The sdA problem – I. Physical properties

  title={The sdA problem – I. Physical properties},
  author={Ingrid Pelisoli and S. O. Kepler and Detlev Koester},
  journal={Monthly Notices of the Royal Astronomical Society},
The so-called sdA stars are defined by having H-rich spectra and surface gravities similar to hot subdwarf stars, but effective temperature below the zero-age horizontal branch. Their evolutionary history is an enigma: their surface gravity is too high for main sequence stars, but too low for single evolution white dwarfs. They are most likely byproducts of binary evolution, including blue-stragglers, extremely-low mass white dwarf stars (ELMs) and their precursors (pre-ELMs). A small number of… 

The sdA problem – III. New extremely low-mass white dwarfs and their precursors fromGaiaastrometry

The physical nature of the sdA stars---cool hydrogen-rich objects with spectroscopic surface gravities intermediate between main sequence and canonical mass white dwarfs---has been elusive since they

The Formation of Subdwarf A-type Stars

Subdwarf A-type stars (sdAs) are objects that have hydrogen-rich spectra with surface gravity similar to that of hot subdwarf stars but effective temperature below the zero-age horizontal branch.

The McDonald Observatory search for pulsating sdA stars

Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values

The sdA problem – II. Photometric and spectroscopic follow-up

Subdwarf A star (sdA) is a spectral classification given to objects showing H-rich spectra and sub-main sequence surface gravities, but effective temperature lower than the zero-age horizontal

Searching for Low-mass Population III Stars Disguised as White Dwarfs

It is uncertain whether or not low-mass Population III stars ever existed. While limits on the number density of Population III stars with M * ≈ 0.8 M ⊙ have been derived, using Sloan Digital Sky

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

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

The ELM Survey. VIII. Ninety-eight Double White Dwarf Binaries

We present the final sample of 98 detached double white dwarf (WD) binaries found in the Extremely Low Mass (ELM) Survey, a spectroscopic survey targeting <0.3 M⊙ He-core WDs completed in the Sloan

The ELM Survey South. I. An Effective Search for Extremely Low Mass White Dwarfs

We begin the search for extremely low mass (M ≤ 0.3M⊙, ELM) white dwarfs (WDs) in the southern sky based on photometry from the VST ATLAS and SkyMapper surveys. We use a similar color selection

Pulsating white dwarfs: new insights

Stars are extremely important astronomical objects that constitute the pillars on which the Universe is built, and as such, their study has gained increasing interest over the years. White dwarf

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



The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution

What's the nature of sdA stars?

White dwarfs with log g lower than 7.0 are called Extremely Low Mass white dwarfs (ELMs). They were first found as companions to pulsars, then to other white dwarfs and main sequence stars (The ELM

Hot Subluminous Stars

Hot subluminous stars of spectral type B and O are core helium-burning stars at the blue end of the horizontal branch or have evolved even beyond that stage. Most hot subdwarf stars are chemically

The age of the Milky Way inner halo

Observations of newly formed white dwarf stars in the halo of the Milky Way are reported and a mathematical relation is developed that links this final stellar mass to the mass of their immediate progenitors, and therefore to the age of the parent population.


We describe new radial velocity and X-ray observations of extremely low-mass white dwarfs (ELM WDs, ∼0.2 M☉) in the Sloan Digital Sky Survey Data Release 4 and the MMT Hypervelocity Star survey. We


We analyze radial velocity observations of the 12 extremely low-mass (ELM), with ⩽0.25 M☉, white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the twelve WDs are binaries with orbital

Candidate hypervelocity stars of spectral type G and K revisited

Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy. When they were first discovered in 2005, dynamical ejection from the supermassive black hole (SMBH) in the Galactic Centre

The SDSS spectroscopic catalogue of white dwarf-main-sequence binaries: new identifications from DR 9-12

We present an updated version of the spectroscopic catalogue of white dwarf-main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS). We identify 938 WDMS binaries within the data

The field white dwarf mass distribution

We revisit the properties and astrophysical implications of the field white dwarf mass distribution in preparation of Gaia applications. Our study is based on the two samples with the best


We present new radial velocity and X-ray observations of extremely low mass (ELM, ∼0.2 M☉) white dwarf (WD) candidates in the Sloan Digital Sky Survey Data Release 7 area. We identify seven new