553 research outputs found
Metallicities of M Dwarf Planet Hosts from Spectral Synthesis
We present the first spectroscopic metallicities of three M dwarfs with known
or candidate planetary mass companions. We have analyzed high resolution, high
signal-to-noise spectra of these stars which we obtained at McDonald
Observatory. Our analysis technique is based on spectral synthesis of atomic
and molecular features using recently revised cool-star model atmospheres and
spectrum synthesis code. The technique has been shown to yield results
consistent with the analyses of solar-type stars and allows measurements of M
dwarf [M/H] values to 0.12 dex precision. From our analysis, we find [M/H] =
-0.12, -0.32, and -0.33 for GJ 876, GJ 436, and GJ 581 respectively. These
three M dwarf planet hosts have sub-solar metallicities, a surprising departure
from the trend observed in FGK-type stars. This study is the first part of our
ongoing work to determine the metallicities of the M dwarfs included in the
McDonald Observatory planet search program.Comment: 13 pages, 2 figures, accepted for publication in ApJ
The 2MASS Wide-Field T Dwarf Search. IV Unting out T dwarfs with Methane Imaging
We present first results from a major program of methane filter photometry
for low-mass stars and brown dwarfs. The definition of a new methane filter
photometric system is described. A recipe is provided for the differential
calibration of methane imaging data using existing 2MASS photometry. We show
that these filters are effective in discriminating T dwarfs from other types of
stars, and demonstrate this with Anglo-Australian Telescope observations using
the IRIS2 imager. Methane imaging data and proper motions are presented for ten
T dwarfs identified as part of the 2MASS "Wide Field T Dwarf Search" -- seven
of them initially identified as T dwarfs using methane imaging.
We also present near-infrared moderate resolution spectra for five T dwarfs,
newly discovered by this technique. Spectral types obtained from these spectra
are compared to those derived from both our methane filter observations, and
spectral types derived by other observers. Finally, we suggest a range of
future programs to which these filters are clearly well suited: the winnowing
of T dwarf and Y dwarf candidate objects coming from the next generation of
near-infrared sky surveys; the robust detection of candidate planetary-mass
brown dwarfs in clusters; the detection of T dwarf companions to known L and T
dwarfs via deep methane imaging; and the search for rotationally-modulated
time-variable surface features on cool brown dwarfs.Comment: 20 pages. To appear in The Astronomical Journal, Nov. 200
LP 349-25: a new tight M8V binary
We present the discovery of a tight M8V binary, with a separation of only 1.2
astronomical units, obtained with the PUEO and NACO adaptive optics systems,
respectively at the CFHT and VLT telescopes. The estimated period of LP 349-25
is approximately 5 years, and this makes it an excellent candidate for a
precise mass measurement.Comment: Accepted by Astronomy and Astrophysics Letter
A Flaring L5 Dwarf: The Nature of H\alpha Emission in Very Low Mass (Sub)Stellar Objects
Time series spectrophotometry of the L5 dwarf 2MASS 01443536-0716142 showed
strong H\alpha emission which declined by nearly 75% in four consecutive
exposures. The line was not detected in emission on a spectrum obtained eleven
months later. This behavior constrasts with that of 2MASSI J1315309-264951, an
L5 dwarf which has shown even stronger H\alpha emission on four separate
occasions. The observational database suggests that L dwarfs can be found in
such strong flares only occasionally, with a duty cycle of order 1%. In
contrast, the few, continuously-strong H\alpha emitters, including PC 0025+0447
and 2MASSI J1237392+652615, must either be (1) objects no older than 10-100
Myrs with continuously-active accretion and/or chromospheres, but which
apparently formed in isolation from known young stellar clusters and
associations, or (2) objects empowered by a different and unknown mechanism for
the H\alpha energy.Comment: 11 pages, 2 figures, Astronomical Journal in press -- Jan 2003 issu
Infrared Spectra and Spectral Energy Distributions of Late-M- and L-Dwarfs
We have obtained 1.0-2.5um spectra at R~600 of 14 disk dwarfs with spectral
types M6 to L7. For four of the dwarfs we have also obtained infrared spectra
at R~3000 in narrow intervals. In addition, we present new L' photometry for
four of the dwarfs in the sample, which allows improved determinations of their
bolometric luminosities. We resolve the L-dwarf Denis-P J 0205-1159 into an
identical pair of objects separated by 0.35". The spectra, with the published
energy distribution for one other dwarf, are compared to synthetic spectra
generated by upgraded model atmospheres. Good matches are found for 2200> Teff
K>1900 (spectral types around M9 to L3), but discrepancies exist at Teff> 2300
K (M8) and for Teff<1800 K (L4-L7). At the higher temperatures the mismatches
are due to incompleteness in the water vapor linelist. At the lower
temperatures the disagreement is probably due to our treatment of dust: we
assume a photospheric distribution in equilibrium with the gas phase. We derive
effective temperatures for the sample from the comparison with synthetic
spectra and also by comparing our observed total intrinsic luminosities to
structural model calculations (which are mostly independent of the atmosphere
but are dependent on the unknown masses and ages of the targets). The two
derivations agree to ~200 K except for the faintest object in the sample where
the discrepancy is larger. Agreement with other temperature determinations is
also ~200 K, except for the L7 dwarf.Comment: 31 pages incl. 5 Tables and 12 Figures, accepted by ApJ for Feb 2001
issu
The role of planets in shaping planetary nebulae
In 1997 Soker laid out a framework for understanding the formation and
shaping of planetary nebulae (PN). Starting from the assumption that
non-spherical PN cannot be formed by single stars, he linked PN morphologies to
the binary mechanisms that may have formed them, basing these connections
almost entirely on observational arguments. In light of the last decade of
discovery in the field of PN, we revise this framework, which, although
simplistic, can still serve as a benchmark against which to test theories of PN
origin and shaping. Within the framework, we revisit the role of planets in
shaping PN. Soker invoked a planetary role in shaping PN because there are not
enough close binaries to shape the large fraction of non-spherical PN. In this
paper we adopt a model whereby only ~20% of all 1-8 solar mass stars make a PN.
This reduces the need for planetary shaping. Through a propagation of
percentages argument, and starting from the assumption that planets can only
shape mildly elliptical PN, we conclude, like in Soker, that ~20% of all PN
were shaped via planetary and other substellar interactions but we add that
this corresponds to only ~5% of all 1-8 solar mass stars. This may be in line
with findings of planets around main sequence stars. PN shaping by planets is
made plausible by the recent discovery of planets that have survived
interactions with red giant branch (RGB) stars. Finally, we conclude that of
the ~80% of 1-8 solar mass stars that do not make a PN, about one quarter do
not even ascend the AGB due to interactions with stellar and substellar
companions, while three quarters ascend the AGB but do not make a PN. Once
these stars leave the AGB they evolve normally and can be confused with
post-RGB, extreme horizontal branch stars. We propose tests to identify them.Comment: 23 pages, accepted by PAS
Five new very low mass binaries
We report the discovery of companions to 5 nearby late M dwarfs (>M5),
LHS1901, LHS4009, LHS6167, LP869-26 and WT460, and we confirm that the recently
discovered mid-T brown dwarf companion to SCR1845-6357 is physically bound to
that star. These discoveries result from our adaptive optics survey of all M
dwarfs within 12 pc. The new companions have spectral types M5 to L1, and
orbital separations between 1 and 10 AU. They add significantly to the number
of late M dwarfs binaries in the immediate solar neighbourhood, and will
improve the multiplicity statistics of late M dwarfs. The expected periods
range from 3 to 130 years. Several pairs thus have good potential for accurate
mass determination in this poorly sampled mass range.Comment: 5 pages, 2 figures, submitted to Astronomy & Astrophysic
The M Dwarf GJ 436 and its Neptune-Mass Planet
We determine stellar parameters for the M dwarf GJ 436 that hosts a
Neptune-mass planet. We employ primarily spectral modeling at low and high
resolution, examining the agreement between model and observed optical spectra
of five comparison stars of type, M0-M3. Modeling high resolution optical
spectra suffers from uncertainties in TiO transitions, affecting the predicted
strengths of both atomic and molecular lines in M dwarfs. The determination of
Teff, gravity, and metallicity from optical spectra remains at ~10%. As
molecules provide opacity both in lines and as an effective continuum,
determing molecular transition parameters remains a challenge facing models
such as the PHOENIX series, best verified with high resolution and
spectrophotometric spectra. Our analysis of GJ 436 yields an effective
temperature of Teff = 3350 +/- 300 K and a mass of 0.44 Msun. New Doppler
measurements for GJ 436 with a precision of 3 m/s taken during 6 years improve
the Keplerian model of the planet, giving a minimum mass, M sin i = 0.0713 Mjup
= 22.6 Mearth, period, P = 2.6439 d, and e = 0.16 +/- 0.02. The noncircular
orbit contrasts with the tidally circularized orbits of all close-in
exoplanets, implying either ongoing pumping of eccentricity by a more distant
companion, or a higher Q value for this low-mass planet. The velocities indeed
reveal a long term trend, indicating a possible distant companion.Comment: 27 pages, 7 figures, accepted to PAS
Precision Masses of the low-mass binary system GJ 623
We have used Aperture Masking Interferometry and Adaptive Optics (AO) at the
Palomar 200'' to obtain precise mass measurements of the binary M dwarf GJ 623.
AO observations spread over 3 years combined with a decade of radial velocity
measurements constrain all orbital parameters of the GJ 623 binary system
accurately enough to critically challenge the models. The dynamical masses
measured are m_{1}=0.371\pm0.015 M_{\sun} (4%) and m_{2}=0.115\pm0.0023
M_{\sun} (2%) for the primary and the secondary respectively. Models are not
consistent with color and mass, requiring very low metallicities.Comment: 7 pages, 5 figures. Accepted for Ap
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