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198 9Apj. . .340. .518B the Astrophysical Journal, 340:518-536,1989 May 1 © 1989. the American Astronomical Society. All Rights

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198 9Apj. . .340. .518B the Astrophysical Journal, 340:518-536,1989 May 1 © 1989. the American Astronomical Society. All Rights .518B The Astrophysical Journal, 340:518-536,1989 May 1 © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .340. 9ApJ. 198 A SURVEY OF RADIO EMISSION FROM GALACTIC OB STARS J. H. Bieging Radio Astronomy Laboratory, University of California, Berkeley David C. Abbott Joint Institute for Laboratory Astrophysics, University of Colorado AND E. B. Churchwell Astronomy Department, University of Wisconsin-Madison Received 1988 June 13 ; accepted 1988 October 3 ABSTRACT Radio continuum observations were made of 88 O-type and early B-type stars selected to provide complete coverage of this portion of the H-R diagram. A subset of these stars forms a distance-limited sample of all 25 OB stars within 2.5 kpc of the Sun, with declinations north of —45°, and with luminosities greater than 106 Lq. Among these very luminous stars, strong nonthermal radio emission is common, clearly exceeding free- free emission in six (24%) of the cases. The nonthermal sources are spatially unresolved by the VLA and are characterized by a negative spectral index, luminosities at 5 GHz on the order of 1019 ergs s-1 Hz-1, mild variability, and no measurable polarization. Nonthermal radiation was not detected from the less luminous stars, suggesting that the nonthermal mechanism is less efficient at lower luminosities. We derive mass loss rates from the radio fluxes of HD 15570 (04f), HD 166734 (07 If + 09 I), HD 151804 (08 If), HD 152408 (08 Ifp), a Cam (09.5 la), Í1 Sco (B1 Ia + ), and HD 169454 (B1 la), all of which are thought to be free-free radio sources. All of these very luminous OB stars are losing mass at a rate on the 5 1 order of 10“ M0 yr“ , a value consistent with previous results. Multifrequency observations confirm the free-free interpretation for the previously detected star Ç Pup (04f) and provide further evidence for variability in the known free-free sources P Cygni (B1 la) and Cyg OB2 No. 12 (B8 la). Subject headings : stars : early-type — stars : mass loss — stars : radio radiation — stars : winds I. INTRODUCTION Early-type stars are expected to be detectable as sources of free-free radio emission, because their stellar winds form a large volume of dense, ionized gas (Wright and Barlow 1975; Panagia and Felli 1975). Quite unexpectedly, some early-type stars are now known to be strong sources of nonthermal emission. The nonthermal sources detected to date have either been very luminous O-type or Wolf-Rayet (WR) stars (e.g., Abbott, Bieging, and Churchwell 1984; Abbott et al 1986), or the highly magnetic, helium peculiar, Bp-type stars (Drake et al 1987). The very luminous O and WR stars all have winds so dense that they are opaque to free-free radiation out to at least 100 stellar radii. The observed nonthermal radiation must therefore be emitted by energetic particles in the outer stellar wind. We refer to these stars as “stellar wind” nonthermal sources. White (1985) has proposed that this nonthermal emission results from a small population of relativistic electrons which are accelerated by Fermi processes in the strong shocks that are thought to permeate hot star winds. In this model, only very weak (few gauss) surface magnetic fields are necessary to produce optically thin synchrotron emission of relatively low brightness temperature, but large emitting volume, which accounts for the observed radio flux. The Bp stars possess a rather weak stellar wind at best (see, e.g. Barker 1986). Ordered surface magnetic fields of a kilogauss or more characterize these stars (Bohlender et al 1987), and the closed field lines must extend out to at least 15-20 stellar radii, forming a magnetosphere. The nonthermal emission is thought to be optically thick synchrotron emission from energetic particles trapped in the magnetosphere (Drake et al 1987). These stars are thus “surface” or “magnetospheric” nonthermal emitters. The relation (if any) between the radio emission and a stellar wind is unclear. The detection of two other early-type stars may indicate the existence of other types of radio emission among the hot stars. The star O1 Ori A (B0.5 V eclipsing binary) was detected by Garay, Moran, and Reid (1985). This source is highly variable and has an angular size ~0,.'004 (Felli, Massi, and Churchwell 1988). The high implied brightness temperature and lack of X-ray emission indicate a nonthermal mechanism for the emission. Strom and Harris (1977) reported a radio source which they identified with the star HD 26676 (B8 Vn), but little else is known about this source. Many types of OB stars have never been observed with high sensitivity at radio wavelengths. We present here radio continuum measurements of 88 OB stars selected to provide complete coverage of the O and early-B star region of the H-R diagram. One goal of this survey was to search for more “ surprises,” i.e., additional types of radio emitters among the hot stars. None were found. A second goal was to determine the frequency and character of the “stellar wind” nonthermal sources. A subset of25 of our program stars forms a distance-limited sample defined as all OB stars within 2.5 kpc of the Sun, with luminosity greater than 106 Lq, and with declinations north of —45°. We found that some 25% of these most luminous stars exhibit nonthermal emission. A preliminary report of this result was given by Abbott, Bieging, and Churchwell (1985). The model of White (1985) predicts that measurable nonthermal fluxes should also have been detected from the OB stars of lower luminosity, if they possess magnetic fields 518 © American Astronomical Society • Provided by the NASA Astrophysics Data System .518B SURVEY OF RADIO EMISSION 519 .340. comparable to those inferred for the high-luminosity sources. In fact, we detected emission from only three of the lower luminosity OB stars surveyed, and it is unclear whether any of these is a stellar wind nonthermal source. A final goal was to provide further high-quality rates of mass loss for OB stars. From the distance-limited survey we derive new 9ApJ. mass-loss rates for the following stars whose radio emission is thought to be free-free: C1 Sco, HD 151804, HD 152408, HD 166734, 198 and HD 15570. A preliminary version of these rates was given by Abbott (1985). Among the less luminous stars we detected a Cam and we derive a new mass-loss rate for this star on the assumption that it is a free-free source. II. THE SURVEY a) The Stellar Sample The stellar samples are given in Tables 1 and 2. The data for each star were taken from the OB star catalog of Garmany, Conti, and Chiosi (1982), with some additions described by Abbott (1982a). The stars in Table 1 form the distance-limited sample of all OB 6 stars within 2.5 kpc of the Sun, north of declination —45°, with bolometric stellar luminosities greater than 10 L0. The stars in Table 2 were chosen to fill in four categories of spectral types: early-O, mid-O, late-O, and early-B; and three luminosity classes: V or dwarf, II-III or giant, and If or supergiant, giving 12 categories in all. Within a category the stars were selected according to distance, except where limitations in the scheduled observing time prevented us from observing the closest stars. Tables 1 and 2 contain a total of 90 stars. Of these, one was not observed for technical reasons, and one was badly confused by a nearby H n region. Thus, there are 88 stars for which we have reliable radio data. Also given in Tables 1 and 2 are the adopted stellar parameters of luminosity, eifective temperature, radius, mass, and distance from the Sun. For binaries, the parameters refer to the primary. For stars within recognized clusters, the absolute visual magnitudes were derived from the distance moduli given by Humphreys (1978). For field stars, the absolute visual magnitudes and the implied distances came from the spectral type calibration of Conti et al (1983) for the O-type stars and Lesh (1968) for the B-type stars. Effective temperatures were assigned on the basis of spectral types and the calibrations given by Conti (1975) for the O-type stars and Böhm-Vitense (1981) for the B-type stars. Bolometric luminosities were calculated using the absolute visual magnitude and the bolometric corrections for the spectral types assigned by Conti (1975) for the O-type stars and from the appropriate model atmosphere of Kurucz (1979) for the B-type stars. Stellar masses were interpolated from the star’s location in the H-R diagram and the evolutionary tracks of Maeder and Meynet (1987). Note that these calibrations differ somewhat from those adopted at the beginning of this project, so some of the stars in Table 1 now have adopted stellar luminosities that are slightly below our cut-off 6 value of 10 Lo. Every star in Tables 1 and 2 is expected to possess a strong stellar wind. In many cases the maximum, or terminal, velocity of the wind has been measured from UV spectroscopy, and these values are also summarized in the tables from the references given by Abbott (1982a). For stars without UY measurements, the terminal velocities have been estimated from the ratio of terminal velocity to escape velocity found in other stars of similar spectral classification (e.g., Abbott 1978; Garmany and Conti 1984). These estimates are indicated by parentheses in Tables 1 and 2. We also list values of v sin i for all stars with measurements from rotational broadening of the photospheric lines, taken from Conti and Ebbets (1977), Hutchings (1981), and Uesugi and Fukuda (1982).
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