George H. Herbig

The diffuse interstellar bands. IX: Constraints on the identification

Equivalent widths of the diffuse interstellar bands (DIBs) λλ5780, 5797 and of the interstellar D1 and D2 lines have been measured on Lick and Canada-France-Hawaii Telescope coude spectrograms of 93 stars

The peculiar object HD 44179 /'The red rectangle'/

A strong infrared source detected in the AFCRL sky survey is confirmed, and is identified with the binary star HD 44179, embedded in a peculiar nebula. UBVRI and broad-band photometry between 2.2 and 27 microns are combined with blue, red, and near-infrared spectra, polarimetry and spectrophotometry of the star, and a range of direct and image-tube photographs of the nebula, to suggest a composite model of the system. In this model, the infrared radiation derives from thermal emission by dust grains contained in a disklike geometry about the central object, which appears to be of spectral type B9-A0 III and which may be in pre-main-sequence evolution. Two infrared emission features are found, peaking at 8.7 and 11.3 microns, the latter corresponding to the feature seen in the spectrum of the planetary nebula NGC 7027. The complex nebular structure is discussed on the basis of photographs through narrow-band continuum and emission-line filters. The polarization data support the suggestion of a disk containing some large particles. No radio continuum emission is detected.

High-Resolution Spectroscopy of FU Orionis Stars

High-resolution spectroscopy was obtained of the FUors FU Ori and V1057 Cyg between 1995 and 2002 with the SOFIN spectrograph at NOT and with HIRES at Keck I. During these years FU Ori remained about 1 mag. (in B) below its 1938–39 maximum brightness, but V1057 Cyg (B ≈ 10.5 at peak in 1970–71) faded from about 13.5 to 14.9 and then recovered slightly. Their photospheric spectra resemble that of a rotationally broadened, slightly veiled supergiant of about type G0 Ib, with veq sin i = 70 km s −1 for FU Ori, and 55 km s −1 for V1057 Cyg. As V1057 Cyg faded, P Cyg structure in Hα and the IR Ca II lines strengthened, and a complex shortwarddisplaced shell spectrum of low-excitation lines of the neutral metals (including Li I and Rb I) increased in strength, disappeared in 1999, and reappeared in 2001. Several SOFIN runs extended over a number of successive nights so that a search for rapid and cyclic changes in the spectra was possible. These spectra show rapid night-to-night changes in the wind structure of FU Ori at Hα, including clear evidence of sporadic infall. The equivalent width of the P Cyg absorption varied cyclically with a period of 14.8 days, with phase stability maintained over 3 seasons. This is believed to be the rotation period of FU Ori. The internal structure of its photospheric lines also varies cyclically, but with a period of 3.54 days. A similar variation may be present in V1057 Cyg, but the data are much noisier and that result uncertain. As V1057 Cyg has faded and the continuum level fallen, the emission lines of a pre-existing low-excitation chromosphere have emerged. Therefore we believe that the ‘line doubling’ in V1057 Cyg is produced by these central emission cores in the absorption lines, not by orbital motion in an inclined Keplerian disk. No dependence of veq sin i on wavelength or excitation potential was detected in either FU Ori or V1057 Cyg, again contrary to expectation for a self-luminous accretion disk. It was found also that certain critical lines in the near infrared are not accounted for by synthetic disk spectra. It is concluded that a rapidly rotating star near the edge of stability, as proposed by Larson (1980), can better account for these observations. The possibility is also considered that FUor eruptions are not a property of ordinary T Tauri stars, but may be confined to a special sub-species of rapidly rotating pre-main sequence stars having powerful quasi-permanent winds. Subject headings: stars: evolution — stars: Pre-Main-Sequence — stars: individual (FU Ori, V1057 Cyg)

The evolution of chromospheric activity and the spin-down of solar-type stars

IUE data for 31 solar-type stars are compared with observations of T Tauri stars in order to determine whether the pattern of main sequence chromospheric decay shown by stars older than about 100 million yr extends back to ages of 100 million yr appropriate for T Tauri stars. An analysis of the time decay of stellar ultraviolet and X-ray emission establishes a relationship between emission level and axial rotation, which is expressed in terms of the Rossby parameter. It is shown that the intensity of ultraviolet chromospheric and transition region lines of solar-type stars declines with age. The activity-age relation for main sequence stars older than about 100 million yr is fitted best by an exponential law whose rate of falloff with age depends on temperature.

The Search for Interstellar C60

The optical region of a number of reddened O-type stars has been examined on Keck I HIRES spectrograms (R = 45,000) for evidence of interstellar C60. No absorption features were detected near the laboratory C60 wavelengths 3857 and 3980 A. An interstellar feature is present at 6220.8 A, but it is unacceptably far from the laboratory gas-phase wavelength of 6217.5 A. It is probably just another of the weak diffuse interstellar bands (DIBs), which are numerous in that spectral region. The most astronomically promising C60 feature was measured in the laboratory at 3284 A in liquid or solid matrices. Its gas-phase wavelength can be inferred either from matrix shifts of C60 features at longer wavelengths or from high-temperature gas-phase measurements. On that evidence, the interstellar feature could fall anywhere between about 3244 and 3306 A. Its width is uncertain but here is taken to be about 1 A . No interstellar absorption fitting these specifications and as strong as 16 mA has been detected in the stars observed, including Cyg OB2/8A of E(B-V) = 1.60. It follows that in that particular line of sight and for the assumed FWHM of 1 A , N(C60) < 4.5 × 1011 cm-2. However, some recent laboratory spectroscopy suggests that its width may be very much larger, in which case this limit would be invalid. At this upper limit, the corresponding number of carbon atoms contained in neutral C60 indicates that that molecule would be only a minor contributor to the total amount of C in that direction, and would be less than 1% of the amount that may be tied up in the DIBs. Stronger C60 bands are known in the laboratory at approximately 2110 and 2566 A, but the upper limit on 3284 A suggests that they will not be easy to detect without high resolution, high signal-to-noise ratio (S/N) satellite spectroscopy and better laboratory gas-phase wavelengths. An estimate of the column density of C, under the assumption that the 9577, 9632 A bands are indeed due to C and that the laboratory f-value is correct, indicates that the C/C60 abundance in that line of sight is greater than 100.

History and Spectroscopy of EXor Candidates

The EXors are T Tauri stars (TTSs) that occasionally flare up from minimum light, apparently as the result of a massive infall of circumstellar material. The prototype, EX Lupi, is the only example that has been examined spectroscopically in any detail, so this paper surveys what can be gleaned from the literature about five candidate EXors, and describes new observations. The aim is to clarify the nature of these objects, and to determine whether they bear a convincing resemblance to EX Lup itself. The spectroscopy was carried out with the HIRES spectrograph (R = 48,000) at the Keck I telescope between 2004 and 2007. Three of the stars examined are in or near the Orion Nebula (NY, V1118, and V1143 Ori), while V1184 Tau is in the molecular cloud CB34, and V350 Cep is at the edge of the young cluster NGC 7129. The spectrograms were obtained at random times, and there was no coordinated photometry, but it was possible to conclude that the three Orion stars can be considered conventional EXors. At minimum light, they resemble K- or early M-type dwarfs plus a T Tauri-like emission spectrum, but no spectral characteristic was found that set them aside from ordinary TTSs. Such a signature might be found in deep-infrared (IR) photometry, but they do not stand out at 2 μm. Several interesting spectroscopic phenomena were observed, however, notably the appearance of the Li I λ6707 line in emission in V1118 Ori near maximum, and the detection of simultaneous infall and outflow at the Na I D12 lines in several EXors (although that has also been reported in several Classical TTSs). V1184 Tau is not an EXor: it is unclear if its activity is repetitive. Nor is V350 Cep: after recovering from a deep minimum about 1975, it has since remained near maximum for nearly three decades.

Near-simultaneous ultraviolet and optical spectrophotometry of T Tauri stars

A set of near-simultaneous ultraviolet and optical spectra and UBVR(J)I(J) photometry of five T Tauri stars has been analyzed for the shape of the energy distribution shortward of 3000 A. The far-ultraviolet continua of these stars are very much stronger than the level of light scattered from longer wavelengths in the IUE spectrograph. The results, expressed as two-color plots, show that the UV colors of T Tauri stars differ significantly from those expected from their optical spectral types. Although these particular K-type T Tauri stars are not extreme members of the class, they have the UV colors of A stars. The spectral shape of this UV excess is approximately that expected from published chromospheric models of T Tauri stars.

The 1993–1994 Activity of EX Lupi

EX Lupi is a classical T Tauri star (and the EXor prototype) subject to sporadic outbursts. The historic record shows that it remains at about V = 13.2 (or mpg = 14.7:) for extended periods, from which it has been observed to brighten to as much as V = 8.4 (on one occasion in 1955-1956). During 1993-1994 the star remained slightly above normal minimum, at about V = 12.8, and from that level rose to three maxima at V = 11.4 and on other occasions to about V = 12.0. At minimum light an M0 V absorption spectrum is present. At outburst this spectrum is veiled by a hot continuum (well shown by an IUE exposure), the equivalent widths of the optical-region emission lines decrease, and reverse P Cygni absorption components appear at the higher Balmer lines. The outbursts are believed to be due to episodic infall onto the M0 star.

On the interpretation of the spectrum of FU Orionis

Several difficulties with the accretion disk interpretation of the spectra of FU Orionis objects are point out. The central contention of the paper is that the double absorption lines observed in FU Ori and other members of the group, which have been claimed to be the signature of a self-luminous Keplerian disk, can be produced equally well by a single rotating star. Direct support for this assertion comes from observations of similar line structure in several high-luminosity stars (ρ Cas and RW Cep in particular) in which there is no reason to think that a disk exists. The properties of the absorption-line spectrum of FU Ori as observed at a resolution of about 5 × 10 4 are described and compared to simple models of a single rotator with a stratified atmosphere

ON THE NATURE OF THE SMALL DARK GLOBULES IN THE ROSETTE NEBULA

The small dark globules seen against the nebulosity in the northwest quadrant of NGC 2237-2244 are shown on a 120-inch direct photograph to have an elongated, tear-drop form with the symmetry axes and sharper edges directed toward the central star cluster. This orientation is shared by the well-known elephant trunk structure, which in general lies farther from the center than these isolated dark spots. Examples of globules still connected with large dark masses by dust filaments are also present. It is suggested that these globules represent a late stage in the pinching-off and dissipation of elephant trunks as the central cavity of the H II region expands into the peripheral dust clouds, and that these globules are not protostars. It is estimated that the age of a typical small isolated globule in this region of NGC 2237-2244 is of the order of 10,000 years.