65 research outputs found
High Resolution Spectroscopy during Eclipse of the Young Substellar Eclipsing Binary 2MASS 0535-0546. I. Primary Spectrum: Cool Spots versus Opacity Uncertainties
We present high-resolution Keck optical spectra of the very young substellar
eclipsing binary 2MASS J05352184-0546085, obtained during eclipse of the
lower-mass (secondary) brown dwarf. The observations yield the spectrum of the
higher-mass (primary) brown dwarf alone, with negligible (~1.6%) contamination
by the secondary. We perform a simultaneous fine-analysis of the TiO-epsilon
band and the red lobe of the KI doublet, using state-of-the-art PHOENIX Dusty
and Cond synthetic spectra. Comparing the effective temperature and surface
gravity derived from these fits to the {\it empirically} determined surface
gravity of the primary (logg=3.5) then allows us to test the model spectra as
well as probe the prevailing photospheric conditions. We find that: (1) fits to
TiO-epsilon alone imply Teff=2500 \pm 50K; (2) at this Teff, fits to KI imply
logg=3.0, 0.5 dex lower than the true value; and (3) at the true logg, KI fits
yield Teff=2650 \pm 50K, ~150K higher than from TiO-epsilon alone. On the one
hand, these are the trends expected in the presence of cool spots covering a
large fraction of the primary's surface (as theorized previously to explain the
observed Teff reversal between the primary and secondary). Specifically, our
results can be reproduced by an unspotted stellar photosphere with Teff=2700K
and (empirical) logg=3.5, coupled with axisymmetric cool spots that are 15%
cooler (2300K), have an effective logg=3.0 (0.5 dex lower than photospheric),
and cover 70% of the surface. On the other hand, the trends in our analysis can
also be reproduced by model opacity errors: there are lacks in the synthetic
TiO-epsilon opacities, at least for higher-gravity field dwarfs. Stringently
discriminating between the two possibilities requires combining the present
results with an equivalent analysis of the secondary (predicted to be
relatively unspotted compared to the primary).Comment: To appear in ApJ. 11 pages, 5 figure
A Hot Gap Around Jupiter's Orbit in the Solar Nebula
The Sun was an order of magnitude more luminous during the first few hundred
thousand years of its existence, due in part to the gravitational energy
released by material accreting from the Solar nebula. If Jupiter was already
near its present mass, the planet's tides opened an optically-thin gap in the
nebula. We show using Monte Carlo radiative transfer calculations that sunlight
absorbed by the nebula and re-radiated into the gap raised temperatures well
above the sublimation threshold for water ice, with potentially drastic
consequences for the icy bodies in Jupiter's feeding zone. Bodies up to a meter
in size were vaporized within a single orbit if the planet was near its present
location during this early epoch. Dust particles lost their ice mantles, and
planetesimals were partially to fully devolatilized, depending on their size.
Scenarios in which Jupiter formed promptly, such as those involving a
gravitational instability of the massive early nebula, must cope with the high
temperatures. Enriching Jupiter in the noble gases through delivery trapped in
clathrate hydrates will be more difficult, but might be achieved by either
forming the planet much further from the star, or capturing planetesimals at
later epochs. The hot gap resulting from an early origin for Jupiter also would
affect the surface compositions of any primordial Trojan asteroids.Comment: 25 pages, 10 figures. ApJ in press. Discussion of Jupiter's volatile
enrichment revised in sec. 4.
The onset of X-ray emission in young stellar objects: a Chandra observation of the Serpens star-forming region
AIMS: To study the properties of X-ray emissions from young stellar objects
(YSOs), through their evolution from Class I to Class III and determine whether
Class 0 protostars emit in X-rays. METHODS: A deep Chandra X-ray observation of
the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally
suited for this type of investigation, being populated by a dense and extremely
young cluster whose members are found in all different evolutionary stages,
including six well studied Class 0 sources. RESULTS: None of the six Class 0
protostars is detected in our observations, excluding the presence of sources
with X-ray luminosities > 0.4 10^30 erg/s (for column densities of the order of
4 10^{23} cm^-2, or A_V ~ 200). A total of 85 X-ray sources are detected and
the light curves and spectra of 35 YSOs are derived. There is a clear trend of
decreasing absorbing column densities as one moves from Class I to Class III
sources, and, possibly, evidence of decreasing plasma temperatures, too. We
observe a strong, long-duration, flare from a Class II low-mass star, for which
we derive a flaring loop length of the order of 20 stellar radii. We interpret
the flaring event as originating from a magnetic flux tube connecting the star
to its circumstellar disk. The presence of such a disk is supported by the
detection, in the spectrum of this star, of 6.4 keV Fe fluorescent emission.Comment: Accepted for publication in A&
A survey for Fe 6.4 keV emission in young stellar objects in rho Oph: the strong fluorescence from Elias 29
We report the results of a search for 6.4 keV Fe fluorescent emission in
Young Stellar Objects (YSOs) with measured accretion luminosities in the rho
Oph cloud, using all existing chandra and XMM-Newton observations of the
region. A total of nine such YSOs have X-ray data with sufficiently high S/N
for the 6.4 keV line to be potentially detected if present. A positive
detection of the Fe 6.4 keV line is reported for one object, Elias 29, in both
the XMM-Newton and the chandra data. The 6.4 keV line is detected in Elias 29
both during quiescent and flaring emission, unlikely all previously reported
detections of 6.4 keV Fe fluorescence in YSOs which were made during intense
flaring. The observed equivalent width of the fluorescent line is large, at
W_alpha approx 140 eV, ruling out fluorescence from diffuse circumstellar
material. It is also larger than expected for simple reflection from a
solar-composition photosphere or circumstellar disk, but it is compatible with
being due to fluorescence from a centrally illuminated circumstellar disk. The
X-ray spectrum of Elias 29 is also peculiar in terms of its high (ionized) Fe
abundance, as evident from the very intense Fe xxv 6.7 keV line emission; we
speculate on the possible mechanism leading to the observed high abundance.Comment: Accepted by A&
High resolution spectroscopy of Ne II emission from young stellar objects
Constraining the spatial and thermal structure of the gaseous component of
circumstellar disks is crucial to understand star and planet formation. Models
predict that the [Ne II] line at 12.81 {\mu}m detected in young stellar objects
with Spitzer traces disk gas and its response to high energy radiation, but
such [Ne II] emission may also originate in shocks within powerful outflows. To
distinguish between these potential origins for mid-infrared [Ne II] emission
and to constrain disk models, we observed 32 young stellar objects using the
high resolution (R~30000) mid-infrared spectrograph VISIR at the VLT. We
detected the 12.81 {\mu}m [Ne II] line in 12 objects, tripling the number of
detections of this line in young stellar objects with high spatial and spectral
resolution spectrographs. We obtain the following main results: a) In Class I
objects the [Ne II] emission observed from Spitzer is mainly due to gas at a
distance of more than 20-40 AU from the star, where neon is, most likely,
ionized by shocks due to protostellar outflows. b) In transition and
pre-transition disks, most of the emission is confined to the inner disk,
within 20-40 AU from the central star. c) Detailed analysis of line profiles
indicates that, in transition and pre-transition disks, the line is slightly
blue-shifted (2-12 km s{^-1}) with respect to the stellar velocity, and the
line width is directly correlated with the disk inclination, as expected if the
emission is due to a disk wind. d) Models of EUV/X-ray irradiated disks
reproduce well the observed relation between the line width and the disk
inclination, but underestimate the blue-shift of the line.Comment: 35 pages, 7 figures, accepted for publication on Ap
Preliminary Orbit of the Young Binary Haro 1-14c
Using the Keck Interferometer, we spatially resolved the orbit of the
pre-main sequence binary, Haro 1-14c, for the first time. We present these
interferometric observations along with additional spectroscopic radial
velocity measurements of the components. We performed a simultaneous orbit fit
to the interferometric visibilities and the radial velocities of Haro 1-14c.
Based on a statistical analysis of the possible orbital solutions that fit the
data, we determined component masses of M_1 = 0.96 (+0.27/-0.08) Msun and M_2 =
0.33 (+0.09/-0.02) Msun for the primary and secondary, respectively, and a
distance to the system of 111 (+19/-18) pc. The distance measurement is
consistent with the close distance estimates of the Ophiuchus molecular cloud.
Comparing our results with evolutionary tracks suggests an age of 3-4 Myr for
Haro 1-14c. With additional interferometric measurements to improve the
uncertainties in the masses and distance, we expect the low-mass secondary to
provide important empirical data for calibrating the theoretical evolutionary
tracks for pre-main sequence stars.Comment: 28 pages, 9 figures, accepted for publication in A
Demonstration of a near-IR line-referenced electro-optical laser frequency comb for precision radial velocity measurements in astronomy
An important technique for discovering and characterizing planets beyond our solar system relies upon measurement of weak Doppler shifts in the spectra of host stars induced by the influence of orbiting planets. A recent advance has been the introduction of optical frequency combs as frequency references. Frequency combs produce a series of equally spaced reference frequencies and they offer extreme accuracy and spectral grasp that can potentially revolutionize exoplanet detection. Here we demonstrate a laser frequency comb using an alternate comb generation method based on electro-optical modulation, with the comb centre wavelength stabilized to a molecular or atomic reference. In contrast to mode-locked combs, the line spacing is readily resolvable using typical astronomical grating spectrographs. Built using commercial off-the-shelf components, the instrument is relatively simple and reliable. Proof of concept experiments operated at near-infrared wavelengths were carried out at the NASA Infrared Telescope Facility and the Keck-II telescope
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