Search for IR Excess in White Dwarf Stars

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The only known pulsating white dwarf star with an IR excess has been the large amplitude pulsating DAV, G29-38 (Zuckerman & Becklin 1987). It is believed ...
14th European Workshop on White Dwarfs ASP Conference Series, Vol. 334, 2005 D. Koester, S. Moehler

Search for IR Excess in White Dwarf Stars A. Nitta,1 J. L. Provencal,2 M. Kilic,3 S. J. Kleinman,1 J. Liebert,4 and T. von Hippel3 1 Apache

Point Observatory, 2001 Apache Point Rd., Sunspot, NM 88339, U.S.A. 2 Mt. Cuba Observatory, Dept. of Physics and Astronomy, University of Delaware, Newark, DE 19716, U.S.A. 3 Dept. of Astronomy and McDonald Observatory, University of Texas at Austin, Austin, TX 78712, U.S.A. 4 University of Arizona, Steward Observatory, Tucson, AZ 86721, U.S.A. Abstract. Using the Two-Micron-All-Sky-Survey database, we searched for IR fluxes of DAs and DBs reported in the Sloan Digital Sky Survey White Dwarf Catalog (Kleinman et al. 2004) as well as for the previously known DBVs.

1.

Motivations

There are two different reasons why we are interested in the excess IR flux of white dwarf stars. (1) Can large amplitude pulsation eject dust around the white dwarf pulsator? The only known pulsating white dwarf star with an IR excess has been the large amplitude pulsating DAV, G29-38 (Zuckerman & Becklin 1987). It is believed that the G29-38 IR flux is caused from surrounding dust although the origin of the dust has yet to be determined. One of the possible explanations is that G29-38’s large amplitude pulsations have pushed material off from the stellar surface and into space. In order to help understand if this is the case, we looked at the 2MASS database for J, H, and K magnitudes for the SDSS DAs and DBs in Kleinman et al. (2004) as well as the known DBVs. We searched for G29-38 in 2MASS just as a sanity check and indeed we found excess IR flux. (2) Searching for direct evidence linking the evolution between interacting binary white dwarf stars and DBs. Interacting binary white dwarf stars (IBWDs) consist of two white dwarf stars (WDs) with an extreme mass ratio which are going through mass transfer. The less massive WD transfers its mass to the more massive one. From their observed spectra, we know that He is being transferred. We believe the mass transfer will continue until the less massive WD loses its mass completely and the end result is a single He–layered WD, a DB (Nather et al. 1981). The well studied IBWD, AM CVn, has an effective temperature of about 25,000K. If AM 589

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CVn keeps this temperature until it produces a DB, its Tef f would be around 25,000K, a temperature which incidentally is right around the DB instability strip. If the mass transfer is not 100% efficient, there might be some small loss from the outer Lagrangian point causing some dust surrounding the system. With this in mind, we decided to search for IR flux using the 2MASS database in all the known IBWDs and the previously known DBVs as well as all the DBs hotter than 20,000K in the Kleinman et al. (2004) WD catalog in the 2MASS database. Upon finding any unusual IR flux, we intend to obtain IR spectra to help us identify the source of the IR flux.

2.

Results

So far, we have found 45 DAs and 4 DBs with IR excesses. Considering the brightness limits of 2MASS (J < 16.0, H < 15.5, K < 15.0), we are finding IR flux that are relatively bright compared to the optical light emitted by the white dwarf stars. Among the 45 DAs with IR excess, 7 DAs lie in the temperature range around the DAV instability strip (11000 K to 13000 K) including two new DAVs reported in Mukadam et al. (2004), two DAs that are mentioned in Mukadam et al. (2004) as non-pulsators and 3 other DAs which are not mentioned in Mukadam et al. (2004), i.e. waiting to be observed to search for pulsations. There are 38 other DAs with 2MASS-detected IR fluxes which are not listed as WD+M system in the Kleinman et al. (2004) catalog. We have not yet obtained follow up spectroscopy to explore the sources of these IR flux excesses. We also found two known DBVs with excess IR fluxes. For one of them, PG1654+160, we obtained an IR spectrum with the IRTF telescope and found the source of the IR flux to be an M-dwarf (Figure 1). This is the first known pulsating DB in a binary system. The prior asteroseismological analysis of PG1654+160 showed no indication of a companion. We have not yet obtained an IR spectrum for the other DBV.

3.

Future

There are another 38 DAs and 3 DBs with IR excesses that appear normal in the optical. We welcome any interested collaborators to help us obtain and analyze IR spectroscopy for these objects. Acknowledgments. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. AN thanks Rich Kron and Bruce Gillespie for their support of this project and my attendance at this meeting.

IR Excess in WDs

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Figure 1. The PG1654+160 IR spectrum from the IRTF telescope. The IR excess is caused by an M-dwarf. The figure shows the data along with the M-dwarf model (the smooth curve). The strong emission lines in the data come from the imperfect telluric correction during the data reduction process. Table 1. DAs around the DAV instability strip with excess IR fluxes. The magnitudes with * have bad SDSS photometric pipeline quality control flags set. DAV (SDSS)

g

J

H

J101548.01+030648.4 J135459.89+010819.3 Non-DAV (SDSS) J015259.19+010018.5 J110515.32+001626.1

15.66 0.02 *16.36 0.01 g 16.43 0.02 15.20 0.02

15.752 16.373 J 16.661 15.455

No Data (SDSS) J130110.51+010739.9 J135211.00+652457.1 J165935.58+620933.9

g 16.30 0.02 15.44 0.01 *16.25 0.02

J 16.286 0.121 15.688 0.068 16.661 0.196

0.073 0.132 0.132 0.070

15.590 16.235 H 15.951 15.283

K 0.129 0.219 null 0.094

H 16.085 null 15.419 0.136 16.545 null

15.421 16.493 K 16.230 15.041

null null null 0.165

K 16.138 null 15.304 0.175 15.882 null

References Kleinman, S. J., Harris, H. C., Eisenstein, D. J., et al. 2004, ApJ, 607, 426

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Mukadam, A., Mullally, F., Nather, R. E., et al. 2004, ApJ, 607, 982 Nather, R. E, Robinson, E. L., & Stover, R. J. 1981, ApJ, 244, 269 Zuckerman, B., & Becklin, E. E. 1987, Nat, 330, 138