795 research outputs found
Organic Molecules and Water in the Inner Disks of T Tauri Stars
We report high signal-to-noise Spitzer IRS spectra of a sample of eleven
classical T Tauri stars. Molecular emission from rotational transitions of H2O
and OH and ro-vibrational bands of simple organic molecules (CO2, HCN, C2H2) is
common among the sources in the sample. The gas temperatures (200-800 K) and
emitting areas we derive are consistent with the emission originating in a warm
disk atmosphere in the inner planet formation region at radii < 2 AU. The H2O
emission appears to form under a limited range of excitation conditions, as
shown by the similarity in relative strengths of H2O features from star to star
and the narrow range in derived temperature and column density. Emission from
highly excited rotational levels of OH is present in all stars; the OH emission
flux increases with the stellar accretion rate, and the OH/H2O flux ratio shows
a relatively small scatter. We interpret these results as evidence for OH
production via FUV photo-dissociation of H2O in the disk surface layers. No
obvious explanation is found for the observed range in the relative emission
strengths of different organic molecules or in their strength with respect to
water. We put forward the possibility that these variations reflect a diversity
in organic abundances due to star-to-star differences in the C/O ratio of the
inner disk gas. Stars with the largest HCN/H2O flux ratios in our sample have
the largest disk masses. We speculate that such a trend could result if higher
mass disks are more efficient at planetesimal formation and sequestration of
water in the outer disk, leading to enhanced C/O ratios and abundances of
organic molecules in the inner disk. A comparison of our derived HCN to H2O
column density ratio to comets, hot cores, and outer T Tauri star disks
suggests that the inner disks are chemically active.Comment: Accepted for publication in The Astrophysical Journa
Other Visions of Virtue in Ordinary Economies in Japan: A Historical Perspective, 1750-1950
Opinion Foru
Spatially Resolved Spectroscopy of Sub-AU-Sized Regions of T Tauri and Herbig Ae/Be Disks
We present spatially resolved near-IR spectroscopic observations of 15 young
stars. Using a grism spectrometer behind the Keck Interferometer, we obtained
an angular resolution of a few milli-arcseconds and a spectral resolution of
230, enabling probes of both gas and dust in the inner disks surrounding the
target stars. We find that the angular size of the near-IR emission typically
increases with wavelength, indicating hot, presumably gaseous material within
the dust sublimation radius. Our data also clearly indicate Brackett-gamma
emission arising from hot hydrogen gas, and suggest the presence of water vapor
and carbon monoxide gas in the inner disks of several objects. This gaseous
emission is more compact than the dust continuum emission in all cases. We
construct simple physical models of the inner disk and fit them to our data to
constrain the spatial distribution and temperature of dust and gas emission
components.Comment: 40 pages, 8 figures. Accepted for publication in Ap
Demographics of Transition Objects
The unusual properties of transition objects (young stars with an optically
thin inner disc surrounded by an optically thick outer disc) suggest that
significant disc evolution has occured in these systems. We explore the nature
of these systems by examining their demographics, specifically their stellar
accretion rates (Mdot) and disc masses (Mdisc) compared to those of accreting T
Tauri stars of comparable age. We find that transition objects in Taurus occupy
a restricted region of the Mdot vs. Mdisc plane. Compared to non-transition
single stars in Taurus, they have stellar accretion rates that are typically
~10 times lower at the same disc mass and median disc masses ~4 times larger.
These properties are anticipated by several proposed planet formation theories
and suggest that the formation of Jovian mass planets may play a significant
role in explaining the origin of at least some transition objects. Considering
transition objects as a distinct demographic group among accreting T Tauri
stars leads to a tighter relationship between disc masses and stellar accretion
rates, with a slope between the two quantities that is close to the value of
unity expected in simple theories of disc accretion.Comment: 11 pages, 4 figures, to appear in MNRA
The narrow, inner CO ring around the magnetic Herbig Ae star, HD 101412
We describe and model emission lines in the first overtone band of CO in the
magnetic Herbig Ae star HD 101412. High-resolution CRIRES spectra reveal
unusually sharp features which suggest the emission is formed in a thin disk
centered at 1 AU with a width 0.32 AU or less. A wider disk will not fit the
observations. Previous observations have reached similar conclusions, but the
crispness of the new material brings the emitting region into sharp focus.Comment: Accepted as Astronomy and Astrophysics Letter; 4 pages, 5 figure
- …