Short period sdB binaries with cool companions are crucial to understand
pre-CV evolution, because they will evolve into cataclysmic variables, when the
sdB will have left the extended horizontal branch. Recently we discovered the
sixth such system, HS 2333+3927, consisting of an sdB star and an M dwarf
(period: 0.172 d) with a very strong reflection effect, but no eclipses. The
reflection is stronger than in any of the other similar systems which renders a
quantitative spectral analysis very difficult because the Balmer line profiles
may be disturbed by the reflected light. A spectroscopic analysis results in
Teff = 36500 K, log g = 5.70, and log (n_He/n_H) = -2.15. Mass-radius relations
were derived from the results of the analysis of light and radial-velocity
curves. Comparison with the mass-radius relation derived from the surface
gravity of the sdB star favours a rather low mass of 0.38 Msun for the primary.
The mass of the companion is 0.29 M_sun. HS 2333+3927 is the only known sdB+dM
system with a period above the CV period gap.Comment: 6 pages, 3 Fig., to appear in 14th European Workshop on White Dwarfs,
  ASP Conference Series, eds. D. Koester, S. Moehle

B. Voss

C. Karl

D. Koester

H. Drechsel

J. -e. Solheim

M. Altmann

O. Cordes

R. Napiwotzki

R. Østensen

S. Folkes

S. Moehler

U. Heber

English

arXiv

Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the PacificShort period sdB binaries with cool companions are crucial to understand pre-CV evolution, because they will evolve into cataclysmic variables, when the sdB will have left the extended horizontal branch. Recently we discovered the sixth such system, HS 2333+3927, consisting of an sdB star and an M dwarf (period: 0.172 d) with a very strong re°ection e®ect, but no eclipses. The re°ection is stronger than in any of the other similar systems which renders a quantitative spectral analysis very di±cult because the Balmer line pro¯les may be disturbed by the re°ected light. A spectroscopic analysis results in Te® = 36 500 K, log g = 5.70, and log(nHe=nH) = ¡2:15. Mass-radius relations were derived from the results of the analysis of light and radial-velocity curves. Comparison with the mass-radius relation derived from the surface gravity of the sdB star favours a rather low mass of 0.38 M¯ for the primary. The mass of the companion is 0.29 M¯. HS 2333+3927 is the only known sdB+dM system with a period above the CV period gap

Heber, U.

Drechsel, H.

Karl, C.

Ostensen, R.

Folkes, S.

Napiwotzki, R.

Altmann, M.

Cordes, O.

Solheim, J.E.

Voss, B.

Koester, D.

University of Hertfordshire Research Archive

14th European Workshop on White DwarfsASP Conference Series, Vol. 334, 2005D. Koester, S. MoehlerThe Mass of the sdB Primary of the Binary HS 2333+3927U. Heber, H. Drechsel, and C. KarlDr. Remeis-Sternwarte, Astronomisches Institut der Universita¨tErlangen-Nu¨rnberg, Sternwartstraße 7, D-96049 Bamberg, GermanyR. Østensen, S. FolkesIsaac Newton Group of Telescopes, E-37800 Santa Cruz de La Palma,Canary Islands, SpainR. NapiwotzkiDepartment of Physics & Astronomy, University of Leicester, UniversityRoad, Leicester LE1 7RH, UKM. AltmannDepartamento de Astronomia, Universidad de Chile, Camino ElObservatorio 1515, Las Condes, ChileO. CordesSternwarte der Universita¨t, Auf dem Hu¨gel 71, D53121 Bonn, GermanyJ.-E. SolheimInstitute of Theoretical Astrophysics University of Oslo, p.box 1029,N-0315 Blindern-Oslo, NorwayB. Voss, D. KoesterInstitut fu¨r Theoretische Physik und Astrophysik, Universita¨t Kiel,24098 Kiel, GermanyAbstract. Short period sdB binaries with cool companions are crucial to un-derstand pre-CV evolution, because they will evolve into cataclysmic variables,when the sdB will have left the extended horizontal branch. Recently we dis-covered the sixth such system, HS 2333+3927, consisting of an sdB star and anM dwarf (period: 0.172 d) with a very strong reflection effect, but no eclipses.The reflection is stronger than in any of the other similar systems which rendersa quantitative spectral analysis very difficult because the Balmer line profilesmay be disturbed by the reflected light. A spectroscopic analysis results in Teff= 36 500 K, log g = 5.70, and log(nHe/nH) = −2.15. Mass-radius relations werederived from the results of the analysis of light and radial-velocity curves. Com-parison with the mass-radius relation derived from the surface gravity of thesdB star favours a rather low mass of 0.38 M¯ for the primary. The mass of thecompanion is 0.29 M¯. HS 2333+3927 is the only known sdB+dM system witha period above the CV period gap.357358 Heber et al.1. IntroductionSdB binaries are important to clarify the evolutionary origin of sdB stars becausethe analysis of light and radial velocity curves can constrain their dimensionsand masses. However, only five suitable systems are known up to now.There is general consensus that the sdB stars can be identified with modelsfor Extreme Horizontal Branch (EHB) stars (Heber 1986). Like all HB starsthey are core helium burning objects. However, their internal structure differsfrom typical HB stars, because their hydrogen envelope is very thin (<1% bymass) and therefore inert. As a consequence EHB stars evolve directly to thewhite dwarf cooling sequence, thus avoiding a second red giant phase. How theyevolve to the EHB configuration is controversial. The problem is how the massloss mechanism of the progenitor manages to remove all but a tiny fraction ofthe hydrogen envelope at precisely the same time as the He core has attainedthe mass (∼0.5 M¯) required for the He flash.Considerable evidence is accumulating that a significant fraction of the sdBstars reside in close binaries (Maxted et al. 2001; Napiwotzki et al., 2004, see alsoKarl et al, these proceedings). Therefore mass transfer should have played animportant role in the evolution of such binary systems. Detailed investigationsof sdB binaries, in particular eclipsing systems, are crucial to determine theirmasses. However, only three such eclipsing binaries, HW Vir, PG 1336-013,and HS 0705+6700 (see Drechsel et al. 2001) and two non-eclipsing ones areknown up to now, which consist of an sdB star and an M dwarf companionrevealed by reprocessed light from the primary (reflection effect). Recently,Heber et al. (2004) discovered another related system, HS 2333+3927, which is,however, not eclipsing, but otherwise possesses very similar system parametersand configuration.2. ObservationsThe first hint for light variations emerged when HS 2333+3927 was monitoredat the Nordic Optical Telescope on October 19, 1999 in order to search forpulsations. The light curve of HS 2333+3927 (see Fig. 1) was measured fromCCD photometry in the B, V , and R bands at four telescopes (JKT 1.0m, CalarAlto 1.23 m, IAC 0.8 m Hoher List 1.06 m) in 15 nights between July 2 andNovember 11, 2002.The radial velocity curve (see Fig. 2) was measured during an observingrun at the Calar Alto Observatory with the TWIN spectrograph at the 3.5mtelescope. A total of 18 spectra were taken from 11 to 18 August 2002 coveringthe wavelength ranges from 3900 A˚ to 5000 A˚ at a resolution of 1.3 A˚ in theblue part of the spectrum, and from 6000 A˚ to 7000 A˚ at a resolution of 1.2 A˚in the red part.HS 2333+3927 was also observed at the Calar Alto Observatory with theCAFOS spectrograph at the 2.2m telescope in order to derive atmospheric pa-rameters by Balmer line fitting. A total of 13 low resolution spectra were takenon August 31, 2003 covering the wavelength range from 3300 A˚ to 6000 A˚ at aspectral resolution of 4.5 A˚. This allowed to measure the entire Balmer series(except Hα) up to the Balmer jump.The Mass of the sdB Primary of the Binary HS 2333+3927 359Figure 1. BV R light curves of HS 2333+3927 (Heber et al. 2004).3. Analysis of Light and Radial Velocity CurvesThe numerical solution of the light curves was performed with the Wilson-Devinney (1971) based light curve program MORO (Drechsel et al. 1995). Thebest fit to the B, V , R light curves is shown in Fig. 1. A strong reflection effect isvisible (∆B = 0.21, ∆V = 0.28, ∆R = 0.33 mag), but no eclipses are apparent.We determined the period to be P=0.1718023 d.The spectrum of HS 2333+3927 is single-lined and the radial velocity curveis sinusoidal (Fig. 2) indicating that the orbits are circular. The semi-amplitudeis K1 = 89.6 km/s and the mass function follows as f(m) = 0.0128 M¯.The light curve analysis provided us with an estimate of the system’s in-clination. For any assumed primary mass we can calculate the secondary massfrom the mass function. In addition the orbital radius a1 of the primary can becalculated. The separation a of the components can then be calculated from themass ratio q. The light curve allows us to determine the ratio of the radii in unitsof the separation. Hence we can determine the radii of both components for anygiven sdB mass, i.e we can derive mass-radius relations for both components.4. Spectroscopic AnalysisThe Balmer and helium lines in the blue spectra can be used to determine theatmospheric parameters by performing a quantitative spectral analysis. Becausethe medium resolution TWIN spectra cover only few of the Balmer lines, we usedthe CAFOS spectra which cover the Balmer series to its limit. Since the highBalmer lines are probably least affected by reprocessed light from the secondary,much emphasis was put in the fit procedure to reproduce these lines well. Since360 Heber et al.-0.4 -0.2 0 0.2 0.4-150-100-50050Figure 2. Radial velocity curve of HS 2333+3927 (Heber et al. 2004).the spectrum displays helium lines from two stages of ionization, we made alsouse of the ionization equilibrium of helium to determine Teff .The atmospheric parameters determined from spectra taken near lower con-junction should be least affected by reprocessed light from the secondary and,therefore, the atmospheric parameters derived from them should be the closestapproximation to the true atmospheric parameters of the sdB star.Based on the above considerations we derived Teff = 36 500 ± 1000 K, log g= 5.70 ± 0.1 and log(nHe/nH) = −2.15 ± 0.15 for the atmospheric parametersof HS 2333+3927.5. Masses and RadiiThe mass-radius relations derived from the analysis of the light and radial ve-locity curves can be compared to independently determined relations. For thesdB star such a relation follows from the gravity (Newton’s law). For the coolcompanion a theoretical mass-radius relation for M-dwarfs may be appropriate.In principle the masses and radii of the sdB star and the M dwarf, respectively,could be determined by requesting these mass-radius relations to be consistent.Fig. 3a compares the mass-radius relation of the sdB derived from the lightand radial velocity curves to that derived from gravity. The gravity of log g = 5.7is too low to match the M-R-relation from light and radial velocity curve forreasonable masses. The intersection of both M-R-relations in Fig. 3a wouldgive a sdB mass of 0.2 M¯, clearly too low for a core helium burning star.Evolutionary scenarios by Han et al. (2003) suggest that the most likely mass is≈0.47 M¯, but possible masses range from 0.38 to 0.8 M¯. As can be seen fromFig. 3a the sdB gravity should be log g ≈ 5.86 if the sdB star is of canonical mass.This is significantly higher than derived from a quantitative spectral analysis ofthe Balmer lines: log g = 5.7±0.1. However, if we adopt the lowest mass coreThe Mass of the sdB Primary of the Binary HS 2333+3927 361Figure 3. a) left: Comparison of the sdB mass-radius relation from theanalysis of light and radial velocity curves (solid line) to those derived fromdifferent gravities. The spectroscopic log g estimate is 5.7±0.1. Dotted linesmark the most probable (0.47 M¯) and the lowest mass (0.38 M¯) accordingto the evolutionary models of Han et al. (2003). Note that the gravity wouldneed to be as high as 5.86 (dashed line) if the star were of canonical mass(0.47 M¯).b) right: Comparison of the companion’s mass-radius relation derived fromthe analysis of light and radial velocity curves (solid line) to relations forM-type dwarfs (short-dashed: observed relation from Clemens et al. 1998;long-dashed: theoretical predictions from Baraffe & Chabrier, 1996). Filledcircle: sdB has canonical mass (0.47 M¯); triangle: sdB has lowest possiblemass.helium burning star that can form in the Han et al. scenario, consistency couldbe achieved if we adopt the largest gravity allowed by the spectroscopic analysis(see Fig. 3a).The mass-radius relation for the companion star is compared to observationsand model predictions for M-type main sequence stars in Fig. 3b. The empiricalrelation for the unseen companion of HS 2333+3927 lies above those for normalM-stars. However, it is possible that the M-star is over-luminous due to thestrong irradiation of its surface by the nearby hot star and hence has a largerradius than a normal M dwarf of the same mass.6. Discussion6.1. ResultsFrom the analysis presented above we conclude that a model for a low mass(≈0.38 M¯) core helium burning sdB star and a 0.29 M¯ M-dwarf fits theobservations best. However, the results have to be taken with a grain of salt.The spectral analysis of the Balmer lines, is plagued by reflected light, as is362 Heber et al.evident from the line profile variations observed in Hα, but less obvious in otherBalmer lines. An improved measurement of the gravity, therefore, is urgentlyneeded.Another important observational constraint would be a precise measure-ment of the projected rotational velocity. The rotation of the sdB star inHS 2333+3927 is very likely tidally locked to the orbital motion. Drechsel et al.(2001) showed this to be true for HS 0705+6700. Determining the projected ro-tation velocity would therefore allow an independent estimate of the inclinationof HS 2333+3927.The spectral lines of the Lyman series are sensitive gravity indicators andplenty of metal lines can be used to measure v sin i. These measurements willallow us to constrain mass and radius much better6.2. SdB Stars and pre-CV EvolutionShort period sdB binaries with main sequence companions, like HS 2333+3927,are important not only to understand the formation and evolution of sdB stars.When the sdB star will have left the EHB, it will evolve into a cataclysmic vari-able. Therefore, these objects are also crucial to understand pre-CV evolution.Schenker (these proceedings) has suggested that cataclysmic variables with shortperiods, i.e. below the CV period gap evolve from sdB + dM binaries. Whilefour other known systems have periods between 1.7 h to 2.8 h, i.e. below theCV period gap, the period (4.13 h) of HS 2333+3927 is a bit above the CVperiod gap. Its orbital separation is so small that the secondary is appreciablydistorted. The shrinkage of the binary orbit by gravitational wave radiationwill initiate mass transfer turning the system into a cataclysmic variable. It islikely that the HS 2333+3927 system will then still have a period larger than theCV period gap. Hence sdB + dM binaries form an evolutionary channel to thepopulation of longer period cataclysmic variables as well as to the short-periodpopulation.Acknowledgments. C.K. gratefully acknowledges financial support by theconference organizers. M.A. is financially supported by FONDAP 1501 0003.ReferencesBaraffe, I., & Chabrier, G. 1996, ApJ, 445, 433Clemens, J. C., Reid, I. N., Gizis, J. E., & O’Brien, M. S. 1998, ApJ, 496, 352Drechsel, H., Haas, S., Lorenz, R., & Gayler, S. 1995, A&A, 294, 723Drechsel, H., Heber, U., Napiwotzki, R., et al. 2001, A&A, 379, 893Han Z., Podsiadlowski, P., Maxted, P. F. L., & Marsh, T. R. 2003, MNRAS, 341, 669Heber, U., 1986, A&A, 155, 33Heber, U., Drechsel, H., Østensen, R., et al. 2004, A&A, 420, 251Maxted, P. F. L., Heber, U., Marsh, T. R., & North, R. C. 2001, MNRAS, 326, 1391Napiwotzki, R., Karl, C. A., Lisker, T., et al. 2004, in “Extreme Horizontal BranchStars and Related Objects”, Kluwer Academic Publishers, ed. P. F. L. Maxted,ApSS 291, 321Wilson, R. E., & Devinney E. J. 1971, ApJ, 166, 605

in \Extreme Horizontal Branch Stars and Related Objects&quot;,

The Mass of the sdB Primary of the Binary HS 2333+3927

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The mass of the sdB primary of the binary HS 2333+3927

The mass of the sdB primary of the binary HS 2333+3927

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