1. Motivation & Approach

FIGURE 2 — Histogram of the orbital periods of all hot subdwarf binaries with known orbital parameters. The black vertical lines at the top indicate the individual periods of all systems. In the rows of vertical lines below the first, we re–mark systems with classified companions, including F–K dwarf (second row, orange marks), M dwarf (third row, red marks), and white dwarf (fourth row, blue marks) companions.

Target Name Type Period [d] Eccentricity ReferencesPG 1018-047 sdB+KV 760 ± 6 0.25 ± 0.05 Deca et al. (2011)

BD-11°162 sdO+GV 491 ± 3 ? Østensen et al. (2011)

PG 1104+243 sdOB+G 753 ± 0.8 ? Barlow et al. (2012)

Balloon 82800003 sdB+FV 887 ± 61 ? Østensen et al. (2011)

BD+29°3070 sdOB+FV 1160 ± 67 0.15 ± 0.01 Østensen et al. (2011); Vos et al. (2013)

BD+34°1543 sdB+FV 818 ± 21 0.16 ± 0.01 Østensen et al. (2011); Vos et al. (2013)

BD-7°5977 sdB+K2III 1194 ± 79 ? Østensen et al. (2011)

PG 1317+123 sdO+GV 1179 ± 12 0.11 ± 0.01 Barlow et al. (2012); Vos et al. (2013)

PG 1338+611 sdB+GV 938 ± 1 0.15 ± 0.02 Barlow et al. (2012)

PG 1449+653 sdB+GV 909 ± 2 0.11 ± 0.02 Barlow et al. (2013)

PG 1701+359 sdB+KV 734 ± 3 < 0.23 Barlow et al. (2013)

PG 1040+243 sdB+GV 928 ± 6 < 0.2 Barlow et al. (2014)

PG 1718+519 sdB+GV 938 ± 1 0.54 ± 0.04 Barlow et al. (2014)

TABLE 1 Solved Long-Period Binaries

• SdB stars with M dwarf and white dwarf companions likely form through different processes than those with F-K dwarf companions, as implied by their differing orbital period distributions.

• Eccentric orbits challenge the prevailing notion that strong binary interactions, involving the filling of the sdB progenitor’s Roche lobe, are required for the formation of hot subdwarfs.

• The sdB binaries with non-circular orbits either (1) are or were hierarchical triple systems in which the sdB star was made in the inner binary; or if these systems have always been binaries, then either (2) circularization was not achieved because the sdB progenitor never filled its Roche lobe or (3) eccentricity was pumped into the system.

• Hot subdwarf formation theories need to be re-assessed as long-period, eccentric orbits continue to be uncovered, and as the predicted short-period sdB+F/G/K binaries fail to be found.

• We have solved the orbits of 9 hot subdwarf binaries with F-K main sequence companions. They tend to have long orbital periods, on the order of 1-3 years (see FIGURE 1); this result is in disagreement with the Han et al. models

• An updated orbital period histogram of sdB binaries (FIGURE 2) shows a clear separation between hot subdwarfs with M-dwarf/white dwarf companions and those with F-K dwarf companions, with a period gap around P = 100 days.

• Many systems have eccentric orbits.

• At least four of the “binaries” studied are members of triple- or quadruple-star systems (see example in FIGURE 3).

• Some of our target systems are likely not members of the thin disk.

• Follow-up RVs need to be collected to further refine orbital periods, eccentricities, and mass ratios.

FIGURE 3 — PG 1629+081, a quadruple-star hot subdwarf system. This system appears on the sky as a resolved visual pair with a 2.1” separation (right); the brighter member is a K-dwarf while the fainter one is the hot subdwarf B star. RV monitoring of both stars reveals each is a member of a binary with an unseen companion; both binaries show consistent systemic velocities and are likely gravitationally bound (see RV curves at left).

This material is based upon work supported by the National Science Foundation under Grant No. AST-0908642. We are also grateful for the help of the resident astronomers at HET, Kim Herrmann and Michele A. Stark for their contributions to the reduction pipeline, Rohit Deshpande, Sharon Wang, and Jason Wright for their assistance with the curve-fitting program RVLIN. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly. This research has made use of NASA’s Astrophysics Data System.

Acknowledgements

Barlow, B.N., et al., 2012, ApJ, 758, 58

Barlow, B.N., Liss, S.E., Wade, R., Green, E., 2013, ApJ, 771, 23

Barlow, B.N., Wade, R.A., Liss, S.E., 2014, ApJ, in preparation

Deca, J., 2012, MNRAS, 421, 2798

Han, Z., et al. 2002, MNRAS, 336, 449

Han, Z., et al. 2003, MNRAS, 341, 669

Maxted, P.F.L., et al. 2001, MNRAS, 326, 1391

Østensen, R. H., & Van Winckel, H. 2012, in ASP Conf. Ser., Vol. 452, Fifth Meeting on Hot Subdwarf Stars and Related Objects, ed. D. Kilkenny, C. S. Jeffery, & C. Koen (San Francisco, CA: ASP), 163

Vos, J., et al., 2013, A&A, 559, A54

References

2. Results & Future Work

Brad N. Barlow1,2, Richard A. Wade2, Sandra E. Liss31Department of Physics, High Point University, 833 Montlieu Ave., High Point, NC 27262

2Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 168023Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325

A Radial Velocity Survey OF Hot Subdwarf StarsWITH THE HOBBY-EBERLY TELESCOPE

3. Summary & Implications

FIGURE 1 — Example radial velocity curves of selected targets in our survey. The orbits of the main sequence companions are shown in orange, while those of the hot subdwarfs are shown in blue. Measurements come from HET/HRS, HET/MRS, and MMT.

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• Hot subdwarf B (sdB) stars are evolved, core He-burning objects that lost their outer hydrogen envelopes while ascending the red giant branch.

• They are astrophysically interesting because they:✦ dominate surveys of faint blue objects in the Galaxy✦ are found in all Galactic populations, elliptical galaxies, and globular

clusters✦ are the primary contributors to the “UV-upturn” in old galaxies✦ hold clues to understanding the “second parameter” problem in

globular clusters

• Observations of hot subdwarfs in the field tend to support a binary formation scenario (Maxted et al., 2001).

• Binary population synthesis (BPS) models are able to explain their formation via (i) Roche lobe overflow interactions, (ii) common envelope evolution, and (iii) the merger of two He white dwarfs (Han et al. 2002, 2003).

• BPS models originally predicted short (<20 d) periods for hot subdwarfs with F-K main sequence companions; however, no periods shorter than 100 days have been observed for such systems.

• To help address the contradiction between observations and theory, we selected 15 moderately-bright (V~13) hot subdwarfs with F-K dwarf companions and monitored their radial velocities (RVs) from 2005-2013 using the Hobby–Eberly Telescope.

What are hot subdwarf stars? Need for better model constraints

PG 1040+234