We present new photometric and astrometric data available for S Ori 70 and 73, the two T-type planetary-mass member candidates in the σ Orionis cluster (~3 ± 2 Myr, d~360 pc). S Ori 70 (J ~ 19.9 mag) ...has a spectral type of T5.5 ± 1.0 measured from published near-infrared spectra, while no spectroscopic data are available for S Ori 73 (J ~ 21 mag). We estimate the spectral type of S Ori 73 by using J, H, and CH4off (λc=1.575 μm, Δλ=0.112 μm) photometry and comparing the H-CH4off index of S Ori 73 with the colors of field stars and brown dwarfs of spectral types in the range F to late T. The locations of S Ori 70 and 73 in the J-H vs H-CH4off color-color diagram are consistent with spectral types T8 ± 1 and T4 ± 1, respectively. Proper motion measurements of the two sources are larger than the motion of the central σ Ori star, making their cluster membership somehow uncertain.
We report photometric and spectroscopic observations and analysis of the 2019 superoutburst of TCP J21040470+4631129. This object showed a 9-mag superoutburst with early superhumps and ordinary ...superhumps, which are the features of WZ Sge-type dwarf novae. Five rebrightenings were observed after the main superoutburst. The spectra during the post-superoutburst stage showed the Balmer, He I and possible sodium doublet features. The mass ratio is derived as 0.0880(9) from the period of the superhump. During the third and fifth rebrightenings, growing superhumps and superoutbursts were observed, which have never been detected during a rebrightening phase among WZ Sge-type dwarf novae with multiple rebrightenings. To induce a superoutburst during the brightening phase, the accretion disk was needed to expand beyond the 3:1 resonance radius of the system again after the main superoutburst. These peculiar phenomena can be explained by the enhanced viscosity and large radius of the disk suggested by the higher luminosity and the presence of late-stage superhumps during the post-superoutburst stage, plus by more mass supply from the cool mass reservoir and/or from the secondary because of the enhanced mass transfer than those of other WZ Sge-type dwarf novae.
The origin of the very red optical and infrared colours of intermediate-age (\(\sim\)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 protoplanetary and debris discs around G196-3B, which is an L3 young brown dwarf with a mass of \(\sim 15\) \(M_{\rm Jup}\) and an age in the interval 20 - 300 Myr. The best-fit solution to G196-3B's photometric spectral energy distribution from optical wavelengths through 24 \(\mu\)m corresponds to the combination of an unreddened L3 atmosphere (\(T_{\rm eff} \approx 1870\)~K) and a warm (\(\approx\) 1280 K), narrow (\(\approx\) 0.07 - 0.11 R\(_{\odot}\)) debris disc located at very close distances (\(\approx\) 0.12 - 0.20 R\(_{\odot}\)) 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 \(\ge 7\times 10^{-10}\) M\(_{\oplus}\) comprised of sub-micron/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 G196-3B may resemble the rings of Neptune and Jupiter, except for its high temperature and thick vertical height (\(\approx 6 \times 10^3\) km). Our inferred debris disc model is able to reproduce G196-3B's spectral energy distribution to a satisfactory level of achievement.
Context. The TESS and PLATO missions are expected to find vast numbers of new transiting planet candidates. However, only a fraction of these candidates will be legitimate planets, and the candidate ...validation will require a significant amount of follow-up resources. Radial velocity follow-up can be carried out only for the most promising candidates around bright, slowly rotating, stars. Thus, before devoting RV resources to candidates, they need to be vetted using cheaper methods, and, in the cases for which an RV confirmation is not feasible, the candidate's true nature needs to be determined based on these alternative methods alone. Aims. We study the applicability of multicolour transit photometry in the validation of transiting planet candidates when the candidate signal arises from a real astrophysical source. We seek to answer how securely can we estimate the true uncontaminated star-planet radius ratio when the light curve may contain contamination from unresolved light sources inside the photometry aperture when combining multicolour transit observations with a physics-based contamination model. Methods. The study is based on simulations and ground-based transit observations. The analyses are carried out with a contamination model integrated into the PyTransit v2 transit modelling package, and the observations are carried out with the MuSCAT2 multicolour imager installed in the 1.5 m TCS in the Teide Observatory. Results. We show that multicolour transit photometry can be used to estimate the amount of flux contamination and the true radius ratio. Combining the true radius ratio with an estimate for the stellar radius yields the true absolute radius of the transiting object, which is a valuable quantity in statistical candidate validation, and enough in itself to validate a candidate whose radius falls below the theoretical lower limit for a brown dwarf.
Abstract
We report the discovery and characterization of TOI-1759 b, a temperate (400 K) sub-Neptune-sized exoplanet orbiting the M dwarf TOI-1759 (TIC 408636441). TOI-1759 b was observed by TESS to ...transit in Sectors 16, 17, and 24, with only one transit observed per sector, creating an ambiguity regarding the orbital period of the planet candidate. Ground-based photometric observations, combined with radial-velocity measurements obtained with the CARMENES spectrograph, confirm an actual period of 18.85019 ± 0.00014 days. A joint analysis of all available photometry and radial velocities reveals a radius of 3.17 ± 0.10
R
⊕
and a mass of 10.8 ± 1.5
M
⊕
. Combining this with the stellar properties derived for TOI-1759 (
R
⋆
= 0.597 ± 0.015
R
⊙
;
M
⋆
= 0.606 ± 0.020
M
⊙
;
T
eff
= 4065 ± 51 K), we compute a transmission spectroscopic metric (TSM) value of over 80 for the planet, making it a good target for transmission spectroscopy studies. TOI-1759 b is among the top five temperate, small exoplanets (
T
eq
< 500 K,
R
p
< 4
R
⊕
) with the highest TSM discovered to date. Two additional signals with periods of 80 days and >200 days seem to be present in our radial velocities. While our data suggest both could arise from stellar activity, the later signal’s source and periodicity are hard to pinpoint given the ∼200 days baseline of our radial-velocity campaign with CARMENES. Longer baseline radial-velocity campaigns should be performed in order to unveil the true nature of this long-period signal.
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.
K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet which was first discovered by the {\it K2} mission and later confirmed with {\it Spitzer Space Telescope} observations. ...With a radius of \(\sim 2 \, R_{\oplus}\) and an orbital period of \(\sim 33\) days, the planet lies in the temperate zone of its host star and receives stellar irradiation similar to 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 \(K_b \sim 3.5\) \mpersec\ and a mass of \(M_b \sim 9 \, M_{\oplus}\), yielding a bulk density around \(\rho_b \sim 4 \, \mathrm{g \,cm^{-3}}\). 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. \ktwo\ 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 {\it James Webb Space Telescope}.
Abstract We present new photometric and astrometric data available for S Ori 70 and 73, the two T-type planetary-mass member candidates in the σ Orionis cluster (~3 ± 2 Myr, d~360 pc). S Ori 70 (J ~ ...19.9 mag) has a spectral type of T5.5 ± 1.0 measured from published near-infrared spectra, while no spectroscopic data are available for S Ori 73 (J ~ 21 mag). We estimate the spectral type of S Ori 73 by using J, H, and CH4off (λc=1.575 μm, Δλ=0.112 μm) photometry and comparing the H-CH4off index of S Ori 73 with the colors of field stars and brown dwarfs of spectral types in the range F to late T. The locations of S Ori 70 and 73 in the J-H vs H-CH4off color-color diagram are consistent with spectral types T8 ± 1 and T4 ± 1, respectively. Proper motion measurements of the two sources are larger than the motion of the central σ Ori star, making their cluster membership somehow uncertain. PUBLICATION ABSTRACT
Abridged: We measured photometric and spectroscopic \(P_{\rm rot}\) for a large sample of nearby bright M dwarfs with spectral types from M0 to M9, as part of our continual effort to fully ...characterize the Guaranteed Time Observation programme stars of the CARMENES survey. We determine \(P_{\rm rot}\) for 129 stars. Combined with the literature, we tabulate \(P_{\rm rot}\) for 261 stars, or 75% of our sample. We evaluate the plausibility of all periods available for this sample by comparing them with activity signatures and checking for consistency between multiple measurements. We find that 166 of these stars have independent evidence that confirmed their \(P_{\rm rot}\). There are inconsistencies in 27 periods, which we classify as debated. A further 68 periods are identified as provisional detections that could benefit from independent verification. We provide an empirical relation for the \(P_{\rm rot}\) uncertainty as a function of the \(P_{\rm rot}\) value, based on the dispersion of the measurements. We show that published formal errors seem to be often underestimated for periods \(\gtrsim 10\) d. We highlight the importance of independent verification on \(P_{\rm rot}\) measurements, especially for inactive M dwarfs. We examine rotation-activity relations with emission in X-rays, H\(\alpha\), Ca II H & K, and surface magnetic field strengths. We find overall agreement with previous works, as well as tentative differences in the partially versus fully convective subsamples. We show \(P_{\rm rot}\) as a function of stellar mass, age, and galactic kinematics. With the notable exception of three transiting planet systems and TZ Ari, all known planet hosts in this sample have \(P_{\rm rot} \gtrsim 15\) d. This indicates that important limitations need to be overcome before the radial velocity technique can be routinely used to detect and study planets around young and active stars.
We present a deep I, Z photometric survey (Icompl = 22 mag) covering a total area of 1.12 deg2 in the σ Orionis cluster. Using I, I − Z color-magnitude diagrams, we have selected 165 photometric ...candidates (I = 16–24 mag), which correspond to a mass interval from 0.11 down to 0.006 M ⊙. Using J-band photometry, we have found that 120 of 147 candidates within the completeness of the survey ( ∼ 80%) follow the infrared photometric sequence of the cluster; we will consider them as likely members. Using J, H, K-band data from UKIDSS and mid-infrared data from IRAC/Spitzer, we have found that 6–11% of the bona fide candidates have near-infrared flux excesses, and about 34 ± 8% show mid-infrared flux excesses, probably related to the presence of disks. We have determined that the mass spectrum from 0.11 to 0.013 M ⊙ is a rising function that can be represented by a power-law distribution (dN/dm ∼ m− α) with an exponent α = 0. 4–1.1.
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