The Pictoris Phenomenon in A-Shell Stars
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THE ASTROPHYSICAL JOURNAL, 471 : L49–L52, 1996 November 1 q 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE b PICTORIS PHENOMENON IN A-SHELL STARS: DETECTION OF ACCRETING GAS C. A. GRADY1, 2 Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA 94602 MARIO R. PEREZ´ 1 Applied Research Corporation, Landover, MD 20785 A. TALAVERA1 Laboratorio de Astrofı´sica Espacial y Fı´sica Fundamental, I.N.T.A., Apartado 50727, 28080-Madrid, Spain AND B. MCCOLLUM,L.A.RAWLEY,M.N.ENGLAND, AND M. SCHLEGEL Science Programs, Computer Sciences Corporation, 10,000A Aerospace Road, Lanham-Seabrook, MD 20706 Received 1996 July 5; accepted 1996 August 22 ABSTRACT We present the results of an expanded survey of A-shell stars using IUE high-dispersion spectra and find accreting, circumstellar gas in the line of sight to nine stars, in addition to the previously identified b Pic, HR 10, and 131 Tau, which can be followed to between 170 and 100 km s21 relative to the star. Two of the program stars, HD 88195 and HD 148283, show variable high-velocity gas. Given the small number of IUE spectra for our program stars, detection of high-velocity, accreting gas in 2/3 of the A-shell stars sampled indicates that accretion is an intrinsic part of the A-shell phenomenon and that b Pic is not unique among main-sequence A stars in exhibiting such activity. Our program stars, as a group, have smaller column densities of high-velocity gas and smaller near-IR excesses compared with b Pic. These features are consistent with greater central clearing of a remnant debris disk, compared with b Pic, and suggest that the majority of field A-shell stars are older than b Pic. Subject headings: accretion: accretion disks — circumstellar matter — line: profiles — ultraviolet: stars 1. INTRODUCTION Over the course of its lifetime, the International Ultraviolet Explorer (IUE) has amassed an extensive archive of 25 km s21 One of the more important astronomical discoveries of the resolution spectra, including many of the brighter A-shell 1980s was the detection of a large circumstellar dust disk stars. Slettebak & Carpenter (1983) demonstrated the pres- around b Pictoris (Aumann 1985; Smith & Terrile 1984). ence of extensive line blanketing due primarily to Fe II in the Subsequently, accreting, circumstellar gas has been detected mid-UV spectra of eight A- to F-shell stars including b Pic. In toward this star (Kondo & Bruhweiler 1985; Hobbs 1986) and the intervening 11 years an additional 10 stars, drawn either has been interpreted as the gaseous comae of infalling, from Slettebak (1982), Andrillat, Jaschek, & Jaschek (1986), evaporating bodies dynamically similar to short-period comets or Lagrange-Henri et al. (1990b), have been observed in high (Ferlet et al. 1987; Beust et al. 1989; Beust & Lissauer 1994). dispersion. These data can be used to determine whether Historically, however, b Pic was identified as an A-shell star, accretion is commonly observed in UV spectra of A stars that is, a rapidly rotating A star with additional superposed, known to possess circumstellar gas and that are viewed at sharp, low-ionization absorption features in transitions such as inclination angles close to 908. Ca II or the Balmer series, especially Ha. The relation of b Pic to other A-shell stars remains uncertain (Beust 1995) since 2. PROGRAM STARS AND OBSERVATIONS optical surveys detected accreting gas only for HR 10 (Lagrange- Our program stars are listed in Table 1. Data reduction for Henri et al. 1990a, 1990b). The lack of accretion detections in the spectra is described in Pe´rez, Grady, & The´ (1993). The optical A-shell star spectra has fueled the suspicion that the long-wavelength camera high-dispersion wavelength calibra- b Pic disk might be unique and thus atypical of circumstellar tion is systematically offset by 8–10 km s21 relative to helio- material around intermediate-mass stars. centric velocities (Boggess et al. 1991). Apart from noting the Optical surveys for circumstellar gas are limited to a few offset, we have made no explicit correction for this effect, since transitions of comparatively low-abundance elements, usually all of our measurements are restricted to the IUE data. Ca II and Na I. In contrast, the mid- and far-UV spectrum is Measured radial velocities for interstellar and low-velocity rich in transitions from the common ionization stages of the circumstellar gas are estimated to be accurate to H5kms21 in more abundant elements. Many of these same transitions are this frame. The centroid velocities for broader features are seen in the UV spectrum of b Pic and other A-shell stars, estimated to be accurate to H10–15 km s21 . The brighter together with numerous transitions to high-lying metastable program stars have IUE high-dispersion spectra with levels (Slettebak & Carpenter 1983; Kondo & Bruhweiler S/N 5 10–12 near 2800 Å (the region of peak sensitivity) with 1985), which enables us not only to identify circumstellar data processed under the IUESIPS processing system (Turn- features but also to map their velocity extent much more rose & Thompson 1984). In Table 1 we list optical spectral sensitively than in the optical. types, and mid-UV (MUV) spectral types estimated by com- 1 Guest Observer, International Ultraviolet Explorer, operated by NASA at parison of our high-dispersion spectra binned to 3.5–4 Å Goddard Space Flight Center. resolution with standard stars from Wu et al. (1983). The 2 Send offprint requests to [email protected]. difference between the optical and MUV type is a measure of L49 L50 GRADY ET AL. Vol. 471 TABLE 1 PROGRAM A-SHELL STARS g SPECTRAL TYPE VELOCITIES DATE OF a f STAR Optical MUVOBSERVATION RV IS LVF HVF Maximum Stars with High-Velocity Gas HD 15004............ A0IIIb A2 1995 Aug 1 2 10 10 70 100 HD 15253............ A2psheb A5 1994 Sep 1 1.5 5 15 65 100 HD 112028........... A1 IV–V A5–7 1994 Feb 3 Var. 5 20 55 90 HD 88195............ A1Vsh A5 1991 Jun 16 22 20 30 70 100 1982 Dec 19 20 40 70: 100 HD 168646........... A2IIIb A5–7 1995 Jul 31 ... 215 10 50 75 HD 99022............ A3–5sh A7–8 1991 Jun 26 25 40 90 120 HD 24863............ A4b A5–7 1995 Aug 1 ... 30 50 75 120 HD 148283........... A5Vsh A8 1986 Oct 9 21.3 10 25 70 70 1979 Jul 26 0 20 70 100 HD 192518........... A5IVsh A7–9 1992 Sep 11 5. 15 30 75 100 Stars with Low-Velocity Circumstellar Gas Only HD 38090............ A3III-sh A5 1992 Feb 3 Var. 30 55 ... 70 HD 42111A .......... A3Vc A2–3 1994 Feb 3 34 40 40 ... 60 HD 98058............ A7IVsh F0 1994 Apr 13 1.6 10 15 ... ... Stars Lacking UV Shell Features HD 138629........... A5sh A5 1986 Nov 26 216 25 ... ... ... HD 118232........... A4V A5 1987 Nov 10 218 5 ... ... ... HD 158352........... A8Vsh A7–8 1994 Sep 11 36 ... ... ... ... Stars with Previously Identified Accreting Gas HD 256d ............. A2IV–Vshd A4 1990 May 31 ... 00... 40 HD 38545h ........... A2Va1e A3–4 1995 Sep 21 21 30 25 75 100 HD 39060i ........... A5IVsh A5–8 1984 Nov 5 21 30 35: 100 200 a Slettebak 1982, unless noted. b Spectral type from SIMBAD. c Lagrange-Henri et al. 1990b. d Lagrange-Henri et al. 1990a. e Bohlender & Walker 1994. f Heliocentric radial velocities. g Velocities in IUE LW Spectrograph frame, which is 8–10 km s21 offset from true heliocentric velocities. h Grady et al. 1996a. i Kondo & Bruhweiler 1985. the Fe II line blanketing and hence the richness of the shell as relatively narrow features superposed on the broad, photo- spectrum. The program stars span the range of minimal shell spheric profiles. The contrast between the photospheric and blanketing to comparatively rich shell spectra. “shell” features is greatest for the earlier spectral types where King & Patten (1992) have suggested that shell stars like b the photospheric Fe II absorption is minimal and is least for Pic might be unrecognized l Boo stars. With the exception of the latest spectral types in our sample. Kondo & Bruhweiler HD 168646, our program stars have 1150–2000 Å low-disper- (1985) noted that the presence of similar absorption features sion spectra that can be directly compared with the Wu et al. in transitions to the ground configuration of Fe II, excited (1983) standard stars. FUV spectral types for our program J-levels in the Fe II ground configuration, and to high-lying stars prior to correction for foreground interstellar extinction metastable levels demonstrates a circumstellar origin for the are within one subtype of the optical type for all of the stars, absorption. The transitions to the 0 eV level of the ground with the exception of HD 99022. The FUV spectrum of that configuration of Fe II are also produced in the diffuse inter- star matches A2 rather than the A3–5 optical type (Slettebak stellar medium. At the epoch of the IUE observations, two of 1982). We find no indication of the systematic FUV shift to our optically identified A-shell stars, HD 138629 and HD later spectral types relative to the optical spectral type, char- 158352, showed only these transitions and thus have no acteristic of extreme l Boo stars. We also do not detect the detectable circumstellar gas. 1600 Å feature characteristic of extreme l Boo stars (Hol- weger, Koester, & Alard 1994). These data suggest, therefore, 3.1. The Low-Velocity Gas that the field A-shell stars are not extreme l Boo stars but cannot rule out milder manifestations of l Boo activity such as The rest of our program stars show prominent absorption in that seen in 131 Tau (Bohlender & Walker 1994).