Abstract
Determination of the lithium depletion boundary (LDB), i.e., the observational limit below which the cores of very low-mass objects do not reach high enough temperatures for Li destruction, ...has been used to obtain ages for several open clusters and stellar associations younger than 200 Myr—which until now has been considered the practical upper limit on the range of applicability of this method. In this work, we show that the LDB method can be extended to significant older ages than previously thought. Intermediate resolution optical spectra of six L-type candidate members in the Hyades cluster obtained using Optical System for Imaging and Low Resolution Integrated Spectroscopy at the 10.4 m Gran Telescopio Canarias are presented. The
670.8 nm resonance doublet is clearly detected only in the two faintest and coolest of these objects, which are classified as L3.5 to L4 brown dwarf (BD) cluster members with luminosities around 10
−4
solar. Lithium depletion factors are estimated for our targets with the aid of synthetic spectra and they are compared with predictions from evolutionary models. An LDB age of 650 ± 70 Myr for the Hyades provides a consistent description of our data using a set of state-of-the-art evolutionary models for BDs calculated by Baraffe et al.
In a search for common proper motion companions using the VISTA Hemisphere Survey (VHS) and the 2MASS catalogs we have identified a very red (J - K sub(S) = 2.47 mag) late-L dwarf companion of a ...previously unrecognized M dwarf VHS J125601.92-125723.9 (hereafter VHS 1256-1257), located at a projected angular separation of 8''.06 + or - 0''.03. In this work we present a suite of astrometric, photometric, and spectroscopic observations of this new pair in an effort to confirm the companionship and characterize the components. From low-resolution (R ~ 130-600) optical and near-infrared spectroscopy we classified the primary and the companion as M7.5 + or - 0.5 and L7 + or - 1.5, respectively. The primary shows slightly weaker alkali lines than field dwarfs of similar spectral type, but still consistent with either a high-gravity dwarf or a younger object of hundreds of millions of years. The secondary shows spectral features characteristic for low surface gravity objects at ages below several hundred million years, like the peaked triangular shape of the H-band continuum and alkali lines weaker than in field dwarfs of the same spectral type. The absence of lithium in the atmosphere of the primary and the likely kinematic membership to the Local Association allowed us to constrain the age of the system to the range of 150-300 Myr. We report a measurement of the trigonometric parallax pi = 78.8 + or - 6.4 mas, which translates into a distance of 12.7 + or - 1.0 pc; the pair thus has a projected physical separation of 102 + or - 9 AU. We derived the bolometric luminosities of the components and compared them with theoretical evolutionary models to estimate the masses and effective temperatures. For the primary, we determined a luminosity of log(L sub(bol)/L sub(middo t in circle)) = -3.14 + or - 0.10, and inferred a mass of 73 super(+20) sub(-15) M sub(Jup) at the boundary between stars and brown dwarfs and an effective temperature of 2620 + or - 140 K. For the companion we obtained a luminosity of log(L sub(bol)/L sub(middo t in circle)) = -5.05 + or - 0.22 and a mass of 11.2 super(+9.7) sub(-1.8) M sub(Jup), placing it near the deuterium-burning mass limit. The effective temperature derived from evolutionary models is 880 super(+140) sub(-110) K, about 400-700 K cooler than the temperature expected for field late-L dwarfs.
K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet that was first discovered by the K2 mission and later confirmed with Spitzer Space Telescope observations. With a radius ...of ∼2 R⊕ and an orbital period of ∼33 days, the planet lies in the temperate zone of its host star and receives stellar irradiation similar to that of Earth. Here we perform radial velocity follow-up observations with the visual channel of CARMENES with the goal of determining the mass and density of the planet. We measure a planetary semi-amplitude of Kb ∼ 3.5 and a mass of Mb ∼ 9 M⊕, yielding a bulk density around . This indicates a low-mass planet with a composition consistent with a solid core and a volatile-rich envelope. A signal at 9 days was recently reported using radial velocity measurements taken with the HARPS spectrograph. This was interpreted as being due to a second planet. We see a weaker, time- and wavelength-dependent signal in the CARMENES data set and thus favor stellar activity for its origin. K2-18 b joins the growing group of low-mass planets detected in the temperate zone of M dwarfs. The brightness of the host star in the near-infrared makes the system a good target for detailed atmospheric studies with the James Webb Space Telescope.
Abstract
Ahead of upcoming space missions intending to conduct observations of low-mass stars in the ultraviolet (UV) spectral region it becomes imperative to simultaneously conduct atmospheric ...modeling from the UV to the visible (VIS) and near-infrared (NIR). Investigations on extended spectral regions will help to improve the overall understanding of the diversity of spectral lines arising from very different atmospheric temperature regions. Here we investigate atmosphere models with a chromosphere and transition region for the M2.5V star GJ 436, which hosts a close-in Hot Neptune. The atmosphere models are guided by observed spectral features from the UV to the VIS/NIR originating in the chromosphere and transition region of GJ 436. High-resolution observations from the Hubble Space Telescope and Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES) are used to obtain an appropriate model spectrum for the investigated M dwarf. We use a large set of atomic species considered in nonlocal thermodynamic equilibrium conditions within our PHOENIX model computations to approximate the physics within the low-density atmospheric regions. In order to obtain an overall match for the nonsimultaneous observations, it is necessary to apply a linear combination of two model spectra, where one of them better reproduces the UV lines while the other better represents the lines from the VIS/NIR range. This is needed to adequately handle different activity states across the observations.
Abstract
In this work we present the results of a direct imaging survey for brown dwarf companions around the nearest stars at the mid-infrared 10 micron range (
λ
c
= 8.7
μ
m, Δ
λ
= 1.1
μ
m) using ...the CanariCam instrument on the 10.4 m Gran Telescopio Canarias (GTC). We imaged the 25 nearest stellar systems within 5 pc of the Sun at declinations
δ
> −25° (at least half have planets from radial-velocity studies), reaching a mean detection limit of 11.3 ± 0.2 mag (1.5 mJy) in the Si-2 8.7
μ
m band over a range of angular separations from 1″ to 10″. This would have allowed us to uncover substellar companions at projected orbital separations between ∼2 and 50 au, with effective temperatures down to 600 K and masses greater than 30
M
Jup
assuming an average age of 5 Gyr and masses down to the deuterium-burning mass limit for objects with ages <1 Gyr. From the nondetection of such companions, we determined upper limits on their occurrence rate at depths and orbital separations yet unexplored by deep imaging programs. For the M dwarfs, the main component of our sample, we found with a 90% confidence level that fewer than 20% of these low-mass stars have L- and T-type brown dwarf companions with
m
≳ 30
M
Jup
and
T
eff
≳ 600 K at ∼3.5–35 au projected orbital separations.
Context.
Double-lined spectroscopic (SB2) binaries allow us to obtain a direct determination of the masses of their components, which is essential to test stellar models. Although these objects only ...provide a lower limit for the mass, they are more abundant than their eclipsing counterparts as they are not as strongly limited by the inclination of their orbit.
Aims.
Our aim is to derive the orbital and physical parameters of GJ 1284, a young SB2. We also revise the membership of this system and its two wide co-moving companions, GJ 898 and GJ 897AB, to a young moving group to assess, along with other youth indicators, their age. Afterwards, we compare the results from these analyses and the photometry of these systems with several pre-main-sequence evolutionary models.
Methods.
We use high-resolution spectra to determine the radial velocity of each component of GJ 1284 and the orbit of the system alongside its systemic velocity. Additionally, we use TESS photometry to derive the rotational period of the GJ 1284 and its two wide companions.
Results.
GJ 1284 is a binary system located at approximately 16 pc with an eccentric orbit (
e
= 0.505) of 11.83 d period made up of an M2–M2.5 + M3–M3.5 with minimum masses of
M
sin
3
i
= 0.141 ± 0.003 and 0.1189 ± 0.003
M
⊙
, respectively. The revised systemic velocity of
γ
= 0.84 ± 0.14 km s
−1
suggests that it is a member of the Local Association. The kinematics together with other activity and youth indicators imply an age of 110–800 Myr for this system and its two companions.
Conclusions.
The isochronal ages derived from the comparison of the photometry with several evolutionary models are younger than the age estimated from the activity indicators for the three co-moving systems. The masses for the components of GJ 1284, derived from their luminosity and age using the different models, are not consistent with the masses derived from the photometry, except for the PARSEC models, but are compatible with dynamical masses of double-lined eclipsing binaries with similar ages and spectral types. The effect of magnetic activity in the form of spots can reconcile to some extent the photometric and dynamical masses, but is not considered in most of the evolutionary models.
Aims.
Our objective is to identify analogues of gas giant planets, but located as companions at wide separations of very young stars. The main purpose is to characterise the binarity frequency and ...the properties of these substellar objects, and to elucidate their early evolutionary stages.
Methods.
To identify these objects, we cross correlated the Visible and Infrared Survey Telescope for Astronomy Hemisphere Survey and the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Clusters Survey catalogues to search for common proper motion companions to 1195 already known members of Upper Scorpius (USco; age ~5–10 Myr, distance ~145 pc). We present the discovery and spectroscopic characterisation of two very wide substellar companions of two early-M stars in Upper Scorpius: USco1621 B and USco1556 B. We obtained optical and near-infrared low-resolution spectroscopy of the candidates to characterise their spectral energy distribution and confirm their youth and membership to the association. We also acquired adaptive optics images of the primaries and secondaries to search for signs of binarity and close companions.
Results.
By comparison with field dwarfs and other young members of USco, we determined a spectral type of M8.5 in the optical for both companions, along with L0 and L0.5 in the near-infrared for USco1621 B and USco1556 B, respectively. The spectra of the two companions show evident markers of youth, such as weak alkaline Na I and K I lines, along with the triangular shape of the
H
-band. The comparison with theoretical evolutionary models gives estimated masses of 0.015 ± 0.002 and 0.014 ± 0.002
M
⊙
, with temperatures of 2270 ± 90 and 2240 ± 100 K, respectively. The physical separations between the components of both systems are 2880 ± 20 and 3500 ± 40 AU for USco1621 and USco1556 systems, respectively. We did not find any additional close companion in the adaptive optics images. The probability that the two secondaries are physically bound to their respective primaries, and not chance alignments of USco members, is 86%, and the probability that none of them are physically related is 1.0%.
Context.
The existence of warm (protoplanetary) disks around very young isolated planetary and brown dwarf mass objects is known based on near- and mid-infrared flux excesses and millimeter ...observations. These disks may later evolve into debris disks or rings, although none have been observed or confirmed so far. Little is known about circum(sub)stellar and debris disks around substellar objects.
Aims.
We aim to investigate the presence of debris disks around two of the closest (~20 pc), young substellar companions, namely G196-3 B and VHS J125601.92–125723.9 b (VHS J1256–1257 b), whose masses straddle the borderline between planets and brown dwarfs. Both are companions at wide orbits (≥100 au) of M-type dwarfs and their ages (50–100 Myr and 150–300 Myr, respectively) are thought to be adequate for the detection of second-generation disks.
Methods.
We obtained deep images of G196-3 B and VHS J1256–1257 b with the NOrthern Extended Millimeter Array (NOEMA) at 1.3 mm. These data were combined with recently published Atacama Large Millimeter Array (ALMA) and Very Large Array (VLA) data of VHS J1256–1257 b at 0.87 mm and 0.9 cm, respectively.
Results.
Neither G196-3 B nor VHS J1256–1257 b were detected in the NOEMA, ALMA, and VLA data. At 1.3 mm, we imposed flux upper limits of 0.108 mJy (G196-3 B) and 0.153 mJy (VHS J1256–1257 b) with a 3-
σ
confidence. Using the flux upper limits at the millimeter and radio wavelength regimes, we derived maximum values of 1.38×10
−2
M
Earth
and 5.46 × 10
−3
M
Earth
for the mass of any cold dust that might be surrounding G196-3 B and VHS J1256–1257 b, respectively.
Conclusions.
We put our results in the context of other deep millimeter observations of free-floating and companion objects with substellar masses smaller than 20
M
Jup
and ages between approximately one and a few hundred million years. Only two very young (2–5.4 Myr) objects are detected out of a few tens of them. This implies that the disks around these very low-mass objects must have small masses, and possibly reduced sizes, in agreement with findings by other groups. If debris disks around substellar objects scale down (in mass and size) in a similar manner as protoplanetary disks do, millimeter observations of moderately young brown dwarfs and planets must be at least two orders of magnitude deeper to be able to detect and characterize their surrounding debris disks.
The origin of the very red optical and infrared colours of intermediate-age (~10-500 Myr) L-type dwarfs remains unknown. It has been suggested that low-gravity atmospheres containing large amounts of ...dust may account for the observed reddish nature. We explored an alternative scenario by simulating debris disc around G 196-3 B, which is an L3 young brown dwarf with a mass of ~15 M sub( Jup) and an age in the interval 20-300 Myr. The best-fit solution to G 196-3 B's photometric spectral energy distribution from optical wavelengths through 24 ...m corresponds to the combination of an unreddened L3 atmosphere (T sub( eff) ... 1870 K) and a warm (...1280 K), narrow (...0.07-0.11 R...) debris disc located at very close distances (...0.12-0.20 R...) from the central brown dwarf. This putative, optically thick, dusty belt, whose presence is compatible with the relatively young system age, would have a mass greater than or equal to 7 x 10 super( -10) M... comprised of submicron/micron characteristic dusty particles with temperatures close to the sublimation threshold of silicates. Considering the derived global properties of the belt and the disc-to-brown dwarf mass ratio, the dusty ring around G 196-3 B may resemble the rings of Neptune and Jupiter, except for its high temperature and thick vertical height (...6 x 10 super( 3) km). Our inferred debris disc model is able to reproduce G 196-3 B's spectral energy distribution to a satisfactory level of achievement. (ProQuest: ... denotes formulae/symbols omitted.)
(ProQuest: ... denotes formulae and/or non-USASCII text omitted)We report on the discovery and characterization of the transiting planet K2-39b (EPIC 206247743b). With an orbital period of 4.6 days, ...it is the shortest-period planet orbiting a subgiant star known to date. Such planets are rare, with only a handful of known cases. The reason for this is poorly understood but may reflect differences in planet occurrence around the relatively high-mass stars that have been surveyed, or may be the result of tidal destruction of such planets. K2-39 (EPIC 206247743) is an evolved star with a spectroscopically derived stellar radius and mass of ... and ... , respectively, and a very close-in transiting planet, with a/Rlow * = 3.4. Radial velocity (RV) follow-up using the HARPS, FIES, and PFS instruments leads to a planetary mass of ... In combination with a radius measurement of 8.3 + or - 1.1 R+ in circle, this results in a mean planetary density of ... g cm super(-3). We furthermore discover a long-term RV trend, which may be caused by a long-period planet or stellar companion. Because K2-39b has a short orbital period, its existence makes it seem unlikely that tidal destruction is wholly responsible for the differences in planet populations around subgiant and main-sequence stars. Future monitoring of the transits of this system may enable the detection of period decay and constrain the tidal dissipation rates of subgiant stars.
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