ABSTRACT 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 . 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 , this results in a mean planetary density of g cm−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.
Abstract
We present updated radial-velocity (RV) analyses of the AU Mic system. AU Mic is a young (22 Myr) early-M dwarf known to host two transiting planets—
P
b
∼ 8.46 days,
R
b
=
4.38
−
0.18
+
...0.18
R
⊕
,
P
c
∼ 18.86 days,
R
c
=
3.51
−
0.16
+
0.16
R
⊕
. With visible RVs from Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical echelle Spectrographs (CARMENES)-VIS, CHIRON, HARPS, HIRES, M
inerva
-Australis, and Tillinghast Reflector Echelle Spectrograph, as well as near-infrared (NIR) RVs from CARMENES-NIR, CSHELL, IRD, iSHELL, NIRSPEC, and SPIRou, we provide a 5
σ
upper limit to the mass of AU Mic c of
M
c
≤ 20.13
M
⊕
and present a refined mass of AU Mic b of
M
b
=
20.12
−
1.57
+
1.72
M
⊕
. Used in our analyses is a new RV modeling toolkit to exploit the wavelength dependence of stellar activity present in our RVs via wavelength-dependent Gaussian processes. By obtaining near-simultaneous visible and near-infrared RVs, we also compute the temporal evolution of RV “color” and introduce a regressional method to aid in isolating Keplerian from stellar activity signals when modeling RVs in future works. Using a multiwavelength Gaussian process model, we demonstrate the ability to recover injected planets at 5
σ
significance with semi-amplitudes down to ≈10 m s
−1
with a known ephemeris, more than an order of magnitude below the stellar activity amplitude. However, we find that the accuracy of the recovered semi-amplitudes is ∼50% for such signals with our model.
ABSTRACT In a search for common proper motion companions using the VISTA Hemisphere Survey (VHS) and the 2MASS catalogs we have identified a very red ( 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 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 0.5 and L7 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 = 78.8 6.4 mas, which translates into a distance of 12.7 1.0 pc; the pair thus has a projected physical separation of 102 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 0.10, and inferred a mass of 73 MJup at the boundary between stars and brown dwarfs and an effective temperature of 2620 140 K. For the companion we obtained a luminosity of and a mass of , placing it near the deuterium-burning mass limit. The effective temperature derived from evolutionary models is 880 K, about 400-700 K cooler than the temperature expected for field late-L dwarfs.
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 (\(\lambda_{c}\)=8.7\(\mu\)m, ...\(\Delta\lambda\)=1.1\(\mu\)m) using the CanariCam instrument at the 10.4 m Gran Telescopio Canarias (GTC). We imaged the 25 nearest stellar systems within 5 pc of the Sun at declinations \(\delta > -25^{\circ}\) (at least half have planets from radial velocity), reaching a mean detection limit of 11.3\(\pm\)0.2 mag (1.5 mJy) in the Si-2 8.7\(\mu\)m band over a range of angular separations from 1 to 10 arcsec. This would have allowed us to uncover substellar companions at projected orbital separations between \(\sim\)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 down to the deuterium-burning mass limit for objects with ages \(<\)1 Gyr. From the non-detection 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, main components of our sample, we found with a 90% confidence level that less than 20% of these low-mass stars have L and T-type brown dwarf companions with \(m \gtrsim 30 M_{Jup}\) and \(T_{eff} \gtrsim\) 600 K at \(\sim\)3.5--35 au projected orbital separations.
Double-lined spectroscopic binaries (SB2) allow us to determine a lower limit of the masses of their components directly to test stellar models. In this work, our aim is to derive the orbital and ...physical parameters of GJ1284, a young SB2. We also revise the membership of this system and its two wide co-moving companions, GJ898 and GJ897AB, 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. We determine the orbit of the GJ1284 system alongside its systemic velocity from high resolution spectra. Additionally, we use TESS photometry to derive the rotational period of the GJ1284 and its two wide companions. GJ1284 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. The revised systemic velocity of \( \gamma = 0.84 \pm 0.14\,\mathrm{km\,s}^{-1} \) 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. 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 GJ1284, 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.
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 temperature 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 as 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 OSIRIS at the 10.4-m Gran Telescopio Canarias are presented. The Li~I 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 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. A LDB age of 650\(\pm\)70 Myr for the Hyades provides a consistent description of our data using a set of state-of-the-art evolutionary models for brown dwarfs calculated by \citet{2015A&A..577..42}.
(Abridged) Our aim is to identify analogues of gas giant planets, as companions at wide separations of very young stars. To identify these objects, we cross correlated the Visible and Infrared Survey ...Telescope for Astronomy (VISTA) Hemisphere Survey (VHS) and the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Clusters Survey (UKIDSS GCS) catalogues to search for common proper motion companions to 1195 already known members of Upper Scorpius (USco; age \(\sim\)5-10 Myr, distance \(\sim\)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 membership to the association. We also acquired adaptive optics images of the primaries and secondaries to search for signs of binarity and close companions. 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. The comparison with theoretical evolutionary models gives estimated masses of 0.015\(\pm\)0.002 and 0.014\(\pm\)0.002 M\(_{\odot}\), with temperatures of 2270\(\pm\)90 and 2240\(\pm\)100 K, respectively. The physical separations between the components of both systems are 2880\(\pm\)20 and 3500\(\pm\)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%.
We present updated radial-velocity (RV) analyses of the AU Mic system. AU Mic is a young (22 Myr) early M dwarf known to host two transiting planets - \(P_{b}\sim8.46\) days, ...\(R_{b}=4.38_{-0.18}^{+0.18}\ R_{\oplus}\), \(P_{c}\sim18.86\) days, \(R_{c}=3.51_{-0.16}^{+0.16}\ R_{\oplus}\). With visible RVs from CARMENES-VIS, CHIRON, HARPS, HIRES, {\sc {\textsc{Minerva}}}-Australis, and TRES, as well as near-infrared (NIR) RVs from CARMENES-NIR, CSHELL, IRD, iSHELL, NIRSPEC, and SPIRou, we provide a \(5\sigma\) upper limit to the mass of AU Mic c of \(M_{c}\leq20.13\ M_{\oplus}\) and present a refined mass of AU Mic b of \(M_{b}=20.12_{-1.57}^{+1.72}\ M_{\oplus}\). Used in our analyses is a new RV modeling toolkit to exploit the wavelength dependence of stellar activity present in our RVs via wavelength-dependent Gaussian processes. By obtaining near-simultaneous visible and near-infrared RVs, we also compute the temporal evolution of RV-``color'' and introduce a regressional method to aid in isolating Keplerian from stellar activity signals when modeling RVs in future works. Using a multi-wavelength Gaussian process model, we demonstrate the ability to recover injected planets at \(5\sigma\) significance with semi-amplitudes down to \(\approx\) 10\,m\,s\(^{-1}\) with a known ephemeris, more than an order of magnitude below the stellar activity amplitude. However, we find that the accuracy of the recovered semi-amplitudes is \(\sim\)50\% for such signals with our model.
We report the discovery of TOI-2285b, a sub-Neptune-sized planet transiting a nearby (42 pc) M dwarf with a period of 27.3 days. We identified the transit signal from the Transiting Exoplanet Survey ...Satellite photometric data, which we confirmed with ground-based photometric observations using the multiband imagers MuSCAT2 and MuSCAT3. Combining these data with other follow-up observations including high resolution spectroscopy with the Tillinghast Reflector Echelle Spectrograph, high resolution imaging with the SPeckle Polarimeter, and radial velocity (RV) measurements with the InfraRed Doppler instrument, we find that the planet has a radius of 1.74 \(\pm\) 0.08 \(R_\oplus\), a mass of \(<\) 19.5 \(M_\oplus\) (95\% c.l.), and an insolation flux of 1.54 \(\pm\) 0.14 times that of the Earth. Although the planet resides just outside the habitable zone for a rocky planet, if the planet harbors an H\(_2\)O layer under a hydrogen-rich atmosphere, then liquid water could exist on the surface of the H\(_2\)O layer depending on the planetary mass and water mass fraction. The bright host star in the near infrared (\(K_s=9.0\)) makes this planet an excellent target for further RV and atmospheric observations to improve our understanding on the composition, formation, and habitability of sub-Neptune-sized planets.
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