[Paper Review] Hot subdwarf binaries from the MUCHFUSS project - Analysis of 12 new systems and a study of the short-period binary population

Thomas Kupfer, S. Geier+7 more|Keele Research Repository (Keele University)|Jan 15, 2015

Stellar, planetary, and galactic studies120 references47 citations

TL;DR

This study presents orbital and atmospheric parameters for 12 new hot subdwarf B star binaries discovered in the MUCHFUSS survey, identifying three with likely white dwarf companions, including a rare low-mass helium-core white dwarf. It reveals a bimodal minimum companion mass distribution peaking at 0.1 M⊙ (dM/substellar) and 0.4 M⊙ (white dwarfs), with 16 systems expected to become cataclysmic variables and 12 white dwarf binaries destined to merge within a Hubble time, including potential Type Ia supernova progenitors and AM CVn systems.

ABSTRACT

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M>1.0 M$_\odot$), neutron stars, or stellar-mass black holes. We present orbital and atmospheric parameters and put constraints on the nature of the companions of 12 close hot subdwarf B star (sdB) binaries found in the course of the MUCHFUSS project. The systems show periods between 0.14 and 7.4 days. Three systems most likely have white dwarf companions. SDSS J083006.17+475150.3 is likely to be a rare example of a low-mass helium-core white dwarf. SDSS J095101.28+034757.0 shows an excess in the infrared that probably originates from a third companion in a wide orbit. SDSS J113241.58-063652.8 is the first helium deficient sdO star with a confirmed close companion. This study brings to 142 the number of sdB binaries with orbital periods of less than 30 days and with measured mass functions. We present an analysis of the minimum companion mass distribution and show that it is bimodal. One peak around 0.1 M$_\odot$ corresponds to the low-mass main sequence and substellar companions. The other peak around 0.4 M$_\odot$ corresponds to the white dwarf companions. The derived masses for the white dwarf companions are significantly lower than the average mass for single carbon-oxygen white dwarfs. In a T$_{ m eff}$-log(g) diagram of sdB+dM companions, we find signs that the sdB components are more massive than the rest of the sample. The full sample was compared to the known population of extremely low-mass white dwarf binaries as well as short-period white dwarfs with main sequence companions. Both samples show a significantly different companion mass distribution. We calculate merger timescales and timescales when the companion will fill its Roche Lobe and the system evolves into a cataclysmic variable.

Motivation & Objective

• To identify and characterize hot subdwarf B stars with massive compact companions, including white dwarfs, neutron stars, or black holes, via spectroscopic analysis.
• To determine the orbital and atmospheric parameters of 12 newly discovered sdB binaries with periods between 0.14 and 7.4 days.
• To investigate the nature of the unseen companions in these systems, particularly focusing on white dwarf and low-mass stellar companions.
• To analyze the minimum companion mass distribution in short-period sdB binaries and assess its implications for binary evolution and merger pathways.
• To identify systems that will evolve into cataclysmic variables, AM CVn stars, or RCrB stars, and assess their potential as Type Ia supernova progenitors.

Proposed method

• Spectroscopic monitoring of 12 hot subdwarf B stars from the SDSS survey to measure radial velocity variations and derive orbital elements.
• Atmospheric parameter determination using high-resolution spectroscopy to obtain effective temperature (Teff), surface gravity (log g), and metallicity.
• Mass function calculation from radial velocity amplitudes to constrain the minimum mass of the unseen companions.
• Comparison of the observed companion mass distribution with theoretical models of binary evolution and known populations of white dwarfs and cataclysmic variables.
• Infrared photometry and analysis of excess emission to search for third-body companions in wide orbits.
• Evolutionary timescale modeling to predict future evolutionary paths, including Roche lobe overflow and binary mergers.

Figure 1: Radial velocity plotted against orbital phase. The RV data were phase folded with the most likely orbital periods and are plotted twice for better visualisation. The residuals are plotted below. The RVs were measured from spectra obtained with SDSS (squares), CAHA3.5m/TWIN (upward triangle

Experimental results

Research questions

• RQ1 What is the nature of the unseen companions in 12 newly discovered short-period hot subdwarf B star binaries?
• RQ2 How does the distribution of minimum companion masses in sdB binaries inform binary evolution models, particularly regarding the presence of white dwarfs and low-mass stars?
• RQ3 Which of the identified systems are likely to evolve into cataclysmic variables, AM CVn systems, or RCrB stars within a Hubble time?
• RQ4 Are any of the systems potential progenitors of Type Ia supernovae, and what constraints do they place on the single-degenerate channel?
• RQ5 What evidence supports the existence of hierarchical triple systems, such as SDSS J095101.28+034757.0, and how do they affect orbital and evolutionary modeling?

Key findings

• Nine of the 12 systems have companions that cannot be unambiguously classified, while three are likely to host white dwarf companions, including SDSS J083006.17+475150.3, which hosts a rare low-mass helium-core white dwarf.
• The minimum companion mass distribution is bimodal, with peaks at ~0.1 M⊙ (dM/substellar companions) and ~0.4 M⊙ (white dwarf companions), indicating distinct evolutionary pathways.
• The derived white dwarf companion masses are significantly lower than the average for single carbon-oxygen white dwarfs, suggesting a distinct formation channel.
• Sixteen systems are predicted to evolve into cataclysmic variables within a Hubble time, two of which will have brown dwarf donors.
• Twelve systems with confirmed white dwarf companions will merge within a Hubble time, including two that may evolve into stable AM CVn-type binaries and two that are potential Type Ia supernova progenitor systems.
• Eight additional systems are expected to merge and form RCrB stars or massive C/O white dwarfs, depending on the internal structure of the white dwarf companion.

Figure 2: Radial velocity plotted against orbital phase (see Fig 1 ).