[Paper Review] sdA in SDSS DR12 are Overwhelmingly Not Extremely Low-Mass (ELM) White Dwarfs

J. J. Hermes, Boris Gaensicke+1 more|arXiv (Cornell University)|Feb 23, 2017

Stellar, planetary, and galactic studies3 references3 citations

TL;DR

This study analyzes 2,675 cool subdwarf A-type stars (sdA) from SDSS DR12, originally suspected to be extremely low-mass (ELM) white dwarfs. Using radial velocity variability, photometric colors, and proper motions, it finds that over 99% of sdA are not ELM white dwarfs, as they lack the short-period binary signatures required for ELM formation, suggesting they are likely A/F-type stars or other evolved objects with misclassified parameters.

ABSTRACT

In a search for new white dwarfs in DR12 of the Sloan Digital Sky Survey, Kepler et al. (2016) found atmospheric parameters for thousands of objects with effective temperatures below 20,000 K and surface gravities between 5.5 < log(g) < 6.5. They classified these objects as cool subdwarfs -- sdA -- and speculated that many may be extremely low-mass (ELM) white dwarfs (helium-core white dwarfs with masses below 0.3 Msun). We present evidence -- using radial velocities, photometric colors, and reduced proper motions -- that the vast majority (>99%) of these objects are unlikely to be ELM white dwarfs. Their true identity remains an interesting question.

Motivation & Objective

• To test the hypothesis that the sdA stars in SDSS DR12 are extremely low-mass (ELM) white dwarfs with masses <0.3 M☉.
• To investigate whether the sdA population exhibits radial velocity (RV) variability on short timescales, a key signature of close binary systems required for ELM white dwarf formation.
• To compare the photometric colors and reduced proper motions of sdA with known ELM white dwarfs to assess their physical similarity.
• To clarify the true nature of the sdA population, which has been misclassified as low-surface-gravity white dwarfs due to spectral contamination or low signal-to-noise.

Proposed method

• Radial velocity variability was measured using SDSS subspectra (three 15-minute exposures per observation) to detect short-period orbital motion.
• A statistical threshold of >3σ variability was applied to identify significant RV changes indicative of compact binaries.
• Photometric color-color diagrams (u-g, g-r, r-i, i-z) were used to compare the color distributions of sdA and known ELM white dwarfs.
• Reduced proper motions were calculated and compared to known ELM white dwarfs to assess spatial kinematics and infer radii.
• Known ELM white dwarfs with solved orbital parameters (n=30) were used as a benchmark for comparison.
• Objects with significant RV variability in a single night were prioritized, as this is characteristic of ultracompact binaries with periods <1 hr.

Experimental results

Research questions

• RQ1 Do the sdA stars in SDSS DR12 exhibit short-period radial velocity variability indicative of close binary companions, as expected for ELM white dwarfs?
• RQ2 How do the photometric colors of sdA compare to those of confirmed ELM white dwarfs in color-color space?
• RQ3 Do the reduced proper motions of sdA suggest compact stellar radii consistent with white dwarfs or larger radii typical of main-sequence stars?
• RQ4 What is the likelihood that the sdA population consists of misclassified A/F-type stars with artificially elevated surface gravities due to spectral contamination?
• RQ5 Could the sdA be remnants of merged ELM white dwarf binaries, and is this consistent with their kinematic and photometric properties?

Key findings

• Only 31 out of 2,518 sdA with usable subspectra (1.2%) show >3σ radial velocity variability, compared to 50% (15/30) of known ELM white dwarfs.
• Of the 31 variable sdA, only three show significant RV variability within a single night, and none exhibit the high-velocity amplitudes (>200 km s⁻¹) typical of ELM binaries.
• No sdA show RV variability on timescales of less than one hour, which is characteristic of the shortest-period ELM binaries (e.g., J0651+2844 with a 12.75-min period).
• The sdA exhibit significant scatter in color-color diagrams (u-g, g-r, r-i, i-z), unlike the tight sequence followed by known ELM white dwarfs.
• The majority of sdA have low reduced proper motions, inconsistent with the compact radii expected for ELM white dwarfs or their merger remnants.
• The study concludes that >99% of the sdA population are not ELM white dwarfs, and their true nature remains an open question, possibly including A/F-type stars with spectral misclassification or low-metallicity halo stars.