Effects of Substellar Companions on Stellar Evolution

Funded by NSF AST #1812874

Planets and brown dwarfs in close orbits around main sequence stars will interact with their stellar hosts once they ascend the red giant branch. The details of these interactions and their outcomes are currently unclear. Recent discoveries of brown dwarfs orbiting post-red giant branch "hot subdwarf" stars imply that at least some substellar objects are not only sufficient for ejecting the outer layers of a red giant’s atmosphere, they can also survive the engulfment phase. More than 100 new eclipsing hot subdwarf binaries with cool, low-mass companions have been uncovered in recent years from large surveys like OGLE, ATLAS, and the Evryscope, increasing the number of known systems by a factor of nearly ten! Dr. Veronika Schaffenroth (PI; U. of Potsdam) and I started the EREBOS project several years ago to obtain spectroscopic observations of these short-period binaries in order to determine their orbital velocities and, ultimately, the companion masses. This collaboration now consists of more than a dozen astronomers from all around the world. We will use our unique and homogeneously-selected sample of binaries to derive the mass distribution of the companions and determine the minimum mass needed to strip off the red-giant envelope and survive the common envelope phase. Several HPU undergraduates have been involved in this work by collecting observations with SOAR/Goodman, writing observing proposals, and analyzing systems. We have also initiated an outreach program tied to this NSF-funded research, called "Learning Astronomy through Service and Research" (LASeR). In Fall 2020, we taught a service learning section of our introductory astronomy course (PHY 1050), during which undergraduates learned how to use the robotic Skynet telescopes. They paired up with local middle and high school teachers and help their classes carry out short observing projects with Skynet using integration time purchased by our grant.

Relevant Works
The first totally-eclipsing HW Vir found, which was discovered by Kyle Corcoran and Stephen Walser.

Model HVC: column, centroid velocity and velocity dispersion. RV semi-amplitudes of known sdB binaries with red/brown dwarf companions (blue = eclipsing; red = not) plotted against their periods. No systems are seen in the bottom-left, as their companions probably merged with the primary or evaporated. From Schaffenroth et al. 2018.

TESS Observations of Compact Hot Subdwarf Binaries

Funded by NASA TESS GI Program Grants 80NSSC19K1720 & 80NSSC21K0364

In 2019 & 2020 we submitted NASA/TESS Guest Investigator (GI) proposals to obtain Cycles 2, 3, & 4 photometry of known and candidate variable hot subdwarf B (sdB) stars, with an emphasis on eclipsing systems with low-mass companions. From an analysis of the Gaia DR2 flux errors, we identified a large number of sdB stars with inflated flux errors for their magnitudes, strongly indicative of variability (see next section below). Follow-up TESS observations of these systems over Sectors 14-55 has and will continue to lead to the discovery of new eclipsing sdB+dM binaries (HW Vir systems), non-eclipsing reflection effect sdB+dM binaries, ellipsoidally-modulated sdB+WD systems, and other types of variables. In addition to this discovery work, we also requested TESS light curves of all known variable hot subdwarf binaries in order to improve modeling of their parameters, and to constrain their masses and eccentricities from Roemer delay measurements. For the past year, our group has been analyzing light curves as they become available. As expected, we are indeed discovering several new and interesting systems!

Relevant Works
Gaia varindex plot Normalized Gaia flux errors plotted against G magnitudes for all known hot subdwarf stars. Candidate variables show anomalously-high flux errors for their given magnitudes.

New HW Vir light curve Folded TESS light curve of a new HW Vir binary discovered from the Gaia varindex method.

Evryscope Search for Compact Variables

Working with collaborators at UNC-Chapel Hill, we have been conducting a survey of candidate white dwarf and hot subdwarf stars in the southern sky searching for fast transits, eclipses, and sinusoidal like variability in the Evryscope light curves. The survey aims to detect transit signals from Neptune size planets to gas-giants, and eclipses from M-dwarfs and brown dwarfs. The other variability signals are primarily expected to be from compact binaries and reflection effect binaries. Due to the small size of hot subdwarfs, transit and eclipse signals are expected to last only a few minutes, but with large signal depths (up to completely eclipsing if the orientation is edge on). With its 2-minute cadence and continuous observing Evryscope is well placed to recover these fast transits and eclipses. The very large field of view (8150 sq. deg.) is critical to obtain enough hot subdwarf targets, despite their rarity. We identified 11,000 potential hot subdwarfs from the 9.3M Evryscope light curves for sources brighter than G = 15. With our machine learning spectral classifier, we flagged high-confidence targets and estimate the total hot subdwarfs in the survey to be 1400. So far, we have discovered several new compact binaries (including two with unseen degenerate companions, and several others with potentially rare secondaries), two eclipsing binaries with M-dwarf companions, as well as new reflection effect binaries and others with sinusoidal like variability. Four of the discoveries are in the process of being published in separate followup papers, and we discuss the followup potential of several of the other discoveries.

Relevant Works
Evryscope Classifier Machine learning based classifiers used to select hot subdwarf candidates for Evryscope. The black contours result from training data from known giants (red diamonds), main sequence stars (green circles), and white dwarfs (blue stars). The hot subdwarf (HSD) candidates are shown as the yellow grouping.
Light Curve of HW Vir. Phase-folded Evyrscope light curve of HW Vir, the prototype eclipsing sdB+dM binary.

New Variable Hot Subdwarfs Identified from Gaia

Funded by NASA TESS GI

Over the past few years, ESA’s Gaia spacecraft has been obtaining astrometric measurements of more than 1 billion stars with unparalleled precision. The reported Gaia DR2 G magnitudes were determined by combining multiple brightness measurements, with photometric uncertainties being determined empirically. Thus, intrinsically variable sources could have anomalously large uncertainties for their given G magnitude. Leveraging this fact, we have identified more than 1,000 candidate variable hot subdwarfs in Gaia DR2. We have initiated a campaign with the CHIRON spectrograph on the CTIO 1.5-m telescope to monitor the radial velocities of all candidate variable hot subdwarfs with G < 13 mag in the southern hemisphere. We have already identified several new rapid binaries. Here, we discuss the details of this survey and tentative results.