arXiv subjects

Stefan Y. Stefanov

Publications and source records attributed to Stefan Y. Stefanov.

4 recordsLinked to original sources

arXiv2026

A Catalog of Homogeneously Derived Stellar Parameters for Spectroscopic Survey Stars

Uniformly derived stellar parameters are vital for exoplanet demographic studies because they directly influence the inferred planetary masses, radii, and bulk densities. This study presents a new homogeneous catalog of physical stellar parameters for 5533 single stars observed by the HARPS, HIRES, and CARMENES radial-velocity (RV) surveys. Stellar parameters are determined using a Bayesian framework with two independent sets of stellar evolutionary models: MIST and PARSEC, using published spectroscopic parameter estimates and catalog values as priors. The resulting stellar effective temperatures, masses, radii, and surface gravities are compared and evaluated for consistency with stellar parameters listed in major exoplanet catalogs. While our estimates show consistency with those listed in external catalogs, we identify method-dependent differences in stellar masses for low-mass and pre-main-sequence stars, as well as G and K giants, where isochrone-based solutions may be affected by age-mass degeneracies. For low-mass stars such as M-dwarfs, the catalog provides masses derived from established mass-luminosity empirical relations, which tend to be more reliable. This catalog provides the largest uniformly derived stellar reference sample for Doppler survey targets and also illustrates the applicability and limitations of stellar-parameter homogenisation methods for future RV exoplanet demographics studies.

astro-ph.SR↗

arXiv2026

A distant brown dwarf coplanar to a warm Jupiter and a hot super-Earth

In transiting planetary systems, in which planetary sizes are accurately determined from transit observations, the presence of transit-timing variations (TTVs), especially when combined with radial velocity (RV) data, provides powerful constraints on masses and orbital eccentricities. Together, these measurements offer crucial insights into system architecture, formation mechanisms and dynamical evolution. We present long-term RV and transit/TTV monitoring of the relatively young star (age approximately 1 Gyr) TOI-201, revealing an exceptional multi-planet system composed of a hot super-Earth (SE) size planet transiting every 5.8 days, a warm Jupiter (WJ) on a 53-day orbit and an eccentric (e = 0.62) low-mass brown dwarf (BD) on an approximately 8-year orbit, with an estimated mass of about 16 Jupiter masses. The BD is the longest-period transiting substellar object ever characterized by means of RVs and the only one known to be coplanar with inner planets. The architecture of this system suggests that the SE was formed isolated and in the innermost region of the gaseous disk. On the other hand, the orbital configuration of the outer companions suggests a nearly in situ formation of both objects, with the WJ forming in a dense inner disk. Alternatively, the BD might have formed farther out and migrated inward, while increasing its eccentricity owing to interactions with the disk.

astro-ph.EP↗

arXiv2026

Long-period magnetic activity in the K dwarf GJ 1137 and a new super-Earth on a 9-day orbit

Aims: We investigate long-term radial velocity (RV) variability in the K-dwarf star GJ 1137 (HD 93083, HIP52521), a known Saturn-mass exoplanet host, and assess the role of stellar activity in shaping the observed signals. Methods: We analyse 13 years of archival high-precision spectroscopic observations obtained with the High Accuracy Radial velocity Planet Searcher spectrograph (HARPS). We performed an extensive spectroscopic analysis of the stellar activity indicators and applied an RV modelling approach, incorporating Keplerian fits, Gaussian process regression as a proxy for stellar activity, and other stellar activity diagnostics. Furthermore, we refined the orbital parameters and the minimum mass of the known exoplanet GJ 1137 b and searched for additional planetary candidates in the system. Results: We detect a long-period RV signal that, if interpreted as planetary, would suggest the presence of a Jovian analogue companion. However, our spectroscopic activity analysis provides strong evidence that this variability is induced by the star's long-term magnetic cycle ( Pcyc = 5870+(480)-(350) days) rather than by an orbiting planet. The signal is detected in both full width at half maximum (FWHM) of the crosscorrelation function and the chromospheric activity index log R'Hk. We measure the stellar rotation period to Prot = 32.3+(1.2)-(1.3) d and identify a significant short-period RV signal, which we attribute to a Super Earth with a period of 9.6412+(12)-(11) d and a minimum mass of 5.12+(0.70)-(0.69) Earth masses, making GJ 1137 a multiple-planet system.

astro-ph.EP↗

arXiv2023

Tilted discs in six poorly studied cataclysmic variables

In this work, we search for negative superhumps (nSHs) in poorly studied cataclysmic variables using TESS data. We find three eclipsing binaries with nSH signatures: HBHA 4204-09, Gaia DR3 5931071148325476992, and SDSS J090113.51+144704.6. The last one exhibits IW And-like behaviour in archival ZTF data, and appears to have shallow, grazing eclipses. In addition, we detect nSH signatures in two non-eclipsing systems: KQ Mon and Gaia DR3 4684361817175293440, by identifying the orbital period from the superorbital-dependent irradiation of the secondary. We discover nSH signatures in one more system, [PK2008] HalphaJ103959, by using an orbital period from another work. An improved mass ratio - nSH deficit relation $q(\varepsilon_-)$ is suggested by us, which agrees with independent measurements on nova-like variables. With this relation, we estimate the mass ratios of all systems in our sample, and determine the orbital inclinations for the three that are eclipsing. All systems with discovered nSHs in this work are excellent targets for follow-up spectroscopic studies.

astro-ph.SR↗