skip to main content

Attention:

The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 11:00 PM ET on Friday, April 12 until 2:00 AM ET on Saturday, April 13 due to maintenance. We apologize for the inconvenience.


Search for: All records

Creators/Authors contains: "Tkachenko, A."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. null (Ed.)
    Luminous hot stars ( M K s  ≲ 0 mag and T eff  ≳ 8000 K) dominate the stellar energy input to the interstellar medium throughout cosmological time, are used as laboratories to test theories of stellar evolution and multiplicity, and serve as luminous tracers of star formation in the Milky Way and other galaxies. Massive stars occupy well-defined loci in colour–colour and colour–magnitude spaces, enabling selection based on the combination of Gaia EDR3 astrometry and photometry and 2MASS photometry, even in the presence of substantive dust extinction. In this paper we devise an all-sky sample of such luminous OBA-type stars, which was designed to be complete rather than very pure, providing targets for spectroscopic follow-up with the SDSS-V survey. To estimate the purity and completeness of our catalogue, we derive stellar parameters for the stars in common with LAMOST DR6 and we compare the sample to other O and B-type star catalogues. We estimate ‘astro-kinematic’ distances by combining parallaxes and proper motions with a model for the expected velocity and density distribution of young stars; we show that this adds useful constraints on the distances and therefore luminosities of the stars. With these distances we map the spatial distribution of a more stringently selected subsample across the Galactic disc, and find it to be highly structured, with distinct over- and under-densities. The most evident over-densities can be associated with the presumed spiral arms of the Milky Way, in particular the Sagittarius-Carina and Scutum-Centaurus arms. Yet, the spatial picture of the Milky Way’s young disc structure emerging in this study is complex, and suggests that most young stars in our Galaxy ( t age  <  t dyn ) are not neatly organised into distinct spiral arms. The combination of the comprehensive spectroscopy to come from SDSS-V (yielding velocities, ages, etc.) with future Gaia data releases will be crucial in order to reveal the dynamical nature of the spiral arms themselves. 
    more » « less
  2. null (Ed.)
    ABSTRACT We report on the discovery and validation of a two-planet system around a bright (V  = 8.85 mag) early G dwarf (1.43  R⊙, 1.15  M⊙, TOI 2319) using data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Three transit events from two planets were detected by citizen scientists in the month-long TESS light curve (sector 25), as part of the Planet Hunters TESS project. Modelling of the transits yields an orbital period of $11.6264 _{ - 0.0025 } ^ { + 0.0022 }$ d and radius of $3.41 _{ - 0.12 } ^ { + 0.14 }$ R⊕ for the inner planet, and a period in the range 19.26–35 d and a radius of $5.83 _{ - 0.14 } ^ { + 0.14 }$ R⊕ for the outer planet, which was only seen to transit once. Each signal was independently statistically validated, taking into consideration the TESS light curve as well as the ground-based spectroscopic follow-up observations. Radial velocities from HARPS-N and EXPRES yield a tentative detection of planet b, whose mass we estimate to be $11.56 _{ - 6.14 } ^ { + 6.58 }$ M⊕, and allow us to place an upper limit of 27.5 M⊕ (99 per cent confidence) on the mass of planet c. Due to the brightness of the host star and the strong likelihood of an extended H/He atmosphere on both planets, this system offers excellent prospects for atmospheric characterization and comparative planetology. 
    more » « less
  3. ABSTRACT

    We present the first asteroseismic results for δ Scuti and γ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilize the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung–Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, γ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74 per cent for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young δ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler.

     
    more » « less