273 research outputs found
Orbital motion of the young brown dwarf companion TWA 5 B
With more adaptive optics images available, we aim at detecting orbital
motion for the first time in the system TWA 5 A+B. We measured separation and
position angle between TWA 5 A and B in each high-resolution image available
and followed their change in time, because B should orbit around A. The
astrometric measurement precision is about one milli arc sec. With ten year
difference in epoch, we can clearly detect orbital motion of B around A, a
decrease in separation by ~ 0.0054 arc sec per year and a decrease in position
angle by ~ 0.26 degrees per year. TWA 5 B is a brown dwarf with ~ 25 Jupiter
masses (Neuh\"auser et al. 2000), but having large error bars (4 to 145 Jupiter
masses, Neuh\"auser et al. 2009). Given its large projected separation from the
primary star, ~ 86 AU, and its young age ~ 10 Myrs), it has probably formed
star-like, and would then be a brown dwarf companion. Given the relatively
large changes in separation and position angle between TWA 5 A and B, we can
conclude that they orbit around each other on an eccentric orbit. Some evidence
is found for a curvature in the orbital motion of B around A - most consistent
with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are
detected and show a small-amplitude (~ 18 mas) periodic sinusoid with ~ 5.7 yr
period, i.e., fully consistent with the orbit of the inner close pair TWA 5
Aa+b. Measuring these residuals caused by the photocenter wobble - even in
unresolved images - can yield the total mass of the inner pair, so can test
theoretical pre-main sequence models.Comment: 6 pages, 4 figures, accepted for publication in A&A; corrected typo
in amplitude below Fig.
SSPMJ1102-3431 brown dwarf characterization from accurate proper motion and trigonometric parallax
In 2005, Scholz and collaborators (Scholz et al. 2005) discovered, in a
proper motion survey, a young brown dwarf SSSPMJ1102-3431(SSSPMJ1102) of
spectral type M8.5, probable member of the TW Hydrae Association (TWA) and
possible companion of the T Tauri star TW Hya. The physical characterization of
SSSPMJ1102 was based on the hypothesis that it forms a binary system with TW
Hya. The recent discovery of a probable giant planet inside the TW Hya
protoplanetary disk with a very short-period (Setiawan et al. 2008) and a disk
around SSSPMJ1102 (Riaz and Gizis 2008) make it especially interesting and
important to measure well the physical parameters of SSSPMJ1102. Trigonometric
parallax and proper motion measurements of SSSPMJ1102 are necessary to test for
TWA membership and, thus, to determine the mass and age of this young brown
dwarf and the possibility that it forms a wide binary system with TW Hya. Two
years of regular observations at the ESO NTT/SUSI2 telescope, have enabled us
to determine the trigonometric parallax and proper motion of SSSPMJ1102. Our
parallax and proper motion determination allow us to precisely describe the
physical properties of this low mass object and to confirm its TWA membership.
Our results are not incompatible with the hypothesis that SSSPMJ1102 is a
binary companion of the star TW Hya.Comment: accepted for publication in A&
M2000 : an astrometric catalog in the Bordeaux Carte du Ciel zone +11 degrees < {delta} < +18 degrees
During four years, systematic observations have been conducted in drift scan
mode with the Bordeaux automated meridian circle in the declination band [+11 ;
+18]. The resulting astrometric catalog includes about 2 300 000 stars down to
the magnitude limit V_M=16.3. Nearly all stars (96%) have been observed at
least 6 times, the catalog being complete down to V_M=15.4. The median internal
standard error in position is about 35 mas in the V_M magnitude range [11 ;
15], which degrades to about 50 mas when the faintest stars are considered.
M2000 provides also one band photometry with a median internal standard error
of 0.04 mag. Comparisons with the Hipparcos and bright part of Tycho-2 catalogs
have enabled to estimate external errors in position to be lower than 40 mas.
In this zone and at epoch 1998, the faint part of Tycho-2 is found to have an
accuracy of 116 mas in alpha instead of 82 mas deduced from the model-based
standard errors given in the catalog.Comment: The catalogue can be fetched directly from:
ftp://cdsarc.u-strasbg.fr/cats/I/272 or queried from:
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/272 More information at :
http://www.observ.u-bordeaux.fr/~soubiran/m2000.ht
Large Amplitude Photometric Variability of the Candidate Protoplanet TMR-1C
In their HST/NICMOS observations, Terebey et al. 1998 detected a candidate
protoplanet, TMR-1C, that lies at a separation of about 10" (~1000 AU) from the
Class I protobinary TMR-1 (IRAS 04361+2547). A narrow filament-like structure
was observed extending south-east from the central proto-binary system towards
TMR-1C, suggesting a morphology in which the candidate protoplanet may have
been ejected from the TMR-1 system. Follow-up low-resolution spectroscopy could
not confirm if this object is a protoplanet or a low-luminosity background
star. We present two epochs of near-infrared photometric observations obtained
at the CFHT of TMR-1C. The time span of ~7 years between the two sets of
observations provides with an opportunity to, (a) check for any photometric
variability similar to that observed among young stellar objects, which would
indicate the youth of this source, and, (b) determine the proper motion. TMR-1C
displays large photometric variability between 1 and 2 mag in both the H- and
Ks-bands. From our 2002 observations, we find a (H-Ks) color of 0.3 mag, which
is much bluer than the value of 1.3 mag reported by T98 from HST observations.
Also, we observe brightening in both the H- and Ks-bands when the colors are
bluer, i.e. the object gets redder as it becomes fainter. We have explored the
possible origins for the observed variability, and find extinction due to the
presence of circumstellar material to be the most likely scenario. The observed
large-amplitude photometric variations, and the possible presence of a
circumstellar disk, are strong arguments against this object being an old
background star.Comment: Accepted in A&
An accurate distance to 2M1207Ab
In April 2004 the first image was obtained of a planetary mass companion (now
known as 2M1207 b) in orbit around a self-luminous object different from our
own Sun (the young brown dwarf 2MASSW J1207334-393254, hereafter 2M1207 A).
2M1207 b probably formed via fragmentation and gravitational collapse, offering
proof that such a mechanism can form bodies in the planetary mass regime.
However, the predicted mass, luminosity, and radius of 2M1207 b depend on its
age, distance, and other observables such as effective temperature. To refine
our knowledge of the physical properties of 2M1207 b and its nature, we
obtained an accurate determination of the distance to the 2M1207 A and b system
by measurements of its trigonometric parallax at the milliarcsec level. With
the ESO NTT/SUSI2 telescope, in 2006 we began a campaign of photometric and
astrometric observations to measure the trigonometric parallax of 2M1207 A. An
accurate distance ( pc) to 2M1207A was measured. From distance and
proper motions we derived spatial velocities fully compatible with TWA
membership. With this new distance estimate, we discuss three scenarios
regarding the nature of 2M1207 b: (1) a cool ( K) companion of mass
M, (2) a warmer ( K) and heavier (
M) companion occulted by an edge-on circum-secondary disk or (3) a
hot protoplanet collision afterglow.Comment: 5 pages, 3 figures, accepted for publication as letter in A&A,
6/11/200
Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the Asteroid and Kuiper-Edgeworth belts in the
Solar system - have so far mostly been identified and studied in thermal
emission shortward of 100 um. The Herschel space observatory and the SCUBA-2
camera on the James Clerk Maxwell Telescope will allow efficient photometric
surveying at 70 to 850 um, which allow for the detection of cooler discs not
yet discovered, and the measurement of disc masses and temperatures when
combined with shorter wavelength photometry. The SCUBA-2 Unbiased Nearby Stars
(SUNS) survey and the DEBRIS Herschel Open Time Key Project are complimentary
legacy surveys observing samples of ~500 nearby stellar systems. To maximise
the legacy value of these surveys, great care has gone into the target
selection process. This paper describes the target selection process and
presents the target lists of these two surveys.Comment: 67 pages with full tables, 7 figures, accepted to MNRA
Astrometric confirmation of young low-mass binaries and multiple systems in the Chamaeleon star-forming regions
The star-forming regions in Chamaeleon are one of the nearest (distance ~165
pc) and youngest (age ~2 Myrs) conglomerates of recently formed stars and the
ideal target for population studies of star formation. We investigate a total
of 16 Cha targets, which have been suggested, but not confirmed as binaries or
multiple systems in previous literature. We used the adaptive optics instrument
Naos-Conica (NACO) at the Very Large Telescope Unit Telescope 4 of the Paranal
Observatory, at 2-5 different epochs, in order to obtain relative and absolute
astrometric measurements, as well as differential photometry in the J, H, and K
band. On the basis of known proper motions and these observations, we analyse
the astrometric results in our "Proper Motion Diagram" (PMD: angular separation
/ position angle versus time), to eliminate possible (non-moving) background
stars, establish co-moving binaries and multiples, and search for curvature as
indications for orbital motion. All previously suggested close components are
co-moving and no background stars are found. The angular separations range
between 0.07 and 9 arcseconds, corresponding to projected distances between the
components of 6-845 AU. Thirteen stars are at least binaries and the remaining
three (RX J0919.4-7738, RX J0952.7-7933, VW Cha) are confirmed high-order
multiple systems with up to four components. In 13 cases, we found significant
slopes in the PMDs, which are compatible with orbital motion whose periods
range from 60 to 550 years. However, in only four cases there are indications
of a curved orbit, the ultimate proof of a gravitational bond. Massive primary
components appear to avoid the simultaneous formation of equal-mass secondary
components. (abridged)Comment: 33 pages, 22 figures, accepted for publication in A&A, 2nd version:
typos and measurement unit added in Table
Confirmation of the binary status of Cha Halpha 2 - a very young low-mass binary in Chamaeleon
Neuhaeuser & Comeron (1998, 1999) presented direct imaging evidence, as well
as first spectra, of several young stellar and sub-stellar M6- to M8-type
objects in the Cha I dark cloud. One of these objects is Cha Halpha 2,
classified as brown dwarf candidate in several publications and suggested as
possible binary in Neuhaeuser et al. (2002). We have searched around Cha Halpha
2 for close and faint companions with adaptive optics imaging. Two epochs of
direct imaging data were taken with the Very Large Telescope (VLT) Adaptive
Optics instrument NACO in February 2006 and March 2007 in Ks-band. We retrieved
an earlier image from 2005 from the European Southern Observatory (ESO) Science
Archive Facility, increasing the available time coverage. After confirmation of
common proper motion, we deduce physical parameters of the objects by
spectroscopy, like temperature and mass. We find Cha Halpha 2 to be a very
close binary of ~0.16 arcsec separation, having a flux ratio of ~0.91, thus
having almost equal brightness and indistinguishable spectral types within the
errors. We show that the two tentative components of Cha Halpha 2 form a common
proper motion pair, and that neither component is a non-moving background
object. We even find evidence for orbital motion. A combined spectrum of both
stars spanning optical and near-infrared parts of the spectral energy
distribution yields a temperature of 3000+/-100 K, corresponding to a spectral
type of M6+/-1 and a surface gravity of log g= 4.0 +0.75-0.5, both from a
comparison with GAIA model atmospheres. We derive masses of ~0.110 Msun (>0.070
Msun) and ~0.124 Msun (>0.077 Msun) for the two components of Cha Halpha 2,
i.e., probably low-mass stars, but one component could possibly be a brown
dwarf.Comment: 6 pages, 8 figures, accepted for publication in A&
A new benchmark T8-9 brown dwarf and a couple of new mid-T dwarfs from the UKIDSS DR5+ LAS
Benchmark brown dwarfs are those objects for which fiducial constraints are
available, including effective temperature, parallax, age, metallicity. We
searched for new cool brown dwarfs in 186 sq.deg. of the new area covered by
the data release DR5+ of the UKIDSS Large Area Survey. Follow-up optical and
near-infrared broad-band photometry, and methane imaging of four promising
candidates, revealed three objects with distinct methane absorption, typical of
mid- to late-T dwarfs, and one possibly T4 dwarf. The latest-type object,
classified as T8-9, shares its large proper motion with Ross 458 (BD+13o2618),
an active M0.5 binary which is 102" away, forming a hierarchical low-mass
star+brown dwarf system. Ross 458C has an absolute J-band magnitude of 16.4,
and seems overluminous, particularly in the K band, compared to similar field
brown dwarfs. We estimate the age of the system to be less than 1 Gyr, and its
mass to be as low as 14 Jupiter masses for the age of 1 Gyr. At 11.4 pc, this
new late T benchmark dwarf is a promising target to constrain the evolutionary
and atmospheric models of very low-mass brown dwarfs. We present proper motion
measurements for our targets and for 13 known brown dwarfs. Two brown dwarfs
have velocities typical of the thick disk and may be old brown dwarfs.Comment: 15 pages, 10 figures and 6 tables. Accepted by MNRAS. Uses mn2e.cls
and aas_macr
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