144 research outputs found
Chemical abundances of late-type pre-main sequence stars in the -Orionis cluster
The young -Orionis cluster is an important location for understanding
the formation and evolution of stars, brown dwarfs, and planetary-mass objects.
Its metallicity, although being a fundamental parameter, has not been well
determined yet. We present the first determination of the metallicity of nine
young late-type stars in -Orionis. Using the optical and near-infrared
broadband photometry available in the literature we derive the effective
temperatures for these nine cluster stars, which lie in the interval 4300--6500
K (1--3 \Msuno). These parameters are employed to compute a grid of synthetic
spectra based on the code MOOG and Kurucz model atmospheres. We employ a
-minimization procedure to derive the stellar surface gravity and
atmospheric abundances of Al, Ca, Si, Fe, Ni and Li, using multi-object optical
spectroscopy taken with WYFFOS+AF2 at at the William Herschel Telescope
(). The average metallicity of the
-Orionis cluster is [Fe/H] (random and
systematic errors). The abundances of the other elements, except lithium, seem
to be consistent with solar values. Lithium abundances are in agreement with
the "cosmic" Li abundance, except for two stars which show a in the range 3.6--3.7 (although almost consistent within
the error bars). There are also other two stars with . We derived an average radial velocity of the
-Orionis cluster of km/s. The -Orionis metallicity is
roughly solar.Comment: Accepted for publication in Astronomy and Astrophysic
El espinar esclerófilo de Aspargo Albi-Rhamnetum "bethurici" en el subsector Ribaduriense
The present work contributes ecological, floristic and phytosociological data about the sclerophilus thombush found in the association Asparago albi-Rhamnetum "bethurici" Ladero 1970 of the lower stratum of dry mesomediterranean bioclimatic level of the Ribaduriense subsector (Lusitano-Duriense sector), the nothermostlimit of its area.Se aportan datos ecológicos, floristicos y fitosociológicos acerca del espinar esclerófilo comprendido en la asociación Asparago albi-Rhannetum "bethurici" Ladero 1970 del horizonte inferior del piso bioclimático mesomediterráneo seco en el Subsector Ribaduriense (Sector Lusitano-Duriense), limite septentrional de su área
Candidate free-floating super-Jupiters in the young sigma Orionis open cluster
Free-floating substellar candidates with estimated theoretical masses of as
low as ~5 Jupiter masses have been found in the ~3 Myr old sigma Orionis open
cluster. As the overlap with the planetary mass domain increases, the question
of how these objects form becomes important. The determination of their number
density and whether a mass cut-off limit exists is crucial to understanding
their formation. We propose to search for objects of yet lower masses in the
cluster and determine the shape of the mass function at low mass. Using new-
and (re-analysed) published IZJHKs[3.6]-[8.0]-band data of an area of 840
arcmin2, we performed a search for LT-type cluster member candidates in the
magnitude range J=19.5-21.5 mag, based on their expected magnitudes and
colours. Besides recovering the T type object S Ori 70 and two other known
objects, we find three new cluster member candidates, S Ori 72-74, with J=21
mag and within 12 arcmin of the cluster centre. They have theoretical masses of
4 (-2,+3) M_Jup and are among the least massive free-floating objects detected
by direct imaging outside the Solar System. The photometry in archival Spitzer
[3.6]-[5.8]-band images infers that S Ori 72 is an L/T transition candidate and
S Ori 73 a T-type candidate, following the expected cluster sequence in the
mid-infrared. Finally, the L-type candidate S Ori 74 with lower quality
photometry is located at 11.8 arcsec (~4250 AU) of a stellar member of sigma
Orionis and could be a companion. After contaminant correction in the area
complete to J=21.1 mag, we estimate that there remain between zero and two
cluster members in the mass interval 6-4 M_Jup. Our result suggests a possible
turnover in the substellar mass spectrum below ~6 Jupiter masses, which could
be investigated further by wider and deeper photometric surveys.Comment: 15 pages, 13 figures, 5 tables, and appendix containing 5 figures;
accepted for publication in AA; v2: 2 minor corrections, in abstract and
sect. 2.
New constraints on the membership of the T dwarf S Ori 70 in the sigma Orionis cluster
(Abridged) The nature of S Ori 70, a faint mid-T type object found towards
the direction of the young sigma Orionis cluster, is still under debate. We
intend to disentangle whether it is a field brown dwarf or a 3-Myr old
planetary-mass member of the cluster. We report on near-infrared JHK_s and
mid-infrared [3.6] and [4.5] IRAC/Spitzer photometry recently obtained for S
Ori 70. The new near-infrared images (taken 3.82 yr after the discovery data)
have allowed us to derive a very small proper motion (11.0 +/- 5.9 mas/yr) for
this object, which is consistent with the proper motion of the cluster within
1.5 sigma the astrometric uncertainty. The colors (H-K_s), (J-K_s) and
K_s-[3.6] appear discrepant when compared to T4-T7 dwarfs in the field. This
behavior could be ascribed to a low-gravity atmosphere or alternatively to an
atmosphere with a metallicity significantly different than solar. Taking into
account the small proper motion of S Ori 70 and its new near- and mid-infrared
colors, a low-gravity atmosphere remains as the most likely explanation to
account for our observations. This supports S Ori 70's membership in sigma
Orionis, with an estimated mass in the interval 2-7 Mjup, in agreement with our
previous derivation.Comment: Accepted for publication in A&
Characterization of the K2-38 planetary system: Unraveling one of the densest planets known to date
.-- Toledo-Padrón, B. et al.Context. An accurate characterization of the known exoplanet population is key to understanding the origin and evolution of planetary systems. Determining true planetary masses through the radial velocity (RV) method is expected to experience a great improvement thanks to the availability of ultra-stable echelle spectrographs. Aims. We took advantage of the extreme precision of the new-generation echelle spectrograph ESPRESSO to characterize the transiting planetary system orbiting the G2V star K2-38 located at 194 pc from the Sun with V 11.4. This system is particularly interesting because it could contain the densest planet detected to date. Methods. We carried out a photometric analysis of the available K2 photometric light curve of this star to measure the radius of its two known planets, K2-38b and K2-38c, with Pb = 4.01593 ± 0.00050 d and Pc = 10.56103 ± 0.00090 d, respectively. Using 43 ESPRESSO high-precision RV measurements taken over the course of 8 months along with the 14 previously published HIRES RV measurements, we modeled the orbits of the two planets through a Markov chain Monte Carlo analysis, significantly improving their mass measurements. Results. Using ESPRESSO spectra, we derived the stellar parameters, Teff = 5731 ± 66, log g = 4.38 ± 0.11 dex, and [Fe/H] = 0.26 ± 0.05 dex, and thus the mass and radius of K2-38, Ma = 1.03-0.02+0.04 MaS and Ra = 1.06-0.06+0.09 RaS. We determine new values for the planetary properties of both planets. We characterize K2-38b as a super-Earth with RP = 1.54 ± 0.14 RaS and Mp = 7.3-1.0+1.1 MaS, and K2-38c as a sub-Neptune with RP = 2.29 ± 0.26 RaS and Mp = 8.3-1.3+1.3 MaS. Combining the radius and mass measurements, we derived a mean density of ρp = 11.0-2.8+4.1 g cm-3 for K2-38b and ρp = 3.8-1.1+1.8 g cm-3 for K2-38c, confirming K2-38b as one of the densest planets known to date. Conclusions. The best description for the composition of K2-38b comes from an iron-rich Mercury-like model, while K2-38c is better described by a rocky-model with H2 envelope. The maximum collision stripping boundary shows how giant impacts could be the cause for the high density of K2-38b. The irradiation received by each planet places them on opposite sides of the radius valley. We find evidence of a long-period signal in the RV time-series whose origin could be linked to a 0.25-3 MJ planet or stellar activity.With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737
ESPRESSO: The next European exoplanet hunter
The acronym ESPRESSO stems for Echelle SPectrograph for Rocky Exoplanets and
Stable Spectroscopic Observations; this instrument will be the next VLT high
resolution spectrograph. The spectrograph will be installed at the
Combined-Coud\'e Laboratory of the VLT and linked to the four 8.2 m Unit
Telescopes (UT) through four optical Coud\'e trains. ESPRESSO will combine
efficiency and extreme spectroscopic precision. ESPRESSO is foreseen to achieve
a gain of two magnitudes with respect to its predecessor HARPS, and to improve
the instrumental radial-velocity precision to reach the 10 cm/s level. It can
be operated either with a single UT or with up to four UTs, enabling an
additional gain in the latter mode. The incoherent combination of four
telescopes and the extreme precision requirements called for many innovative
design solutions while ensuring the technical heritage of the successful HARPS
experience. ESPRESSO will allow to explore new frontiers in most domains of
astrophysics that require precision and sensitivity. The main scientific
drivers are the search and characterization of rocky exoplanets in the
habitable zone of quiet, nearby G to M-dwarfs and the analysis of the
variability of fundamental physical constants. The project passed the final
design review in May 2013 and entered the manufacturing phase. ESPRESSO will be
installed at the Paranal Observatory in 2016 and its operation is planned to
start by the end of the same year.Comment: 12 pages, figures included, accepted for publication in Astron. Nach
The CARMENES search for exoplanets around M dwarfs. A sub-Neptunian mass planet in the habitable zone of HN Lib
We report the discovery of HN Lib b, a sub-Neptunian mass planet orbiting the
nearby ( = 6.25 pc) M4.0 V star HN Lib detected by our CARMENES
radial-velocity (RV) survey. We determined a planetary minimum mass of
5.46 0.75 and an orbital period
of 36.116 0.029 d, using 5 yr of CARMENES data, as
well as archival RVs from HARPS and HIRES spanning more than 13 years. The flux
received by the planet equals half the instellation on Earth, which places it
in the middle of the conservative habitable zone (HZ) of its host star. The RV
data show evidence for another planet candidate with
9.7 1.9 and 113.46 0.20 d. The
long-term stability of the signal and the fact that the best model for our data
is a two-planet model with an independent activity component stand as strong
arguments for establishing a planetary origin. However, we cannot rule out
stellar activity due to its proximity to the rotation period of HN Lib, which
we measured using CARMENES activity indicators and photometric data from a
ground-based multi-site campaign as well as archival data. The discovery adds
HN Lib b to the shortlist of super-Earth planets in the habitable zone of M
dwarfs, but HN Lib [c] probably cannot be inhabited because, if confirmed, it
would most likely be an icy giant
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
Automatic model-based telluric correction for the ESPRESSO data reduction software. Model description and application to radial velocity computation
Ground-based high-resolution spectrographs are key instruments for several
astrophysical domains. Unfortunately, the observed spectra are contaminated by
the Earth's atmosphere. While different techniques exist to correct for
telluric lines in exoplanet atmospheric studies, in radial velocity (RV)
studies, telluric lines with an absorption depth of >2% are generally masked,
which poses a problem for faint targets and M dwarfs as most of their RV
content is present where telluric contamination is important. We propose a
simple telluric model to be embedded in the ESPRESSO DRS. The goal is to
provide telluric-free spectra and enable RV measurements, including spectral
ranges where telluric lines fall. The model is a line-by-line radiative
transfer code that assumes a single atmospheric layer. We use the sky
conditions and the physical properties of the lines from HITRAN to create the
telluric spectrum. A subset of selected telluric lines is used to robustly fit
the spectrum through a Levenberg-Marquardt minimization algorithm. When applied
to stellar spectra from A0- to M5-type stars, the residuals of the strongest
H2O lines are below 2% for all spectral types, with the exception of M dwarfs,
which are within the pseudo-continuum. We then determined the RVs from the
telluric-corrected ESPRESSO spectra of Tau Ceti and Proxima. We created
telluric-free masks and compared the obtained RVs with the DRS RVs. In the case
of Tau Ceti, we identified that micro-telluric lines introduce systematics up
to an amplitude of 58 cm/s and with a period of one year. For Proxima, the gain
in spectral content at redder wavelengths is equivalent to a gain of 25% in
photon noise. This leads to better constraints on the semi-amplitude and
eccentricity of Proxima d. We showcase that our model can be applied to other
molecules, and thus to other wavelength regions observed by other
spectrographs, such as NIRPS.Comment: 18 pages, 18 figures, accepted to A&
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