34 research outputs found
Do stellar magnetic cycles influence the measurement of precise radial velocities?
The ever increasing level of precision achieved by present and future
radial-velocity instruments is opening the way to discovering very low-mass,
long-period planets (e.g. solar-system analogs). These systems will be
detectable as low-amplitude signals in radial-velocity (RV). However, an
important obstacle to their detection may be the existence of stellar magnetic
cycles on similar timescales. Here we present the results of a long-term
program to simultaneously measure radial-velocities and stellar-activity
indicators (CaII, H_alpha, HeI) for a sample of stars with known activity
cycles. Our results suggest that all these stellar activity indexes can be used
to trace the stellar magnetic cycle in solar-type stars. Likewise, we find
clear indications that different parameters of the HARPS cross-correlation
function (BIS, FWHM, and contrast) are also sensitive to activity level
variations. Finally, we show that, although in a few cases slight correlations
or anti-correlations between radial-velocity and the activity level of the star
exist, their origin is still not clear. We can, however, conclude that for our
targets (early-K dwarfs) we do not find evidence of any radial-velocity
variations induced by variations of the stellar magnetic cycle with amplitudes
significantly above ~1 m/s.Comment: Accepted for publication in A&A (revised version following minor
language corrections
Reflected Light from Sand Grains in the Terrestrial Zone of a Protoplanetary Disk
We show that grains have grown to ~mm size (sand sized) or larger in the
terrestrial zone (within ~3 AU) of the protoplanetary disk surrounding the 3
Myr old binary star KH 15D. We also argue that the reflected light in the
system reaches us by back scattering off the far side of the same ring whose
near side causes the obscuration.Comment: 22 pages, 5 figures. To be published in Nature, March 13, 2008.
Contains a Supplemen
Evolution in the Cluster Early-type Galaxy Size-Surface Brightness Relation at z =~ 1
We investigate the evolution in the distribution of surface brightness, as a
function of size, for elliptical and S0 galaxies in the two clusters RDCS
J1252.9-2927, z=1.237 and RX J0152.7-1357, z=0.837. We use multi-color imaging
with the Advanced Camera for Surveys on the Hubble Space Telescope to determine
these sizes and surface brightnesses. Using three different estimates of the
surface brightnesses, we find that we reliably estimate the surface brightness
for the galaxies in our sample with a scatter of < 0.2 mag and with systematic
shifts of \lesssim 0.05 mag. We construct samples of galaxies with early-type
morphologies in both clusters. For each cluster, we use a magnitude limit in a
band which closely corresponds to the rest-frame B, to magnitude limit of M_B =
-18.8 at z=0, and select only those galaxies within the color-magnitude
sequence of the cluster or by using our spectroscopic redshifts. We measure
evolution in the rest-frame B surface brightness, and find -1.41 \+/- 0.14 mag
from the Coma cluster of galaxies for RDCS J1252.9-2927 and -0.90 \+/- 0.12 mag
of evolution for RX J0152.7-1357, or an average evolution of (-1.13 \+/- 0.15)
z mag. Our statistical errors are dominated by the observed scatter in the
size-surface brightness relation, sigma = 0.42 \+/- 0.05 mag for RX
J0152.7-1357 and sigma = 0.76 \+/- 0.10 mag for RDCS J1252.9-2927. We find no
statistically significant evolution in this scatter, though an increase in the
scatter could be expected. Overall, the pace of luminosity evolution we measure
agrees with that of the Fundamental Plane of early-type galaxies, implying that
the majority of massive early-type galaxies observed at z =~ 1 formed at high
redshifts.Comment: Accepted in ApJ, 16 pages in emulateapj format with 15 eps figures, 6
in colo
Measuring the Initial Mass Function of Low Mass Stars and Brown Dwarfs
I review efforts to determine the form and any lower limit to the initial
mass function in the Galactic disk, using observations of low-mass stars and
brown dwarfs in the field, young clusters and star forming regions. I focus on
the methodologies that have been used and the uncertainties that exist due to
observational limitations and to systematic uncertainties in calibrations and
theoretical models. I conclude that whilst it is possible that the low-mass
IMFs deduced from the field and most young clusters are similar, there are too
many problems to be sure; there are examples of low-mass cluster IMFs that
appear to be very discrepant and the IMFs for brown dwarfs in the field and
young clusters have yet to be reconciled convincingly.Comment: From a series of lectures presented at the Evry-Schatzman school on
Low-mass stars and the transition from stars to brown dwarfs, edited by C.
Charbonnel, C. Reyle, M. Schultheis. To appear in the EAS Conference Series.
47p
Further deep imaging of HR 7329 A (eta Tel A) and its brown dwarf companion B
About 4" south of the young A0-type star HR 7329, a faint companion candidate
was found by Lowrance et al. (2000). Its spectral type of M7-8 is consistent
with a young brown dwarf companion. Here, we report ten new astrometric imaging
observations of the pair HR 7329 A and B, obtained with the Hubble Space
Telescope and the Very Large Telescope, aimed at showing common proper motion
with high significance and possible orbital motion of B around A. With 11 yrs
of epoch difference between the first and our last image, we can reject by more
than 21 sigma that B would be a non-moving background object unrelated to A. We
detect no change in position angle and small or no change in separation (2.91
+/- 2.41 mas/yr), so that the orbit of HR 7329 B around A is inclined and/or
eccentric and/or the orbital motion is currently only in radial direction. If
HR 7329 B is responsible for the outer radius of the debris disk around HR 7329
A being 24 AU, and if HR 7329 B currently is at its apastron at 200 AU (4.2" at
47.7 pc), we determine its pericenter distance to be 71 AU, its semi-major axis
to be 136 AU, and its eccentricity to be e=0.47. From the magnitude differences
between HR 7329 A and B and the 2MASS magnitudes for the HR 7329 A+B system, we
can estimate the magnitudes of HR 7329 B (J=12.06+/-0.19, H=11.75+/-0.10,
Ks=11.6+/-0.1, L=11.1+/-0.2 mag) and then, with a few otherwise known
parameters, its luminosity and mass (20-50 Jupiter masses). In the deepest
images available, we did not detect any additional companion candidates up to
<=9", but determine upper limits in the planetary mass regime.Comment: 7 pages, 3 figures, accepted for publication in MNRA
Debris Disks: Probing Planet Formation
Debris disks are the dust disks found around ~20% of nearby main sequence
stars in far-IR surveys. They can be considered as descendants of
protoplanetary disks or components of planetary systems, providing valuable
information on circumstellar disk evolution and the outcome of planet
formation. The debris disk population can be explained by the steady
collisional erosion of planetesimal belts; population models constrain where
(10-100au) and in what quantity (>1Mearth) planetesimals (>10km in size)
typically form in protoplanetary disks. Gas is now seen long into the debris
disk phase. Some of this is secondary implying planetesimals have a Solar
System comet-like composition, but some systems may retain primordial gas.
Ongoing planet formation processes are invoked for some debris disks, such as
the continued growth of dwarf planets in an unstirred disk, or the growth of
terrestrial planets through giant impacts. Planets imprint structure on debris
disks in many ways; images of gaps, clumps, warps, eccentricities and other
disk asymmetries, are readily explained by planets at >>5au. Hot dust in the
region planets are commonly found (<5au) is seen for a growing number of stars.
This dust usually originates in an outer belt (e.g., from exocomets), although
an asteroid belt or recent collision is sometimes inferred.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018
In-depth study of moderately young but extremely red, very dusty substellar companion HD206893B
Accepted for publication in Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.The substellar companion HD206893b has recently been discovered by direct imaging of its disc-bearing host star with the SPHERE instrument. We investigate the atypical properties of the companion, which has the reddest near-infrared colours among all known substellar objects, either orbiting a star or isolated, and we provide a comprehensive characterisation of the host star-disc-companion system. We conducted a follow-up of the companion with adaptive optics imaging and spectro-imaging with SPHERE, and a multiinstrument follow-up of its host star. We obtain a R=30 spectrum from 0.95 to 1.64 micron of the companion and additional photometry at 2.11 and 2.25 micron. We carried out extensive atmosphere model fitting for the companions and the host star in order to derive their age, mass, and metallicity. We found no additional companion in the system in spite of exquisite observing conditions resulting in sensitivity to 6MJup (2MJup) at 0.5" for an age of 300 Myr (50 Myr). We detect orbital motion over more than one year and characterise the possible Keplerian orbits. We constrain the age of the system to a minimum of 50 Myr and a maximum of 700 Myr, and determine that the host-star metallicity is nearly solar. The comparison of the companion spectrum and photometry to model atmospheres indicates that the companion is an extremely dusty late L dwarf, with an intermediate gravity (log g 4.5-5.0) which is compatible with the independent age estimate of the system. Though our best fit corresponds to a brown dwarf of 15-30 MJup aged 100-300 Myr, our analysis is also compatible with a range of masses and ages going from a 50 Myr 12MJup planetary-mass object to a 50 MJup Hyades-age brown dwarf...Peer reviewedFinal Accepted Versio
The Classification of T Dwarfs
We discuss methods for classifying T dwarfs based on spectral morphological
features and indices. T dwarfs are brown dwarfs which exhibit methane
absorption bands at 1.6 and 2.2 . Spectra at red optical (6300--10100
{\AA}) and near-infrared (1--2.5 ) wavelengths are presented, and
differences between objects are noted and discussed. Spectral indices useful
for classification schemes are presented. We conclude that near-infrared
spectral classification is generally preferable for these cool objects, with
data sufficient to resolve the 1.17 and 1.25 K I doublets lines being
most valuable. Spectral features sensitive to gravity are discussed, with the
strength of the K-band peak used as an example. Such features may be used to
derive a two-dimensional scheme based on temperature and mass, in analogy to
the MK temperature and luminosity classes.Comment: 15 pages, 6 figures, conference proceedings for IAU Ultracool Dwarf
Stars session, ed. I. Steele & H. Jone
Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers
We present a review of the interplay between the evolution of circumstellar
disks and the formation of planets, both from the perspective of theoretical
models and dedicated observations. Based on this, we identify and discuss
fundamental questions concerning the formation and evolution of circumstellar
disks and planets which can be addressed in the near future with optical and
infrared long-baseline interferometers. Furthermore, the importance of
complementary observations with long-baseline (sub)millimeter interferometers
and high-sensitivity infrared observatories is outlined.Comment: 83 pages; Accepted for publication in "Astronomy and Astrophysics
Review"; The final publication is available at http://www.springerlink.co
First Scattered-light Images of the Gas-rich Debris Disk around 49 Ceti
We present the first scattered-light images of the debris disk around 49 Ceti, a ∼40 Myr A1 main-sequence star at 59 pc, famous for hosting two massive dust belts as well as large quantities of atomic and molecular gas. The outer disk is revealed in reprocessed archival Hubble Space Telescope NICMOS-F110W images, as well as new coronagraphic H-band images from the Very Large Telescope SPHERE instrument. The disk extends from 1.″1 (65 au) to 4.″6 (250 au) and is seen at an inclination of 73°, which refines previous measurements at lower angular resolution. We also report no companion detection larger than 3 M Jup at projected separations beyond 20 au from the star (0.″34). Comparison between the F110W and H-band images is consistent with a gray color of 49 Ceti's dust, indicating grains larger than 2 μm. Our photometric measurements indicate a scattering efficiency/infrared excess ratio of 0.2-0.4, relatively low compared to other characterized debris disks. We find that 49 Ceti presents morphological and scattering properties very similar to the gas-rich HD 131835 system. From our constraint on the disk inclination we find that the atomic gas previously detected in absorption must extend to the inner disk, and that the latter must be depleted of CO gas. Building on previous studies, we propose a schematic view of the system describing the dust and gas structure around 49 Ceti and hypothetical scenarios for the gas nature and origin.E.C. acknowledges support from NASA through Hubble Fellowship grant HST-HF2-51355 awarded by STScI, operated by AURA, Inc. under contract NAS5-26555, and support from HST-AR-12652, for research carried out at the Jet Propulsion Laboratory, California Institute of Technology. J.M. acknowledges ESO through the ESO fellowship program. M.B. acknowledges support from DFG project Kr 2164/15-1. G.M.K. is supported by the Royal Society as a Royal Society University Research Fellow. C.d.B. is supported by Mexican CONACyT research grant CB-2012-183007. L.M. acknowledges support by STFC through a graduate studentship. J.C.A. acknowledges support by the Programme National de Planétologie. We acknowledge support by the European Union through ERC grant 337569 for O.A. and C.A.G.G. and grant 279973 for M.W. and L.M