The Pictoris Phenomenon in A-Shell Stars

THE ␤ 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 ␤ Pic, HR 10, and 131 Tau, which can be followed to between ϩ70 and 100 km sϪ1 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 ␤ 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 ␤ Pic. These features are consistent with greater central clearing of a remnant debris disk, compared with ␤ Pic, and suggest that the majority of field A-shell stars are older than ␤ 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 sϪ1 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 ␤ 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 ␤ 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, ␤ 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 90Њ. Ca II or the Balmer series, especially H␣. The relation of ␤ 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- ␤ Pic disk might be unique and thus atypical of circumstellar tion is systematically offset by 8–10 km sϪ1 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 H5kmsϪ1 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 ␤ Pic and other A-shell stars, estimated to be accurate to H10–15 km sϪ1 . 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 ϭ 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 ... Ϫ15 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 Ϫ1.3 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 Ϫ16 Ϫ5 ...... HD 118232...... A4V A5 1987 Nov 10 Ϫ18 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 ...... A2Vaϩe 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 sϪ1 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 ␤ the photospheric Fe II absorption is minimal and is least for Pic might be unrecognized ␭ 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 ␭ Boo stars. We also do not detect the detectable circumstellar gas. 1600 Å feature characteristic of extreme ␭ 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 ␭ Boo stars but cannot rule out milder manifestations of ␭ Boo activity such as The rest of our program stars show prominent absorption in that seen in 131 Tau (Bohlender & Walker 1994). Fe II transitions with the strongest circumstellar absorption typically concentrated in a feature centered on the stellar 3. DETECTION OF ACCRETING, CIRCUMSTELLAR GAS radial velocity. In four stars, all of the circumstellar absorption In the IUE data, the rapid rotation of our program A-shell detectable at the S/N of the IUE data is confined within stars (Slettebak 1982) facilitates detection of circumstellar gas 40 km sϪ1 of the stellar radial velocity. No. 1, 1996 ␤ PIC PHENOMENON IN A-SHELL STARS L51

FIG. 1.—Transitions of Fe II (63) and (62) for the program A-shell stars FIG. 2.—Fe II (63) and (62) transitions for two observations of HD 88195 together with rapidly rotating comparison stars. The zero point of the radial (from 1991 July 15 and 1982 December 19), HD 168646, and HD 99022. As in velocity scale corresponds to the heliocentric velocity of Fe II ␭2739.546. In Fig. 1, for HD 168646 and HD 99022, leader lines indicate the location of the each case the strongest absorption components are the low-velocity features HVF, and the strongest absorption features are a blend of the LVF and (LVF) blended with circumstellar gas at the system velocity. This transition circumstellar gas at the system velocity. The two spectra of HD 88195 illustrate does not have interstellar absorption components. High-velocity features changing apparent column density in a LVF in addition to the HVF gas, both (HVF) are indicated by leader lines: (top) HD 15004, showing circumstellar of which are marked by leader lines. absorption compared to 68 Oph (flat and comparatively featureless spectrum); (middle) HD 15253; and (bottom) HD 112028. first evidence of accreting, circumstellar gas in a majority of the bright A-shell stars. Optical and UV studies of ␤ Pic have demonstrated that the The data for our program stars have been obtained over the strongest circumstellar feature is characterized by gas at the entire lifetime of the IUE and typically without supporting stellar radial velocity and prominent, discrete absorption com- optical data or optical detection of enhanced shell activity. 1 ponents redshifted some 10–30 km sϪ relative to the star, Each program star is represented in the IUE archives by only termed the low-velocity features (LVF) by Lagrange et al. a few spectra, so detection of accreting gas in 66% of the (1996). At the resolution of our data, LVF gas can be sample suggests, as is the case with ␤ Pic, that accretion events distinguished from the gas at the stellar radial velocity by a are routinely detectable. Three of our program stars (HD progressive offset of the centroid of the absorption with 42111A, HD 138629, and HD 158352) have been historically increasing excitation of the lower level of the Fe II transition. described as having strong to weak shell spectra (Slettebak When the entire sample of stars with shell features in their UV 1982; Abt & Moyd 1973; Dominy & Smith 1977); the IUE data spectra are considered, an offset of the LVF relative to the suggest weak shell absorption in HD 42111A, and no discern- interstellar feature or the stellar radial velocity is seen in 12 of ible shell features in the other two stars. These data suggest our program stars as well as in three shell stars with previously changes in the visibility of shell features on timescales of years, identified accreting gas (HR 10, ␤ Pic, and 131 Tau). The IUE comparable to the observed variability in the visibility of the data, therefore, suggest that systematically redshifted gas may accreting gas toward ␤ Pic (Deleuil et al. 1995) and similar to be routinely detectable in the spectra of A-shell stars, if the long-term variability noted in a number of Herbig Ae stars observed at higher S/N and velocity resolution. (Schevchenko et al. 1994). If long-term changes in the circumstellar gas column toward A-shell stars are present, our 3.2. High-Velocity, Circumstellar Gas detection rate of 66% represents a lower bound to the Nine of our program stars (Figs. 1, 2, and 3), show addi- frequency of high-velocity, accreting gas in A-shell stars. tional absorption on the long-wavelength side of the Fe II profiles. The circumstellar gas can be traced to between 70 and 3.3. Neutral, Atomic Gas 100 km sϪ1 relative to the star and frequently shows a local UV and optical studies of Herbig Ae stars have demon- maximum some 40–60 km sϪ1 longward of the stellar radial strated the presence of accreting, neutral atomic gas (Grinin et velocity, in the same velocity range occupied by the high- al. 1994, 1995; de Winter 1995; Grady et al. 1996b) in elements velocity features (HVF) in ␤ Pic spectra (Lagrange et al. with first ionization potentials below that of hydrogen (e.g., 1996). The comparatively low S/N of the IUE spectra pre- Na I,MgI). Given the short photoionization lifetimes for this cludes detection of higher velocity gas except in the case of atomic gas within a few stellar radii of an A star, Grinin et al. ␤ Pic, where the circumstellar gas is visible to ϩ200 km sϪ1 in (1994) noted that the presence of these species is inconsistent the IUE data and to higher velocities in HST GHRS spectra with transportation of the material in the gas phase or in small, (Boggess et al. 1991; Lagrange-Henri et al. 1995 and refer- refractory grains. The only viable transportation mechanisms ences therein; Vidal-Madjar et al. 1994). While the majority of discussed to date for such gas involve larger, solid parent our program stars have only a few observations in the IUE bodies with gas and potentially dust comae. Prominent Mg I archives, two stars with moderately rich shell spectra, HD 2852 Å absorption is detected toward 12 (75%) of our program 148283 and HD 88195, show changes in their HVF similar to stars. In one case, HD 138629, the lack of detectable absorp- those routinely observed in ␤ Pic. These data thus provide the tion in the Fe II fine-structure lines, coupled with the ratio of L52 GRADY ET AL.

While still limited, the IR data for these objects imply that the presence of circumstellar gas and dust are linked and that high-velocity accretion signatures may be more easily detected in systems with larger near-IR excesses. These data indicate, therefore, that the majority of A-shell stars have spectral similarities with ␤ Pic because they have circumstellar disks and thus do not represent a low-luminosity extension of the classical Be phenomenon, as suggested by Slettebak & Car- penter (1983). Accreting gas is routinely detected in the UV in the spectra of pre–main-sequence (PMS) Herbig Ae/Be stars with either high v sin i or polarimetric and photometric signatures indi- cating that the star is viewed essentially equator-on through a dust and gas disk (Pe´rez et al. 1993; Grady et al. 1993, 1996b; Grinin et al. 1991, 1994). Together with detection of circumstellar gas in the higher v sin i ␭ Boo stars (Holweger & Rentzsch-Holm 1995) and accretion in 131 Tau (Grady et al. 1996a), the IUE observations of A-shell stars imply that accreting gas is preferentially observed in the stellar equatorial plane, where material would be expected to concentrate in FIG. 3.—Fe II (63) and (62) transitions for HD 24863, two observations of either a proto–planetary or remnant debris disk. The com- HD 148283 (1986 October 9 and 1979 July 26), and HD 192518 with HD bined optical, UV, and IR data therefore imply that the field 118232 as a nonshell comparison star (no deep absorption line). As in Fig. 1, A-shell stars are those evolutionary successors of the PMS leader lines indicate the location of the HVF, and the strongest absorption features are the LVF gas blended with circumstellar gas at the system velocity. Herbig Ae/Be stars that are oriented most favorably for coronagraphic imaging. The combined IR and UV data fur- the Fe II ␭2599 equivalent width to that of Mg I, suggests that ther indicate that ␤ Pic is intermediate in character between the feature is interstellar rather than circumstellar; however, the Herbig Ae/Be stars and unambiguously main-sequence for the other stars, a circumstellar origin is likely. The Mg I objects such as the A-shell stars and thus is likely to be feature is resolved by the IUE in the spectrum of HD 88195 intermediate in age, as suggested by Lanz et al. (1995). (17 Sex) and includes absorption at the stellar radial velocity as well as a red wing visible to ϩ30 km sϪ1 relative to the bulk of This study was supported under the NASA Long-Term the atomic gas. Higher S/N spectra may well reveal similar Space Astrophysics program through NASA contract NASW absorption wings in the line of sight to other of our program stars. 4756 to Eureka Scientific. IUE data included in this study were made under several GO programs and as part of the NASA 4. DISCUSSION IUE Observatory Ultraviolet Spectral Survey of Bright Stars Jaschek et al. (1991) noted the unexpectedly high frequency (USSBS) program. The USSBS observations were made as of detection of shell spectral features (45%) in A stars with IR service observations of bright stars lacking previous IUE excesses that cannot be accounted for by binarity or source observations when the regular observing programs could not confusion and a high detection frequency of IR excesses be accommodated. The USSBS observations were made with among A-shell stars compared to the field A-star population. the gracious permission of the project scientist, Yoji Kondo. Two of the stars in their sample with prominent IR excesses, UV data analysis was carried out using the facilities of the IUE ␤ Pic and HD 168646, have prominent, high-velocity accreting Data Analysis Center located in the Laboratory for Astronomy gas. In contrast, the A-shell stars lacking excesses at 12 ␮m, and Solar Physics of Goddard Space Flight Center. We have but with firm IRAS point-source detections, tend to have less also made use of the SIMBAD database located in Strasbourg, prominent shell features in the UV and either smaller column France. We wish to thank David Bohlender for a number of densities of accreting gas or nondetections of accreting gas. suggestions which have improved the readability of the Letter.

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