1,236 research outputs found
A library of near-infrared integral field spectra of young M-L dwarfs
We present a library of near-infrared (1.1-2.45 microns) medium-resolution
(R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of
companions with known ages and of isolated objects. We use it to (re)derive the
NIR spectral types, luminosities and physical parameters of the targets, and to
test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral
types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and
M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B,
DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX
models yield close Teff and log g estimates for each sources. The models seem
to evidence a 600-300+600 K drop of the effective temperature at the M-L
transition. Assuming the former temperatures are correct, we derive new mass
estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4,
OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range.
We combine the empirical luminosities of the M9.5-L0 sources to the Teff to
derive semi-empirical radii estimates that do not match "hot-start"
evolutionary models predictions at 1-3 Myr. We use complementary data to
demonstrate that atmospheric models are able to reproduce the combined optical
and infrared spectral energy distribution, together with the near-infrared
spectra of these sources simultaneously. But the models still fail to represent
the dominant features in the optical. This issue casts doubts on the ability of
these models to predict correct effective temperatures from near-infrared
spectra alone. We advocate the use of photometric and spectroscopic data
covering a broad range of wavelengths to study the properties of very low mass
young companions to be detected with the planet imagers (Subaru/SCExAO,
LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).Comment: 27 pages, 14 tables, 19 figures, accepted for publication in
Astronomy & Astrophysic
High resolution imaging of young M-type stars of the solar neighborhood: Probing the existence of companions down to the mass of Jupiter
Context. High contrast imaging is a powerful technique to search for gas
giant planets and brown dwarfs orbiting at separation larger than several AU.
Around solar-type stars, giant planets are expected to form by core accretion
or by gravitational instability, but since core accretion is increasingly
difficult as the primary star becomes lighter, gravitational instability would
be the a probable formation scenario for yet-to-be-found distant giant planets
around a low-mass star. A systematic survey for such planets around M dwarfs
would therefore provide a direct test of the efficiency of gravitational
instability. Aims. We search for gas giant planets orbiting around late-type
stars and brown dwarfs of the solar neighborhood. Methods. We obtained deep
high resolution images of 16 targets with the adaptive optic system of VLT-NACO
in the Lp band, using direct imaging and angular differential imaging. This is
currently the largest and deepest survey for Jupiter-mass planets around
Mdwarfs. We developed and used an integrated reduction and analysis pipeline to
reduce the images and derive our 2D detection limits for each target. The
typical contrast achieved is about 9 magnitudes at 0.5" and 11 magnitudes
beyond 1". For each target we also determine the probability of detecting a
planet of a given mass at a given separation in our images. Results. We derived
accurate detection probabilities for planetary companions, taking into account
orbital projection effects, with in average more than 50% probability to detect
a 3MJup companion at 10AU and a 1.5MJup companion at 20AU, bringing strong
constraints on the existence of Jupiter-mass planets around this sample of
young M-dwarfs.Comment: Accepted for publication in A&
Search for cool giant exoplanets around young and nearby stars - VLT/NaCo near-infrared phase-coronagraphic and differential imaging
[Abridged] Context. Spectral differential imaging (SDI) is part of the
observing strategy of current and future high-contrast imaging instruments. It
aims to reduce the stellar speckles that prevent the detection of cool planets
by using in/out methane-band images. It attenuates the signature of off-axis
companions to the star, such as angular differential imaging (ADI). However,
this attenuation depends on the spectral properties of the low-mass companions
we are searching for. The implications of this particularity on estimating the
detection limits have been poorly explored so far. Aims. We perform an imaging
survey to search for cool (Teff<1000-1300 K) giant planets at separations as
close as 5-10 AU. We also aim to assess the sensitivity limits in SDI data
taking the photometric bias into account. This will lead to a better view of
the SDI performance. Methods. We observed a selected sample of 16 stars (age <
200 Myr, d < 25 pc) with the phase-mask coronagraph, SDI, and ADI modes of
VLT/NaCo. Results. We do not detect any companions. As for the sensitivity
limits, we argue that the SDI residual noise cannot be converted into mass
limits because it represents a differential flux, unlike the case of
single-band images. This results in degeneracies for the mass limits, which may
be removed with the use of single-band constraints. We instead employ a method
of directly determining the mass limits. The survey is sensitive to cool giant
planets beyond 10 AU for 65% and 30 AU for 100% of the sample. Conclusions. For
close-in separations, the optimal regime for SDI corresponds to SDI flux ratios
>2. According to the BT-Settl model, this translates into Teff<800 K. The
methods described here can be applied to the data interpretation of SPHERE. We
expect better performance with the dual-band imager IRDIS, thanks to more
suitable filter characteristics and better image quality.Comment: 19 pages, 16 figures, accepted for publication in A&A, version
including language editin
Discovery of a Low-Mass Companion to the F7V star HD 984
We report the discovery of a low-mass companion to the nearby (d = 47 pc) F7V
star HD 984. The companion is detected 0.19" away from its host star in the L'
band with the Apodizing Phase Plate on NaCo/VLT and was recovered by L'-band
non-coronagraphic imaging data taken a few days later. We confirm the companion
is co-moving with the star with SINFONI integral field spectrograph H+K data.
We present the first published data obtained with SINFONI in pupil-tracking
mode. HD 984 has been argued to be a kinematic member of the 30 Myr-old Columba
group, and its HR diagram position is not altogether inconsistent with being a
ZAMS star of this age. By consolidating different age indicators, including
isochronal age, coronal X-ray emission, and stellar rotation, we independently
estimate a main sequence age of 11585 Myr (95% CL) which does not rely on
this kinematic association. The mass of directly imaged companions are usually
inferred from theoretical evolutionary tracks, which are highly dependent on
the age of the star. Based on the age extrema, we demonstrate that with our
photometric data alone, the companion's mass is highly uncertain: between 33
and 96 M (0.03-0.09 M) using the COND evolutionary
models. We compare the companion's SINFONI spectrum with field dwarf spectra to
break this degeneracy. Based on the slope and shape of the spectrum in the
H-band, we conclude that the companion is an M dwarf. The age of the
system is not further constrained by the companion, as M dwarfs are poorly fit
on low-mass evolutionary tracks. This discovery emphasizes the importance of
obtaining a spectrum to spectral type companions around F-stars.Comment: Accepted for publication in MNRAS, 10 pages, 5 figure
Confirmation of the planet around HD 95086 by direct imaging
VLT/NaCo angular differential imaging at L' (3.8 microns) revealed a probable
giant planet comoving with the young and early-type HD 95086 also known to
harbor an extended debris disk. The discovery was based on the proper motion
analysis of two datasets spanning 15 months. However, the second dataset
suffered from bad atmospheric conditions, which limited the significance of the
redetection at the 3 sigma level. In this Letter, we report new VLT/NaCo
observations of HD 95086 obtained on 2013 June 26-27 at L' to recover the
planet candidate. We unambiguously redetect the companion HD 95086 b with
multiple independent pipelines at a signal-to-noise ratio greater than or equal
to 5. Combined with previously reported measurements, our astrometry decisively
shows that the planet is comoving with HD 95086 and inconsistent with a
background object. With a revised mass of 5 pm 2 Jupiter masses, estimated from
its L' photometry and "hot-start" models at 17 pm 4 Myr, HD 95086 b becomes a
new benchmark for further physical and orbital characterization of young giant
planets.Comment: accepted for publication to AP
Discovery of a probable 4-5 Jupiter-mass exoplanet to HD 95086 by direct-imaging
Direct imaging has just started the inventory of the population of gas giant
planets on wide-orbits around young stars in the solar neighborhood. Following
this approach, we carried out a deep imaging survey in the near-infrared using
VLT/NaCo to search for substellar companions. We report here the discovery in
L' (3.8 microns) images of a probable companion orbiting at 56 AU the young
(10-17 Myr), dusty, and early-type (A8) star HD 95086. This discovery is based
on observations with more than a year-time-lapse. Our first epoch clearly
revealed the source at 10 sigma while our second epoch lacked good observing
conditions hence yielding a 3 sigma detection. Various tests were thus made to
rule out possible artifacts. This recovery is consistent with the signal at the
first epoch but requires cleaner confirmation. Nevertheless, our astrometric
precision suggests the companion to be comoving with the star, with a 3 sigma
confidence level. The planetary nature of the source is reinforced by a
non-detection in Ks-band (2.18 microns) images according to its possible
extremely red Ks - L' color. Conversely, background contamination is rejected
with good confidence level. The luminosity yields a predicted mass of about
4-5MJup (at 10-17 Myr) using "hot-start" evolutionary models, making HD 95086 b
the exoplanet with the lowest mass ever imaged around a star.Comment: accepted for publication to APJ
OTS44: Disk and accretion at the planetary border
We discover that the very low-mass brown dwarf OTS44 (M9.5, ~12 M_Jup) has
significant accretion and a substantial disk, which demonstrates that the
processes that accompany canonical star formation occur down to a central mass
of a few Jupiter masses. We discover in VLT/SINFONI spectra that OTS44 has
strong, broad, and variable Paschen beta emission that is evidence for active
accretion at the planetary border. We also detect strong Halpha emission of
OTS44 in a literature spectrum and determine an Halpha EW (-141 A) that is
indicative of active accretion. Both the Pa beta and Halpha emission lines have
broad profiles with wings extending to velocities of about +/-200 km/s. We
determine the mass accretion rate of OTS44 based on Halpha to 7.6x10^{-12}
Msun/yr, which shows that OTS44 has a relatively high mass-accretion rate
considering its small central mass. This mass rate is nevertheless consistent
with the general decreasing trend found for stars of several solar masses down
to brown dwarfs. Furthermore, we determine the properties of the disk
surrounding OTS44 through radiative transfer modeling of flux measurement from
the optical to the far-IR (Herschel) by applying a Bayesian analysis. We find
that OTS44 has a highly flared disk (beta >1.2) with a mass of 9.1x10^{-5}
M_Sun, i.e. about 0.1 M_Jup or 30 M_Earth. We show that the ratio of
disk-to-central-mass of about 10^{-2} found for objects between 0.03 Msun and
14 Msun is also valid for OTS44 at a mass of ~0.01 M_Sun. Our observations are
in line with an isolated star-like mode of the formation of brown dwarfs down
to 0.01 M_Sun.Comment: Accepted for publication in Astronomy and Astrophysics Lette
Deep imaging survey of young, nearby austral stars: VLT/NACO near-infrared Lyot-coronographic observations
Context. High contrast and high angular resolution imaging is the optimal search technique for substellar companions to nearby stars at physical separations larger than typically 10 AU. Two distinct populations of substellar companions, brown dwarfs and planets, can be probed and characterized. As a result, fossile traces of processes of formation and evolution can be revealed by physical and orbital properties, both for individual systems and as an ensemble.
Aims. Since November 2002, we have conducted a large, deep imaging, survey of young, nearby associations of the southern hemisphere. Our goal is detection and characterization of substellar companions with projected separations in the range 10–500 AU. We have observed a sample of 88 stars, primarily G to M dwarfs, younger than 100 Myr, and within 100 pc of Earth.
Methods. The VLT/NACO adaptive optics instrument of the ESO Paranal Observatory was used to explore the faint circumstellar environment between typically 0.1 and 10". Diffraction-limited observations in H and K_s-band combined with Lyot-coronagraphy enabled us to reach primary star-companion brightness ratios as small as 10^(-6). The existence of planetary mass companions could therefore be probed. We used a standardized observing sequence to precisely measure the position and flux of all detected sources relative to their visual primary star. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects.
Results. We report the discovery of 17 new close (0.1–5.0") multiple systems. HIP 108195 AB and C (F1 III-M6), HIP 84642 AB (a~14 AU, K0-M5) and TWA22 AB (a~1.8 AU; M6-M6) are confirmed comoving systems. TWA22 AB is likely to be a rare astrometric calibrator that can be used to test evolutionary model predictions. Among our complete sample, a total of 65 targets were observed with deep coronagraphic imaging. About 240 faint companion candidates were detected around 36 stars. Follow-up observations with VLT or HST for 83% of these stars enabled us to identify a large fraction of background contaminants. Our latest results that pertain to the substellar companions to GSC 08047-00232, AB Pic and 2M1207 (confirmed during this survey and published earlier), are reviewed. Finally, a statistical analysis of our complete set of coronagraphic detection limits enables us to place constraints on the physical and orbital properties of giant planets between typically 20 and 150 AU
Deep search for companions to probable young brown dwarfs
We have obtained high contrast images of four nearby, faint, and very low
mass objects 2MASSJ04351455-1414468, SDSSJ044337.61+000205.1,
2MASSJ06085283-2753583 and 2MASSJ06524851-5741376 (here after 2MASS0435-14,
SDSS0443+00, 2MASS0608-27 and 2MASS0652-57), identified in the field as
probable isolated young brown dwarfs. Our goal was to search for binary
companions down to the planetary mass regime. We used the NAOS-CONICA adaptive
optics instrument (NACO) and its unique capability to sense the wavefront in
the near-infrared to acquire sharp images of the four systems in Ks, with a
field of view of 28"*28". Additional J and L' imaging and follow-up
observations at a second epoch were obtained for 2MASS0652-57. With a typical
contrast DKs= 4.0-7.0 mag, our observations are sensitive down to the planetary
mass regime considering a minimum age of 10 to 120 Myr for these systems. No
additional point sources are detected in the environment of 2MASS0435-14,
SDSS0443+00 and 2MASS0608-27 between 0.1-12" (i.e about 2 to 250 AU at 20 pc).
2MASS0652-57 is resolved as a \sim230 mas binary. Follow-up observations reject
a background contaminate, resolve the orbital motion of the pair, and confirm
with high confidence that the system is physically bound. The J, Ks and L'
photometry suggest a q\sim0.7-0.8 mass ratio binary with a probable semi-major
axis of 5-6 AU. Among the four systems, 2MASS0652-57 is probably the less
constrained in terms of age determination. Further analysis would be necessary
to confirm its youth. It would then be interesting to determine its orbital and
physical properties to derive the system's dynamical mass and to test
evolutionary model predictions.Comment: Research note, 5 pages, 2 tables and 3 figures, accepted to A&
The 2008 outburst in the young stellar system ZCMa: I. Evidence of an enhanced bipolar wind on the AU-scale
Accretion is a fundamental process in star formation. Although the time
evolution of accretion remains a matter of debate, observations and modelling
studies suggest that episodic outbursts of strong accretion may dominate the
formation of the protostar. Observing young stellar objects during these
elevated accretion states is crucial to understanding the origin of unsteady
accretion. ZCMa is a pre-main-sequence binary system composed of an embedded
Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. The
Herbig Be component recently underwent its largest optical photometric outburst
detected so far. We aim to constrain the origin of this outburst by studying
the emission region of the HI Brackett gamma line, a powerful tracer of
accretion/ejection processes on the AU-scale in young stars. Using the
AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed
spatially and spectrally resolved interferometric observations of the hot gas
emitting across the Brackett gamma emission line, during and after the
outburst. From the visibilities and differential phases, we derive
characteristic sizes for the Brackett gamma emission and spectro-astrometric
measurements across the line, with respect to the continuum. We find that the
line profile, the astrometric signal, and the visibilities are inconsistent
with the signature of either a Keplerian disk or infall of matter. They are,
instead, evidence of a bipolar wind, maybe partly seen through a disk hole
inside the dust sublimation radius. The disappearance of the Brackett gamma
emission line after the outburst suggests that the outburst is related to a
period of strong mass loss rather than a change of the extinction along the
line of sight. Based on these conclusions, we speculate that the origin of the
outburst is an event of enhanced mass accretion, similar to those occuring in
EX Ors and FU Ors.Comment: Accepted for publication in Astronomy and Astrophysics Letter
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