35 research outputs found
Detection of Molecular Hydrogen Orbiting a "Naked" T Tauri Star
Astronomers have established that for a few million years newborn stars
possess disks of orbiting gas and dust. Such disks, which are likely sites of
planet formation, appear to disappear once these stars reach ages of 5-10 times
10^6 yr; yet, >= 10^7 yr is thought necessary for giant planet formation. If
disks dissipate in less time than is needed for giant planet formation, such
planets may be rare and those known around nearby stars would be anomalies.
Herein, we report the discovery of H_2 gas orbiting a weak-lined T Tauri star
heretofore presumed nearly devoid of circumstellar material. We estimate that a
significant amount of H_2 persists in the gas phase, but only a tiny fraction
of this mass emits in the near-infrared. We propose that this star possesses an
evolved disk that has escaped detection thus far because much of the dust has
coagulated into planetesimals. This discovery suggests that the theory that
disks are largely absent around such stars should be reconsidered. The
widespread presence of such disks would indicate that planetesimals can form
quickly and giant planet formation can proceed to completion before the gas in
circumstellar disks disperses.Comment: latex 12 pages, including 1 figur
Variations of the 10 um Silicate Features in the Actively Accreting T Tauri Stars: DG Tau and XZ Tau
Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we
observed multiple epochs of 11 actively accreting T Tauri stars in the nearby
Taurus-Auriga star forming region. In total, 88 low-resolution mid-infrared
spectra were collected over 1.5 years in Cycles 2 and 3. The results of this
multi-epoch survey show that the 10 um silicate complex in the spectra of two
sources - DG Tau and XZ Tau - undergoes significant variations with the
silicate feature growing both weaker and stronger over month- and year-long
timescales. Shorter timescale variations on day- to week-long timescales were
not detected within the measured flux errors. The time resolution coverage of
this data set is inadequate for determining if the variations are periodic.
Pure emission compositional models of the silicate complex in each epoch of the
DG Tau and XZ Tau spectra provide poor fits to the observed silicate features.
These results agree with those of previous groups that attempted to fit only
single-epoch observations of these sources. Simple two-temperature, two-slab
models with similar compositions successfully reproduce the observed variations
in the silicate features. These models hint at a self-absorption origin of the
diminution of the silicate complex instead of a compositional change in the
population of emitting dust grains. We discuss several scenarios for producing
such variability including disk shadowing, vertical mixing, variations in disk
heating, and disk wind events associated with accretion outbursts.Comment: 6 pages, emulate apj format, accepted for publication in ApJ Letter
Quiescent H2 Emission From Pre-Main Sequence Stars in Chamaeleon I
We report the discovery of quiescent emission from molecular hydrogen gas
located in the circumstellar disks of six pre-main sequence stars, including
two weak-line T Tauri stars (TTS), and one Herbig AeBe star, in the Chamaeleon
I star forming region. For two of these stars, we also place upper limits on
the 2->1 S(1)/1->0 S(1) line ratios of 0.4 and 0.5. Of the 11 pre-main sequence
sources now known to be sources of quiescent near-infrared hydrogen emission,
four possess transitional disks, which suggests that detectable levels of H
emission and the presence of inner disk holes are correlated. These H
detections demonstrate that these inner holes are not completely devoid of gas,
in agreement with the presence of observable accretion signatures for all four
of these stars and the recent detections of [Ne II] emission from three of
them. The overlap in [Ne II] and H detections hints at a possible
correlation between these two features and suggests a shared excitation
mechanism of high energy photons. Our models, combined with the kinematic
information from the H lines, locate the bulk of the emitting gas at a few
tens of AU from the stars. We also find a correlation between H detections
and those targets which possess the largest H equivalent widths,
suggesting a link between accretion activity and quiescent H emission. We
conclude that quiescent H emission from relatively hot gas within the disks
of TTS is most likely related to on-going accretion activity, the production of
UV photons and/or X-rays, and the evolutionary status of the dust grain
populations in the inner disks.Comment: 12 pages, emulateapj, Accepted by Ap
Correlating Changes in Spot Filling Factors with Stellar Rotation: The Case of LkCa 4
We present a multi-epoch spectroscopic study of LkCa 4, a heavily spotted
non-accreting T Tauri star. Using SpeX at NASA's Infrared Telescope Facility
(IRTF), 12 spectra were collected over five consecutive nights, spanning
1.5 stellar rotations. Using the IRTF SpeX Spectral Library, we
constructed empirical composite models of spotted stars by combining a warmer
(photosphere) standard star spectrum with a cooler (spot) standard weighted by
the spot filling factor, . The best-fit models spanned two
photospheric component temperatures, = 4100 K (K7V) and 4400 K
(K5V), and one spot component temperature, = 3060 K (M5V) with an
of 0.3. We find values of to vary between 0.77 and 0.94 with
an average uncertainty of 0.04. The variability of is periodic
and correlates with its 3.374 day rotational period. Using a mean value for
to represent the total spot coverage, we calculated spot
corrected values for and . Placing these values alongside
evolutionary models developed for heavily spotted young stars, we infer mass
and age ranges of 0.45-0.6 and 0.50-1.25 Myr, respectively. These
inferred values represent a twofold increase in the mass and a twofold decrease
in the age as compared to standard evolutionary models. Such a result
highlights the need for constraining the contributions of cool and warm regions
of young stellar atmospheres when estimating and to infer
masses and ages as well as the necessity for models to account for the effects
of these regions on the early evolution of low-mass stars.Comment: 21 pages, 9 Figures; Accepted for publication in Ap
Circumbinary Gas Accretion onto a Central Binary: Infrared Molecular Hydrogen Emission from GG Tau A
We present high spatial resolution maps of ro-vibrational molecular hydrogen
emission from the environment of the GG Tau A binary component in the GG Tau
quadruple system. The H2 v= 1-0 S(1) emission is spatially resolved and
encompasses the inner binary, with emission detected at locations that should
be dynamically cleared on several hundred-year timescales. Extensions of H2 gas
emission are seen to ~100 AU distances from the central stars. The v = 2-1 S(1)
emission at 2.24 microns is also detected at ~30 AU from the central stars,
with a line ratio of 0.05 +/- 0.01 with respect to the v = 1-0 S(1) emission.
Assuming gas in LTE, this ratio corresponds to an emission environment at ~1700
K. We estimate that this temperature is too high for quiescent gas heated by
X-ray or UV emission from the central stars. Surprisingly, we find that the
brightest region of H2 emission arises from a spatial location that is exactly
coincident with a recently revealed dust "streamer" which seems to be
transferring material from the outer circumbinary ring around GG Tau A into the
inner region. As a result, we identify a new excitation mechanism for
ro-vibrational H2 stimulation in the environment of young stars. The H2 in the
GG Tau A system appears to be stimulated by mass accretion infall as material
in the circumbinary ring accretes onto the system to replenish the inner
circumstellar disks. We postulate that H2 stimulated by accretion infall could
be present in other systems, particularly binaries and "transition disk"
systems which have dust cleared gaps in their circumstellar environments.Comment: 18 pages, including 4 figures. Accepted for publication in Ap
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Physical Conditions of Accreting Gas in T Tauri Star Systems
We present results from a low resolution (R~300) near-infrared spectroscopic
variability survey of actively accreting T Tauri stars (TTS) in the
Taurus-Auriga star forming region. Paschen and Brackett series H I
recombination lines were detected in 73 spectra of 15 classical T Tauri
systems. The values of the Pan/PaB, Brn/BrG, and BrG/Pan H I line ratios for
all observations exhibit a scatter of < 20% about the weighted mean, not only
from source to source, but also for epoch-to-epoch variations in the same
source. A representative or `global' value was determined for each ratio in
both the Paschen and Brackett series as well as the BrG/Pan line ratios. A
comparison of observed line ratio values was made to those predicted by the
temperature and electron density dependent models of Case B hydrogen
recombination line theory. The measured line ratios are statistically well-fit
by a tightly constrained range of temperatures (T < 2000 K) and electron
densities 1e9 < n_e < 1e10 cm^-3. A comparison of the observed line ratio
values to the values predicted by the optically thick and thin local
thermodynamic equilibrium cases rules out these conditions for the emitting H I
gas. Therefore, the emission is consistent with having an origin in a non-LTE
recombining gas. While the range of electron densities is consistent with the
gas densities predicted by existing magnetospheric accretion models, the
temperature range constrained by the Case B comparison is considerably lower
than that expected for accreting gas. The cooler gas temperatures will require
a non-thermal excitation process (e.g., coronal/accretion-related X-rays and UV
photons) to power the observed line emission.Comment: 12 pages, emulateapj format, Accepted for publication in Ap
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Linking soil microbial community structure to potential carbon mineralization: A continental scale assessment of reduced tillage
Potential carbon mineralization (Cmin) is a commonly used indicator of soil health, with greater Cmin values interpreted as healthier soil. While Cmin values are typically greater in agricultural soils managed with minimal physical disturbance, the mechanisms driving the increases remain poorly understood. This study assessed bacterial and archaeal community structure and potential microbial drivers of Cmin in soils maintained under various degrees of physical disturbance. Potential carbon mineralization, 16S rRNA sequences, and soil characterization data were collected as part of the North American Project to Evaluate Soil Health Measurements (NAPESHM). Results showed that type of cropping system, intensity of physical disturbance, and soil pH influenced microbial sensitivity to physical disturbance. Furthermore, 28% of amplicon sequence variants (ASVs), which were important in modeling Cmin, were enriched under soils managed with minimal physical disturbance. Sequences identified as enriched under minimal disturbance and important for modeling Cmin, were linked to organisms which could produce extracellular polymeric substances and contained metabolic strategies suited for tolerating environmental stressors. Understanding how physical disturbance shapes microbial communities across climates and inherent soil properties and drives changes in Cmin provides the context necessary to evaluate management impacts on standardized measures of soil microbial activity