Astrometric observations of resolved binaries provide estimates of orbital periods and will eventually lead to measurement of dynamical masses. Only a few very low mass star and brown dwarf masses ...have been measured to date, and the mass-luminosity relation still needs to be calibrated. We have monitored 14 very low mass multiple systems for several years to confirm their multiplicity and, for those with a short period, derive accurate orbital parameters and dynamical mass estimates. We have used high spatial resolution images obtained at the Paranal, Lick and HST observatories to obtain astrometric and photometric measurements of the multiple systems at several epochs. The targets have periods ranging from 5 to 200 years, and spectral types in the range M7.5 - T5.5. All of our 14 multiple systems are confirmed as common proper motion pairs. One system (2MASSW J0920122+351742) is not resolved in our new images, probably because the discovery images were taken near maximum elongation. Six systems have periods short enough to allow dynamical mass measurements within the next 15 to 20years. We estimate that only 8% of the ultracool dwarfs in the solar neighborhood are binaries with separations large enough to be resolved, and yet periods short enough to derive astrometric orbital fits over a reasonable time frame with current instrumentation. A survey that doubles the number of ultracool dwarfs observed with high angular resolution is called for to discover enough binaries for a first attempt to derive the mass-luminosity relationship for very low-mass stars and brown dwarfs.
(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.
We present the discovery of fifteen new T2.5-T7.5 dwarfs (with estimated distances between ~24-93pc, identified in the first three main data releases of the UKIRT Infrared Deep Sky Survey. This ...brings the total number of T dwarfs discovered in the Large Area Survey (to date) to 28. These discoveries are confirmed by near infrared spectroscopy, from which we derive spectral types on the unified scheme of Burgasser et al. (2006). Seven of the new T dwarfs have spectral types of T2.5-T4.5, five have spectral types of T5-T5.5, one is a T6.5p, and two are T7-7.5. We assess spectral morphology and colours to identify T dwarfs in our sample that may have non-typical physical properties (by comparison to solar neighbourhood populations). The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y-J with decreasing effective temperature beyond T8. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity and Malmquist bias, we estimate that there are 17+-4 >=T4 dwarfs in the J<=19 volume of the LAS second data release. Comparing this to theoretical predictions is most consistent with a sub-stellar mass function exponent alpha between -1.0 and 0. This is consistent with the latest 2MASS/SDSS constraint (which is based on lower number statistics), and is significantly lower than the alpha~1.0 suggested by L dwarf field populations, possibly a result of the lower mass range probed by the T dwarf class.
A&A 668, A86 (2022) HE 0107$-$5240 is a hyper metal-poor star with $\rm Fe/H=-5.39$. We
performed high-res observations with the ESPRESSO spectrograph at the VLT to
constrain the kinematical ...properties of the binary system HE 0107$-$5240 and to
probe the binarity of the sample of 8 most metal-poor stars with $\rm
Fe/H<-4.5$. Radial velocities are obtained by using cross-correlation in the
interval 4200$-$4315A, which contains the strong CH band, against a template in
an iterative process. A Bayesian method is applied to calculate the orbit by
using the ESPRESSO measurements and others from the literature. A chemical
analysis has also been performed in HE0107$-$5240 by means of spectral
synthesis. Observations of HE 0107$-$5240 spanning more than 3 years show a
monotonic decreasing trend in radial velocity at a rate of approximately by 0.5
ms$^{-1}$d$^{-1}$. The period is constrained at $P_{\rm orb} =
13009_{-1370}^{+1496}$d. New more stringent upper-limits have been found for
several elements: a)Sr/Fe and Ba/Fe are lower than $-0.76$ and $+0.2$
respectively, confirming the star is a CEMP-no; b)$A(Li)< 0.5$ is well below
the plateau at $A(Li)=1.1$ found in the Lower Red Giant Branch stars,
suggesting Li was originally depleted; and c)the isotopic ratio
$^{12}$C/$^{13}$C is 87$\pm6$ showing very low $^{13}$C in contrast to what
expected from a spinstar progenitor. We confirm that HE 0107$-$5240 is a binary
star with a long period of about 13000d ($\sim36$ years).The carbon isotopic
ratio excludes the possibility that the companion has gone through the AGB
phase and transferred mass to the currently observed star. The binarity of HE
0107$-$5240 implies some of the first generations of low-mass stars form in
multiple systems and indicates that the low metallicity does not preclude the
formation of binaries. Finally, a solid indication of $v_{ rad}$ variation has
been found also in SMSS 1605$-$1443.
We review different surveys, in the optical and infrared, conducted in the very young (age 1-8 Myr), nearby (d 6 350 pc) s Orionis cluster aimed to characterize the substellar population. We describe ...spectral characteristics of very low mass stars, brown dwarfs and planetary mass objects in the cluster with spectral types from K7 to T6. We study the spatial distribution of the substellar population detected in a IZJ survey covering an area of 1.12 deg.2 We find that the radial distribution of substellar objects can be well fitted by an exponential law (r = ro e-r/ro), with a central density (ro) of 0.26 c 0.03 objects/arcmin2 and a characteristic radius (ro) of 8.8 arcmin c 0.6 (equivalent to 0.90 c 0.06 pc at the distance of the cluster). We discuss the presence of possible inhomogeneities in this distribution due to the existence of subclustering. We also compare the spatial distribution of the substellar population with previously known stars in the cluster. We report the initial mass spectrum in the substellar domain.
We investigate the mass function in the substellar domain down to a few Jupiter masses in the young sigma Orionis open cluster (3+/-2 Ma, d = 360^+70_-60 pc). We have performed a deep IJ-band search, ...covering an area of 790 arcmin^2 close to the cluster centre. This survey was complemented with an infrared follow-up in the HKs- and Spitzer 3.6-8.0 mum-bands. Using colour-magnitude diagrams, we have selected 49 candidate cluster members in the magnitude interval 16.1 mag < I < 23.0 mag. Accounting for flux excesses at 8.0 mum and previously known spectral features of youth, 30 objects are bona fide cluster members. Four are first identified from our optical-near infrared data. Eleven have most probable masses below the deuterium burning limit and are classified as planetary-mass object candidates. The slope of the substellar mass spectrum (Delta N / Delta M = a M^-alpha) in the mass interval 0.11 Msol M < 0.006 Msol is alpha = +0.6+/-0.2. Any opacity mass-limit, if these objects form via fragmentation, may lie below 0.006 Msol. The frequency of sigma Orionis brown dwarfs with circumsubstellar discs is 47+/-15 %. The continuity in the mass function and in the frequency of discs suggests that very low-mass stars and substellar objects, even below the deuterium-burning mass limit, may share the same formation mechanism.
We present three transit observations of HD 189733 b obtained with the high-resolution spectrograph CARMENES at Calar Alto. A strong absorption signal is detected in the near-infrared He I triplet at ...10830 \AA{} in all three transits. During mid-transit, the mean absorption level is \(0.88\pm0.04\) % measured in a \(\pm\)10 km s\(^{-1}\) range at a net blueshift of \(-3.5\pm0.4\) km s\(^{-1}\) (10829.84--10830.57 \AA{}). The absorption signal exhibits radial velocities of \(+6.5\pm3.1\) km s\(^{-1}\) and \(-12.6\pm1.0\) km s\(^{-1}\) during ingress and egress, respectively; measured in the planetary rest frame. We show that stellar activity related pseudo-signals interfere with the planetary atmospheric absorption signal. They could contribute as much as 80% of the observed signal and might also affect the radial velocity signature, but pseudo-signals are very unlikely to explain the entire signal. The observed line ratio between the two unresolved and the third line of the He I triplet is \(2.8\pm0.2\), which strongly deviates from the value expected for an optically thin atmospheres. When interpreted in terms of absorption in the planetary atmosphere, this favors a compact helium atmosphere with an extent of only 0.2 planetary radii and a substantial column density on the order of \(4\times 10^{12}\) cm\(^{-2}\). The observed radial velocities can be understood either in terms of atmospheric circulation with equatorial superrotation or as a sign of an asymmetric atmospheric component of evaporating material. We detect no clear signature of ongoing evaporation, like pre- or post-transit absorption, which could indicate material beyond the planetary Roche lobe, or radial velocities in excess of the escape velocity. These findings do not contradict planetary evaporation, but only show that the detected helium absorption in HD 189733 b does not trace the atmospheric layers that show pronounced escape signatures.
White Paper on MAAT@GTC Prada, Francisco; Content, Robert; Goobar, Ariel ...
arXiv.org,
07/2020
Paper, Journal Article
Odprti dostop
MAAT is proposed as a visitor mirror-slicer optical system that will allow the OSIRIS spectrograph on the 10.4-m Gran telescopio CANARIAS (GTC) the capability to perform Integral Field Spectroscopy ...(IFS) over a seeing-limited FoV 14.20''x10'' with a slice width of 0.303''. MAAT@GTC will enhance the resolution power of OSIRIS by 1.6 times as compared to its 0.6'' wide long-slit. All the eleven OSIRIS grisms and volume-phase holographic gratings will be available to provide broad spectral coverage with moderate resolution (R=600 up to 4100) in the 3600 - 10000 Å wavelength range. MAAT unique observing capabilities will broaden its use to the needs of the GTC community to unveil the nature of most striking phenomena in the universe well beyond time-domain astronomy. The GTC equipped with OSIRIS+MAAT will also play a fundamental role in synergy with other facilities, some of them operating on the northern ORM at La Palma. This White Paper presents the different aspects of MAAT@GTC - including scientific and technical specifications, outstanding science cases, and an outline of the instrument concept.
Astron.Astrophys. 419 (2004) 167-180 We explored the regions within a radius of 25 arcsec around 473 nearby,
low-metallicity G- to M-type stars using (VR)I optical filters and
small-aperture ...telescopes. About 10% of the sample was searched up to angular
separations of 90 arcsec. We applied photometric and astrometric techniques to
detect true physical companions to the targets. The great majority of the
sample stars was drawn from the Carney-Latham surveys; their metallicities
range from roughly solar to Fe/H=-3.5 dex. Our I-band photometric survey
detected objects that are between 0 and 5 mag fainter (completeness) than the
target stars; the maximum dynamical range of our exploration is 9 mag. We also
investigated the literature and inspected images from the Digitized Sky Surveys
to complete our search. By combining photometric and proper motion
measurements, we retrieved 29 previously known companions, and identified 13
new proper motion companions. Near-infrared 2MASS photometry is provided for
the great majority of them. Low-resolution optical spectroscopy (386-1000 nm)
was obtained for eight of the new companion stars. These spectroscopic data
confirm them as cool, late-type, metal-depleted dwarfs, with spectral classes
from esdK7 to sdM3. After comparison with low-metallicity evolutionary models,
we estimate the masses of the proper motion companion stars to be in the range
0.5-0.1 Msol. They are orbiting their primary stars at projected separations
between ~32 and ~57000 AU. These orbital sizes are very similar to those of
solar-metallicity stars of the same spectral types. Our results indicate that
about 15% of the metal-poor stars have stellar companions at large orbits,
which is in agreement with the binary fraction observed among main sequence G-
to M-type stars and T Tauri stars.
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