Predicted microlensing events from analysis of Gaia Data Release 2

Context. Astrometric microlensing can be used to make precise measurements of the masses of lens stars that are independent of their assumed internal physics. Such direct mass measurements, obtained purely by observing the gravitational effects of the stars on external objects, are crucial for validating theoretical stellar models. Specifically, astrometric microlensing provides a channel to direct mass measurements of single stars for which so few measurements exist. Microlensing events that also exhibit a detectable photometric signature provide even stronger lens mass constraints. Aims. I use the astrometric solutions and photometric measurements of ~1.7 billion stars provided by Gaia Data Release 2 (GDR2) to predict microlensing events during the nominal Gaia mission and beyond. This will enable astronomers to observe the entirety of each event, including the peak, with appropriate observing resources. The data collected will allow precise lens mass measurements for white dwarfs and low-mass main sequence stars (K and M dwarfs) helping to constrain stellar evolutionary models. Methods. I search for source-lens pairs in GDR2 that could potentially lead to microlensing events between 25th July 2014 and 25th July 2026. I estimate the lens masses using GDR2 photometry and parallaxes, and appropriate model stellar isochrones. Combined with the source and lens parallax measurements from GDR2, this allows the Einstein ring radius to be computed for each source-lens pair. By considering the source and lens paths on the sky, I calculate the microlensing signals that are to be expected. Results. I present a list of 76 predicted microlensing events. Nine and five astrometric events will be caused by the white dwarf stars LAWD 37 and Stein 2051 B, respectively. A further nine events will exhibit detectable photometric and astrometric signatures. Of the remaining events, ten will exhibit astrometric signals with peak amplitudes above 0.5 mas, while the rest are low-amplitude astrometric events with peak amplitudes between 0.131 and 0.5 mas. Five and two events will reach their peaks during 2018 and 2019, respectively. Five of the photometric events have the potential to evolve into high-magnification events, which may also probe for planetary companions to the lenses.

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White Dwarf Stars

International Journal of Modern Physics Conference Series ◽

10.1142/s2010194517600230 ◽

2017 ◽

Vol 45 ◽

pp. 1760023

Author(s):

S. O. Kepler ◽

Alejandra Daniela Romero ◽

Ingrid Pelisoli ◽

Gustavo Ourique

Keyword(s):

White Dwarf ◽

Final Stage ◽

White Dwarfs ◽

Sloan Digital Sky Survey ◽

Dwarf Stars ◽

Multiple Systems ◽

White Dwarf Stars ◽

Sky Survey ◽

Data Release ◽

Low Mass

White dwarf stars are the final stage of most stars, born single or in multiple systems. We discuss the identification, magnetic fields, and mass distribution for white dwarfs detected from spectra obtained by the Sloan Digital Sky Survey up to Data Release 13 in 2016, which lead to the increase in the number of spectroscopically identified white dwarf stars from 5[Formula: see text]000 to 39[Formula: see text]000. This number includes only white dwarf stars with [Formula: see text], i.e., excluding the Extremely Low Mass white dwarfs, which are necessarily the byproduct of stellar interaction.

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Gaia Data Release 2 catalogue of extremely low-mass white dwarf candidates

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1876 ◽

2019 ◽

Vol 488 (2) ◽

pp. 2892-2903 ◽

Cited By ~ 12

Author(s):

Ingrid Pelisoli ◽

Joris Vos

Keyword(s):

White Dwarf ◽

Theoretical Models ◽

Dwarf Stars ◽

Single Star ◽

Common Envelope ◽

White Dwarf Stars ◽

Final Sample ◽

Data Release ◽

Low Mass ◽

Binary Evolution

ABSTRACT Extremely low-mass white dwarf stars (ELMs) are M < 0.3 M⊙ helium-core white dwarfs born either as a result of a common-envelope phase or after a stable Roche lobe overflow episode in a multiple system. The Universe is not old enough for ELMs to have formed through single-star evolution channels. As remnants of binary evolution, ELMs can shed light onto the poorly understood phase of common-envelope evolution and provide constraints to the physics of mass accretion. Most known ELMs will merge in less than a Hubble time, providing an important contribution to the signal to be detected by upcoming space-based gravitational wave detectors. There are currently less than 150 known ELMs; most were selected by colour, focusing on hot objects, in a magnitude-limited survey of the Northern hemisphere only. Recent theoretical models have predicted a much larger space density for ELMs than estimated observationally based on this limited sample. In order to perform meaningful comparisons with theoretical models and test their predictions, a larger well-defined sample is required. In this work, we present a catalogue of ELM candidates selected from the second data release of Gaia (DR2). We have used predictions from theoretical models and analysed the properties of the known sample to map the space spanned by ELMs in the Gaia Hertzsprung–Russell diagram. Defining a set of colour cuts and quality flags, we have obtained a final sample of 5762 ELM candidates down to Teff ≈ 5000 K.

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Comparing the asteroseismic properties of pulsating pre-extremely low mass white dwarf and δ Scuti stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731808 ◽

2018 ◽

Vol 616 ◽

pp. A80 ◽

Cited By ~ 2

Author(s):

Julieta P. Sánchez Arias ◽

Alejandra D. Romero ◽

Alejandro H. Córsico ◽

Ingrid Pelisoli ◽

Victoria Antoci ◽

...

Keyword(s):

White Dwarf ◽

Variable Stars ◽

Period Change ◽

Period Range ◽

Ionization Zone ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Low Mass ◽

The Mean ◽

Scuti Stars

Context. Pulsating extremely low-mass pre-white dwarf stars (pre-ELMV), with masses between ~0.15 M⊙ and ~0.30 M⊙, constitute a new class of variable stars showing g- and possibly p-mode pulsations with periods between 320 and 6000 s (frequencies between 14.4 and 270 c/d), driven by the κ mechanism operating in the second He ionization zone. On the other hand, main sequence δ Scuti stars, with masses between 1.2 and 2.5 M⊙, pulsate in low-order g and p modes with periods in the range [700–28 800] s (frequencies in the range [3–123] c/d), driven by the κ mechanism operating in the He II ionization zone and the turbulent pressure acting in the HI ionization layer. Interestingly enough, the instability strips of pre-ELM white dwarf and δ Scuti stars nearly overlap in the Teff vs. log g diagram, leading to a degeneracy when spectroscopy is the only tool to classify the stars and pulsation periods only are considered. Aims. Pre-ELM white dwarf and δ Scuti stars are in very different stages of evolution and therefore their internal structure is very distinct. This is mirrored in their pulsational behavior, thus employing asteroseismology should allow us to distinguish between these groups of stars despite their similar atmospheric parameters. Methods. We have employed adiabatic and non-adiabatic pulsation spectra for models of pre-ELM white dwarfs and δ Scuti stars, and compare their pulsation periods, period spacings, and rates of period change. Results. Unsurprisingly, we found substantial differences in the period spacing of δ Scuti and pre-ELM white dwarf models. Even when the same period range is observed in both classes of pulsating stars, the modes have distinctive signature in the period spacing and period difference values. For instance, the mean period difference of p-modes of consecutive radial orders for δ Scuti model are at least four times longer than the mean period spacing for the pre-ELM white dwarf model in the period range [2000–4600] s (frequency range [18.78–43.6] c/d). In addition, the rate of period change is two orders of magnitudes larger for the pre-ELM white dwarfs compared to δ Scuti stars. In addition, we also report the discovery of a new variable star, SDSSJ075738.94+144827.50, located in the region of the Teff versus log g diagram where these two kind of stars coexist. Conclusions.The characteristic spacing between modes of consecutive radial orders (p as well as g modes) and the large differences found in the rates of period change for δ Scuti and pre-ELM white dwarf stars suggest that asteroseismology can be employed to discriminate between these two groups of variable stars. Furthermore, we found that SDSSJ075738.94+144827.50 exhibits a period difference between p modes characteristic of a δ Sct star, assuming consecutive radial order for the observed periods.

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Asteroseismological Probing of the Thermal Evolution of White Dwarf Stars

Highlights of Astronomy ◽

10.1017/s1539299600009916 ◽

1992 ◽

Vol 9 ◽

pp. 643-645

Author(s):

G. Fontaine ◽

F. Wesemael

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

Main Sequence ◽

Thermal Evolution ◽

Sequence Evolution ◽

Helium Burning ◽

Dwarf Stars ◽

White Dwarf Stars ◽

The Galaxy ◽

Low Mass

AbstractIt is generally believed that the immediate progenitors of most white dwarfs are nuclei of planetary nebulae, themselves the products of intermediate- and low-mass main sequence evolution. Stars that begin their lifes with masses less than about 7-8 M⊙ (i.e., the vast majority of them) are expected to become white dwarfs. Among those which have already had the time to become white dwarfs since the formation of the Galaxy, a majority have burnt hydrogen and helium in their interiors. Consequently, most of the mass of a typical white dwarf is contained in a core made of the products of helium burning, mostly carbon and oxygen. The exact proportions of C and 0 are unknown because of uncertainties in the nuclear rates of helium burning.

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Molecular Opacities in Cool Dwarf Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100021242 ◽

1994 ◽

Vol 146 ◽

pp. 61-70

Author(s):

James Liebert

Keyword(s):

Main Sequence ◽

Galactic Disk ◽

Small Radius ◽

The Sun ◽

M Dwarfs ◽

Hydrogen Burning ◽

Dwarf Stars ◽

Low Mass ◽

High Space ◽

Lower Luminosity

The term dwarf stars identifies objects of small radius in the Hertzsprung-Russell (H-R) Diagram, but encompasses more than one phase of stellar evolution. The M dwarfs (type dM) populate the main sequence at the low mass end; these are the coolest core hydrogen-burning stars. They belong generally to the Galactic disk, or Population I, have relatively small space motions with respect to the Sun, and have similar metallicities to the Sun (although perhaps only within a factor of several). In particular, this means that the abundance of oxygen is always greater than that of carbon. The M subdwarfs (sdM) are the Population II counterparts, showing low metallicities and high space motions. Because they have smaller radii, they define a main sequence at lower luminosity than the M dwarfs for a given temperature. Hence the term subdwarf.

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Cool Companions to White Dwarf Stars from the Two Micron All Sky Survey All Sky Data Release

The Astronomical Journal ◽

10.1086/517878 ◽

2007 ◽

Vol 134 (1) ◽

pp. 26-42 ◽

Cited By ~ 38

Author(s):

D. W. Hoard ◽

S. Wachter ◽

Laura K. Sturch ◽

Allison M. Widhalm ◽

Kevin P. Weiler ◽

...

Keyword(s):

White Dwarf ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Sky Survey ◽

Data Release

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The seismic properties of low-mass He-core white dwarf stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201220114 ◽

2012 ◽

Vol 547 ◽

pp. A96 ◽

Cited By ~ 30

Author(s):

A. H. Córsico ◽

A. D. Romero ◽

L. G. Althaus ◽

J. J. Hermes

Keyword(s):

White Dwarf ◽

Dwarf Stars ◽

Seismic Properties ◽

White Dwarf Stars ◽

Low Mass

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White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz960 ◽

2019 ◽

Vol 486 (2) ◽

pp. 2169-2183 ◽

Cited By ~ 24

Author(s):

S O Kepler ◽

Ingrid Pelisoli ◽

Detlev Koester ◽

Nicole Reindl ◽

Stephan Geier ◽

...

Keyword(s):

Survey Data ◽

White Dwarf ◽

White Dwarfs ◽

Main Sequence ◽

Sloan Digital Sky Survey ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Sky Survey ◽

Data Release ◽

Hot Subdwarfs

ABSTRACT White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with 3σ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain Teff, log g, and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf stars with continuum spectra (DCs). We find 15 793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below $T_\mathrm{eff}\simeq 10\, 000$ K; most are likely low-mass metal-poor main-sequence stars, but some could be the result of interacting binary evolution.

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Exoplanet characterisation in the longest known resonant chain: the K2-138 system seen by HARPS

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936267 ◽

2019 ◽

Vol 631 ◽

pp. A90 ◽

Cited By ~ 4

Author(s):

T. A. Lopez ◽

S. C. C. Barros ◽

A. Santerne ◽

M. Deleuil ◽

V. Adibekyan ◽

...

Keyword(s):

Bayesian Analysis ◽

Radial Velocity ◽

Bright Star ◽

Planetary Formation ◽

Mass Measurements ◽

Multiple Systems ◽

Upper Limits ◽

Low Mass ◽

Formation And Evolution ◽

The Masses

The detection of low-mass transiting exoplanets in multiple systems brings new constraints to planetary formation and evolution processes and challenges the current planet formation theories. Nevertheless, only a mere fraction of the small planets detected by Kepler and K2 have precise mass measurements, which are mandatory to constrain their composition. We aim to characterise the planets that orbit the relatively bright star K2-138. This system is dynamically particular as it presents the longest chain known to date of planets close to the 3:2 resonance. We obtained 215 HARPS spectra from which we derived the radial-velocity variations of K2-138. Via a joint Bayesian analysis of both the K2 photometry and HARPS radial-velocities (RVs), we constrained the parameters of the six planets in orbit. The masses of the four inner planets, from b to e, are 3.1, 6.3, 7.9, and 13.0 M⊕ with a precision of 34, 20, 18, and 15%, respectively. The bulk densities are 4.9, 2.8, 3.2, and 1.8 g cm−3, ranging from Earth to Neptune-like values. For planets f and g, we report upper limits. Finally, we predict transit timing variations of the order two to six minutes from the masses derived. Given its peculiar dynamics, K2-138 is an ideal target for transit timing variation (TTV) measurements from space with the upcoming CHaracterizing ExOPlanet Satellite (CHEOPS) to study this highly-packed system and compare TTV and RV masses.

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