The 1.4 GHz NRAO VLA Sky Survey (NVSS) and the Villanova Catalog of Spectroscopically Identified White Dwarfs (VCSIWD) were compared to search for radio emission from white dwarfs. Twenty-five coincidences were found with separations less than 30 arcseconds. Eight coincidences were associated with white dwarf instability strips. Thirty-one percent of the known PG 1159 stars associated with a planetary nebula appeared radio-loud. No DA central stars were coincident with NVSS sources.
White Dwarf Candidates for Radio Emission within
WD Name RA J2000 Dec J2000 Separation Flux Spectral Comments
Arcseconds mJy Type
2114+239 21 16 53 24 12 17 2.27 7.9 DAO PNN Abell 74
1204-322 12 06 47 -32 34 32 2.63 25.3 DA
0127-050 01 30 23 -04 47 58 3.78 26.5 DA
0926-039 09 29 21 -04 10 08 5.32 3.5 DA Possible ZZ Ceti
1229+290 12 31 43 28 47 46 6.59 140.5 DC QSO FBQSJ123143.5+284749
0907+336 09 10 36 33 29 20 6.59 104.8 DA QSO FBQS1091037+332924
1958+015 20 00 40 01 43 48 9.7 11.1 PG 1159 (lgE) CSPN NGC 6852
2134+125 21 36 52 12 47 14 11.20 13.2 PG 1159 Hybrid (lgEH) CSPN NGC 7094
1909+304 19 11 06 30 27 36 13.0 10.1 PG 1159 (lgE) CSPN NGC 6765
2304-347 23 07 39 -34 27 45 13.3 3.5 DC Seyfert Galaxy 12” away
0954+342 09 57 48 33 59 41 13.6 3.4 DBV FIRST radio source, UV
0455+424 04 59 27 55 25 18 15.0 3.5 DAV ZZ ceti
Twenty-five WD appear to be associated with NVSS radio sources. Of these, eight lie within the three white dwarf instability strips: five are PG 1159 or PG 1159-like stars, one is a DB variable and two lie within the ZZ Ceti instability strip. Six are planetary nebulae central stars (CSPN), each of which includes helium in its spectrum. Six are associated with UV radiation and two are associated with binary systems.
The results of this research do not identify radio-emitting white dwarfs – only the possibility of their existence. PG 1159 and PG 1159 Hybrid stars are relatively rare (~30) among white dwarfs. Of these, ten are lgE types, low gravity and He II emission in the core (Dreizler et al. 1994). Four are associated with radio emission sources and are CSPN. Dgani and Soker (1998) suggest that nonthermal radio emission may exist from some of these CSPN.
All twenty-five stars require further investigation. Higher resolution radio imaging would help determine if these WD were truly sources of radio emission or simply accidental coincidences.
This project used the VCSIWD (McCook et al.,1999) as input to the NVSS (Condon, 1998) database (NVSSlist) to search for 20 cm radio wavelength emissions from 2074 white dwarf stars north of declination -40 degrees. Both databases were obtained via the internet; a Unix format and File Transfer Protocol (FTP) were used. After obtaining the files and making NVSSlist ready for use, (a fairly complicated endeavor), several Basic programs were written to modify the white dwarf catalog to suit NVSSlist requirements. Once formatted correctly, the white dwarf catalog was input into NVSSlist to search for radio emission coincident with white dwarf positions within 30 arcseconds. A range of 30” for separation seemed appropriate because the coordinates of the WD can be uncertain to 30” and the proper motion of the stars was not considered.
After the white dwarf/radio source matches were found, DSS images were obtained for visual inspection searching for radio emitting objects around each star. Using the astronomical databases, SIMBAD and NED, a region of one arcsecond about each star of was inspected for potential radio emitting sources.
PG 1159 and Hybrid PG 1159 Stars Associated with Radio Sources
Condon, JJ and WD Cotton, EW Greisen, et al., (1998).
The NRAO VLA Sky Survey. The Astronomical Journal.
McCook, GP and EM Sion. (1999). A Catalog of
Spectroscopically Identified White Dwarfs. The Astrophysical
Journal Supplement Series. 121,1-130.
Bradley, P. (2001). Variable White Dwarf Data Tables.
Dreizler S., Werner K., Heber U., 1994, in White Dwarfs
(Eds. Koester, K.Werner), Springer-Verlag, p.160 (Table 2).
Dgani, Ruth and Noam Soker. (1998) Nonthermal Radio Emission from Planetary Nebulae.
This research has made use of the Digitized Sky Survey; SIMBAD database operated at CDS, Strasbourg, France; Skyview Virtual Observatory; NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the NASA; and the NVSS Postage Stamp Server, Condon, et al. (1998), AJ, 115, 1693.
Figure 1. WD 1958+015, lgE
Figure 2. WD2134+125, Hybrid, lgEH
Figure 3. WD1909+304, lgE
Figure 4. WD1751+106. Hybrid, lgEH
Figures 1-4. Images of lgE PG 1159 and lgEH Hybrid PG 1159 stars coincident with radio emission sources. PG 1159 stars show broad absorption troughs due to Helium (He II ) only. PG 1159 Hybrids have Hydrogen (H) and He II in their spectra. NVSS radio images are overlaid (areas of transparent yellow and light blue) on DSS images for each PG 1159 star. The images are 6 x 6 arcminutes and were obtained from the Skyview Virtual Observatory.