk with a
uniform surface density distribution, expected if Poynting-Robertson and
stellar wind drag are the dominant grain removal processes, and using a single
temperature black body, expected if the dust is located in a narrow ring around
the star. The IRS spectra of many objects are better modeled with a single
temperature black body, suggesting that the disks possess inner holes. The
distribution of grain temperatures, based on our black body fits, peaks at Tgr
= 110 - 130 K. Since the timescale for ice sublimation of micron-sized grains
with Tgr > 110 K is a fraction of a Myr, the lack of warmer material may be
explained if the grains are icy. If planets dynamically clear the central
portions of debris disks, then the frequency of planets around other stars is
probably high. We estimate that the majority of debris disk systems possess
parent body masses, MPB < 1 Mearth. The low inferred parent body masses suggest
that planet formation is an efficient process. (abridged abstract)Comment: 66 pages, 18 figures (including 12 color figures), ApJS, in pres
The development of a protoplanetary disk from its natal envelope
Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations, and therefore are studied intensively for details of the formation of protoplanetary disks within protostellar envelopes. At millimetre wavelengths, kinematic signatures of collapse have been observed in several such protostars, through observations of molecular lines that probe their outer envelopes. It has been suggested that one or more components of the proto-multiple system NGC 1333-IRAS 4 (refs 1, 2) may display signs of an embedded region that is warmer and denser than the bulk of the envelope(3,4). Here we report observations that reveal details of the core on Solar System dimensions. We detect in NGC 1333-IRAS 4B a rich emission spectrum of H2O, at wavelengths 20-37 mu m, which indicates an origin in extremely dense, warm gas. We can model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disk, and therefore see the development of a protoplanetary disk. This is the only example of mid-infrared water emission from a sample of 30 class 0 objects, perhaps arising from a favourable orientation; alternatively, this may be an early and short-lived stage in the evolution of a protoplanetary disk.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62894/1/nature06087.pd
State-to-State Vibrational Predissociation
Author Institution: Dept. of Chemistry, University of North CarolinaUsing a variation of the optothermal detection method, photofragment angular distributions for several HF containing van der Waals complexes have been measured. These distributions show resolved structure which can be assigned to individual fragment dissociation channels. This data can be used to establish correlations between the rotational states of the two fragments. The relative state-to-state cross sections for are determined and compared with unimolecular dissociation theories. This comparison clearly shows that the predissociation dynamics of is highly non-statistical. In addition to this information, the experiment also yields an accurate zero point dissociation energy for $(HF)_{2}.