231 research outputs found
Characterization of the known T type dwarfs towards the Sigma Orionis cluster
(Abridged) A total of three T type candidates (SOri70, SOri73, and
SOriJ0538-0213) lying in the line of sight towards Sigma Orionis were
characterized by means of near-infrared photometric, astrometric, and
spectroscopic studies. H-band methane images were collected for all three
sources and an additional sample of 15 field T type dwarfs using LIRIS/WHT.
J-band spectra of resolution of ~500 were obtained for SOriJ0538-0213 with
ISAAC/VLT, and JH spectra of resolution of ~50 acquired with WFC3/HST were
employed for the spectroscopic classification of SOri70 and 73. Proper motions
with a typical uncertainty of +/-3 mas/yr and a time interval of ~7-9 yr were
derived. Using the LIRIS observations of the field T dwarfs, we calibrated this
imager for T spectral typing via methane photometry. The three SOri objects
were spectroscopically classified as T4.5+/-0.5 (SOri73), T5+/-0.5
(SOriJ0538-0213), and T7 (SOri70). The similarity between the
observed JH spectra and the methane colors and the data of field ultra-cool
dwarfs of related classifications suggests that SOri70, 73, and
SOriJ053804.65-021352.5 do not deviate significantly in surface gravity in
relation to the field. Additionally, the detection of KI at ~1.25 microns in
SOriJ0538-0213 points to a high-gravity atmosphere. Only the K-band reddish
nature of SOri70 may be consistent with a low gravity atmosphere. The proper
motions of SOri70 and 73 are measurable and are larger than that of the cluster
by >3.5 sigma. The proper motion of SOriJ0538-0213 is consistent with a null
displacement. These observations suggest that none of the three T dwarfs are
likely Sigma Orionis members, and that either planetary-mass objects with
masses below ~4 MJup may not exist free-floating in the cluster or they may lie
at fainter near-infrared magnitudes than those of the targets (this is H>20.6
mag), thus remaining unidentified to date.Comment: Accepted for publication in A&A (2014), corrected typo
Search and characterization of T-type planetary mass candidates in the sigma Orionis cluster
(Abridged) We aim to: i) confirm the presence of methane absorption in S Ori
73 (a T-type member candidate of the sig Orionis cluster, 3 Myr, 352 pc)
through methane imaging; ii) study S Ori 70 and 73 cluster membership via
photometric colors and accurate proper motion analysis; iii) perform a new
search to identify additional T-type sig Orionis member candidates with likely
masses below 7 Mjup. We obtained HAWK-I (VLT) J, H, and CH4off photometry of an
area of 119.15 sq. arcmin in sig Orionis down to Jcomp = 21.7 and Hcomp = 21
mag. Near-infrared data were complemented with optical photometry using images
acquired with OSIRIS (GTC) and VISTA as part of the VISTA Orion survey. We
derived proper motions by comparison of the new HAWK-I and VISTA images with
published near-infrared data taken 3.4 - 7.9 yr ago. S Ori 73 has a red
H-CH4off color indicating methane absorption in the H-band and a spectral type
of T4 +/- 1. S Ori 70 displays a redder methane color than S Ori 73 in
agreement with its latter spectral classification. Our proper motion
measurements are larger than the motion of sig Orionis, rendering S Ori 70 and
73 cluster membership uncertain. We identified one new photometric candidate
with J = 21.69 +/- 0.12 mag and methane color consistent with spectral type
greater than T8. S Ori 73 has colors similar to those of T3-T5 field dwarfs,
which in addition to its high proper motion suggests that it is probably a
field dwarf located at 170-200 pc. The origin of S Ori 70 remains unclear: it
can be a field, foreground mid- to late-T free-floating dwarf with peculiar
colors, or an orphan planet ejected through strong dynamical interactions from
sig Orionis or from a nearby star-forming region in Orion.Comment: Accepted for publication in A&
Trigonometric parallaxes of young field L dwarfs
(Abridged) We aim to determine the trigonometric parallaxes and proper
motions of a sample of ten field L0-L5 dwarfs with spectroscopic evidence for
low-gravity atmospheres. We obtained J and Ks imaging data using 2-4-m class
telescopes with a typical cadence of one image per month between 2010 January
and 2012 December. We also obtained low resolution optical spectra (R~300,
500-1100 nm) using the 10-m GTCs to assess the presence of lithium absorption
in four targets and confirm their young age. Trigonometric parallaxes and
proper motions were derived to typical accuracies of 1 mas and +/-10 mas/yr.
All ten L dwarfs have large motions, and are located at distances between 9 and
47 pc. They lie above and on the sequence of field dwarfs in the absolute J and
K_s magnitude versus spectral type and luminosity versus Teff diagrams,
implying ages similar to or smaller than those typical of the field. The
detection of atomic lithium in the atmosphere of 2MASS J00452143+1634446 is
reported for the first time. Three dwarfs have locations in the HR diagram
indicative of old ages and high masses consistent with the observed lithium
depletion previously published. We did not find evidence for the presence of
astrometric companions with minimum detectable masses typically >=25 Mjup and
face-on, circular orbits with periods between 60-90 d and 3 yr around eight
targets. The astrometric and spectroscopic data indicate that about 60-70% of
the field L-type dwarfs in our sample with evidence for low-gravity atmospheres
are indeed young-to-intermediate-age brown dwarfs of the solar neighborhood
with expected ages and masses in the intervals 10-500 Myr and 11-45 Mjup. The
peaked-shape of the H-band spectra of L dwarfs, a signpost of youth, appears to
be present up to ages of 120-500 Myr and intermediate-to-high gravities.Comment: Accepted for publication in A&
Linear polarization of rapidly rotating ultracool dwarfs
Abstract. We present imaging linear polarimetry data of a sample of 18 late-M, L-, and early-T type dwarfs taken with the Z-and J-band filters and the LIRIS instrument of the 4.2-m William Herschel Telescope. All of our targets have projected rotational velocities â„ 30 km s â1 and oblate ultracool atmospheres (T eff < 2700 K), which may harbor clouds of condensate particles. Our polarimetric measurements have typical error bars of ±0.13%, i.e., linear polarization degrees larger than 0.4% can be detected with a confidence of â„ 3Ï. Seven dwarfs appear to be polarized in the J-band with indices of P = 0.4-0.7%, suggesting the presence of atmospheric dusty structures. There is a hint that the dwarfs with the largest rotations (v sin i â„ 60 km s â1 ) show higher incidence of detected J-band linear polarization than the dwarfs with smaller projected rotational velocities. We also detect linear polarization variability indicative of "weather"
Near-infrared Linear Polarization of Ultracool Dwarfs
We report on near-infrared J- and H-band linear polarimetric photometry of
eight ultracool dwarfs (two late-M, five L0-L7.5, and one T2.5) with known
evidence for photometric variability due to dust clouds, anomalous red infrared
colors, or low-gravity atmospheres. The polarimetric data were acquired with
the LIRIS instrument on the William Herschel Telescope. We also provide
mid-infrared photometry in the interval 3.4-24 micron for some targets obtained
with Spitzer and WISE, which has allowed us to confirm the peculiar red colors
of five sources in the sample. We can impose modest upper limits of 0.9% and
1.8% on the linear polarization degree for seven targets with a confidence of
99%. Only one source, 2MAS, J02411151-0326587 (L0), appears to be strongly
polarized (P ~ 3%) in the J-band with a significance level of P/sigma_P ~ 10.
The likely origin of its linearly polarized light and rather red infrared
colors may reside in a surrounding disk with an asymmetric distribution of
grains. Given its proximity (66 +/- 8 pc), this object becomes an excellent
target for the direct detection of the disk.Comment: Accepted for publication in Ap
A new L-dwarf member of the moderately metal-poor triple system HD 221356
We report on the discovery of a fourth component in the HD 221356 star
system, previously known to be formed by an F8V, slightly metal-poor primary
([Fe/H]=-0.26), and a distant M8V+L3V pair. In our ongoing common proper motion
search based on VISTA Hemisphere Survey (VHS) and 2MASS catalogues, we have
detected a faint (J=13.76+/-0.04 mag) co-moving companion of the F8 star
located at angular separation of 12.13+/-0.18 arcsec (position angle of
221.8+/-1.7), corresponding to a projected distance of ~312 AU at 26 pc.
Near-infrared spectroscopy of the new companion, covering the 1.5-2.4 micron
wavelength range with a resolving power of R~600, indicates an L1+/-1 spectral
type. Using evolutionary models the mass of the new companion is estimated at
~0.08 solar masses, which places the object close to the stellar-substellar
borderline. This multiple system provides an interesting example of objects
with masses slightly above and below the hydrogen burning mass limit. The low
mass companions of HD 221356 have slightly bluer colours than field dwarfs with
similar spectral type, which is likely a consequence of the sub-solar
metallicity of the system.Comment: 7 pages, 4 figures, accepted for publication in MNRA
Variable stars in the VVV globular clusters. II. NGC6441, NGC6569, NGC6626 (M28), NGC6656 (M22), 2MASS-GC02, and Terzan10
© ESO 2021. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1051/0004-6361/202140546Context. The Galactic globular clusters (GGCs) located in the inner regions of the Milky Way suffer from high extinction that makes their observation challenging. High densities of field stars in their surroundings complicate their study even more. The VISTA Variables in the Via Lactea (VVV) survey provides a way to explore these GGCs in the near-infrared where extinction effects are highly diminished. Aims. We conduct a search for variable stars in several inner GGCs, taking advantage of the unique multi-epoch, wide-field, near-infrared photometry provided by the VVV survey. We are especially interested in detecting classical pulsators that will help us constrain the physical parameters of these GGCs. In this paper, the second of a series, we focus on NGC 6656 (M 22), NGC 6626 (M 28), NGC 6569, and NGC 6441; these four massive GGCs have known variable sources, but quite different metallicities. We also revisit 2MASS-GC 02 and Terzan 10, the two GGCs studied in the first paper of this series. Methods. We present an improved method and a new parameter that efficiently identify variable candidates in the GGCs. We also use the proper motions of those detected variable candidates and their positions in the sky and in the color-magnitude diagrams to assign membership to the GGCs. Results. We identify and parametrize in the near-infrared numerous variable sources in the studied GGCs, cataloging tens of previously undetected variable stars. We recover many known classical pulsators in these clusters, including the vast majority of their fundamental mode RR Lyrae. We use these pulsators to obtain distances and extinctions toward these objects. Recalibrated period-luminosity-metallicity relations for the RR Lyrae bring the distances to these GGCs to a closer agreement with those reported by Gaia, except for NGC 6441, which is an uncommon Oosterhoff III GGC. Recovered proper motions for these GGCs also agree with those reported by Gaia, except for 2MASS-GC 02, the most reddened GGC in our sample, where the VVV near-infrared measurements provide a more accurate determination of its proper motions.Peer reviewe
Dwarf spheroidal satellites of the Milky Way from dark matter free tidal dwarf galaxy progenitors: maps of orbits
The long term time evolution of tidal dwarf satellite galaxies with two
different initial densities orbiting a host galaxy that resembles the Milky Way
has been studied using a large set of Newtonian N-Body simulations. From the
simulations two maps of the orbital conditions that lead to quasi-equilibrium
objects were constructed. It has been found that several orbits of the
satellites allow for the existence, for about 1 Gyr or more, of
out-of-equilibrium bodies with high apparent mass-to-light ratios. Within this
framework the satellites in the quasi-stable phase reproduce the observed
satellite properties for about 16% of the orbit for high density progenitors,
and for about 66% for progenitors with lower densities An additional simulation
for a single satellite with initial mass of 10^7 Msun and Plummer radius of
0.15 kpc leads to remnants in the quasi- equilibrium phase that simultaneously
reproduce remarkably well the observational quantities of the UFDGs of the
Milky Way. This satellite in the quasi-stable phase reproduces the observed
satellite properties for about 42% of the orbit. The results suggest that a
fraction of the observed satellites could plausibly be galaxies without dark
matter that have true M/L ratios much lower than those measured. The inflated
M/L ratios arise because they are observed at the right time, along the right
orbit and during the quasi-equilibrium phase of their evolution. This is a
viable explanation for the high M/L ratios observed in all satellites as long
as the satellites are preferentially on certain orbits and are observed at
certain times. This could arise within the TDG scenario if all satellites are
created at the same time along a few specific orbits that are particularly
susceptible to the quasi-equilibrium phase.Comment: MNRAS, accepted, 11 pages, 8 figure
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