Nancy D. Morrison

High-Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000-2001

We present an overview of the spectral variability of the peculiar F-type hypergiant ρ Cas, obtained from our long-term monitoring campaigns over the past 8.5 yr with four spectrographs in the northern hemisphere. Between 2000 June and September an exceptional variability phase occurred when the V brightness dimmed by about a full magnitude. The star recovered from this deep minimum by 2001 April. It is the third outburst of ρ Cas on record in the last century. We observe TiO absorption bands in high-resolution near-IR spectra obtained with the Utrecht Echelle Spectrograph during the summer of 2000. TiO formation in the outer atmosphere occurred before the deep brightness minimum. Atmospheric models reveal that the effective temperature decreases by at least 3000 K, and the TiO shell is driven supersonically with 5.4 × 10-2 M☉ yr-1. Strong episodic mass loss and TiO have also been observed during the outbursts of 1945-1947 and 1985-1986. A detailed analysis of the exceptional outburst spectra is provided, by comparing with high-resolution optical spectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse (Iab). During the outburst, central emission appears above the local continuum level in the split Na D lines. A prominent optical emission line spectrum appears in variability phases of fast wind expansion. The radial velocity curves of Hα and of photospheric metal absorption lines signal a very extended and velocity-stratified dynamic atmosphere. The outburst spectra indicate the formation of a low-temperature, optically thick circumstellar gas shell of 3 × 10-2 M☉ during 200 days, caused by dynamic instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit. We observe that the mass-loss rate during the outburst is of the same order of magnitude as has been proposed for the outbursts of η Carinae. We present calculations that correctly predict the outburst timescale, whereby the shell ejection is driven by the release of hydrogen ionization recombination energy.

SPECTROSCOPIC AND SPECTROPOLARIMETRIC OBSERVATIONS OF V838 MONOCEROTIS

The spectroscopic and spectropolarimetric variability of the peculiar variable V838 Monocerotis during the brighter phases of its multiple outbursts in 2002 is presented. Significant line profile variability of H alpha and Si II 6347.10 [angstrom] & 6371.36 [angstrom] occurred in spectra obtained between 2002 February 5 and 2002 March 14, and a unique secondary absorption component was observed near the end of this time period. Our observations also suggest that multiple shifts in ionization states occurred during the outbursts. Spectropolarimetric observations reveal that V838 Mon exhibited both intrinsic and interstellar polarization components during the initial stages of the second outburst, indicating the presence of an asymmetric geometry; however, the intrinsic component had significantly declined by February 14. We determine the interstellar polarization to be ...[see article] and we find the integrated intrinsic V band polarization on February 5 to be ...[see article] at a position angle of ...[see article]. The implications of these observations for the nature of V838 Monocerotis, its distance, and its ejecta are discussed.

HIGH-RESOLUTION OPTICAL SPECTRA OF THE HEAD OF THE COMET C/1996 B2 (HYAKUTAKE)

On 23 and 27 March 1996, we obtained high-resolution (R 26,000) echelle spectra by manually guiding on the head of Comet C/1996 B2 (Hyakutake). The two spectra cover different but slightly overlapping sets of 9 disjoint 70-A regions of the green, yellow, and red. The continuum signal-to-noise ratio is in the range 40--100 per spectral resolution element. We used line lists from the standard astronomical literature to identify lines of [O I], Na I, C2, NH2, CN, and H2O+, we unsuccessfully sought evidence for CO+, and we obtained upper limits on HCO line strengths. In addition, we used laboratory data to make new identifications, by wavelength coincidence only, of lines of NH2. For the lines that remain unidentified, we have listed measured wavelengths and relative fluxes, in order to aid future efforts at identification. Our spectra include the green and one of the red lines of [O I], and the indicated photon flux ratio of λ 5577 / (λ 6300 + λ 6364) is in the range 0.12 to 0.16, consistent with formation of the excited O atoms from photodissociation of H2O.

Non-LTE, line-blanketed model atmospheres for late O- and early B-type stars

The use of non-LTE line-blanketed model atmospheres to analyze the spectra of hot stars is reported. The stars analyzed are members of clusters and associations, have spectral types in the range O9-B2 and luminosity classes in the range III-IV, have slow to moderate rotation, and are photometrically constant. Sampled line opacities of iron-group elements were incorporated in the radiative transfer solution; solar abundances were assumed. Good to excellent agreement is obtained between the computed profiles and essentially all the line profiles used to fix the model, and reliable stellar parameters are derived. The synthetic M II 5581 equivalent widths agree well with the observed ones at the low end of the temperature range studied, but, above 25,000 K, the synthetic line is generally stronger than the observed line. The behavior of the observed equivalent widths of N II, N III, C II and C III lines as a function of Teff is studied. Most of the lines show much scatter, with no consistent trend that could indicate abundance differences from star to star.

Spectroscopy of the growing circumstellar disk in the Delta Scorpii Be binary

We present the results of a spectroscopic monitoring program of the binary system Delta Scorpii, whose primary became a Be star after the last periastron encounter in the Summer of 2000. The observations cover a period of 2 years (March 2001–June 2003) and are a continuation of our previous campaign reported in Miroshnichenko et al. (2001). We found that the emission-line spectrum was gradually strengthening over the whole monitoring period.

A new phase of activity of the Herbig Be star HD 200775 in 2001: Evidence for binarity

The results of high-resolution spectroscopy of the Herbig Be star HD 200775 obtained within the framework of a cooperative observing programme in 2000–2002 are presented. A new high-activity phase of the object’s Ha line occurred in the middle of 2001 in full agreement with a 3.68-year periodicity predicted by Miroshnichenko et al. (1998). A complicated picture of the Ha line profile variability near the activity maximum phase turned out to be very similar to that observed during the previous one in 1997. Variations of the radial velocity with the activity phase are detected in He I, Si II, and S II photospheric lines. The observed phenomena are interpreted in the framework of a model in which the star, together with its gaseous envelope, is a component of an eccentric binary system. A preliminary orbital solution is derived, and the system’s parameters are estimated from the radial velocity curves of the Ha emission line. We find that the orbital eccentricity is e ~ 0.3, the mean companion separation is ~1000 R?, and the secondary companion is most likely to be a ~3.5 M, pre-main sequence object. We emphasize the importance of coordinated spectroscopic and interferometric observations at different phases of the object’s activity for further understanding the properties of the system.

High State of Hα Emission Activity of the Herbig Be Star HD 200775

We present the results of high-resolution spectroscopic observations of the pre–main-sequence Herbig Be star HD 200775 obtained between 1994 September and 1998 February. Strong variations of the Ha line associated with an extended period of strong emission during the summer and fall of 1997 have been observed. The line equivalent width took on the highest value reported in the last 20 yr. A review of the observational literature dating back to 1977 indicates periodic behavior of the Ha equivalent width with a period of 3.68 yr. These variations might be due to interaction between the stellar wind and the protostellar envelope, as suggested previously by Beskrovnaya et al., or to an effect of a possible close companion, such as those detected in some classical Be stars. The next high emission state is predicted to occur in the first half of 2001. We emphasize the importance of coordinated photometric and high-resolution spectroscopic observations for further understanding of the star’s behavior.

A SEARCH FOR AP STARS WITH VERY LONG PERIODS

Four-color (uvby) photoelectric observations have yielded new periods for three peculiar A-type stars. For HD 8441, the period P is equal to 69.5 days, for RD 12288, P = 34.9 days, and for RD 216533, P = 17.20 days. The period of 12.448 days derived by Preston (1972) for HD 24712 is confirmed. The periods of HD 18078 and RD 2453 are longer than one year. These results nearly double the number of Ap stars known to have periods longer than 25 days and add weight to the evidence that magnetic stars undergo rotational deceleration during their lffetimes. Key words: photometry - peculiar A-type stars

Spectroscopic study of the long-period dust-producing WC7pd+O9 binary HD 192641

We present the results of an optical spectroscopic study of the massive Wolf-Rayet binary HD192641=WR137. These 1986-2000 data cover the dust-formation maximum in 1997. Combining all available measurements of radial velocities, we derive, for the first time, a spectroscopic orbit with period 4766 ± 66 days (13.05 ± 0.18 years). The resulting masses, adopting i=67 ◦ , are MO=20 ± 2M⊙ for the O component and MW R=4.4 ± 1.5M⊙ for the WR component. These appear, respectively, ∼ normal and on the low side for the given spectral types. Analysis of the intense multi-site spectroscopic monitoring in 1999 shows that the CIII λ5696 and CIV λ5802/12 lines have the highest intrinsic variability levels. The periodogram analysis yields a smallamplitude modulation in the absorption troughs of the CIV λ5802/12 and HeI λ5876 lines with a period of 0.83 days, which could be related either to pulsations or largescale rotating structures as seen in the WN4 star EZ Canis Majoris (WR6). Wavelet analysis of the strong emission lines of CIII λ5696 and CIV λ5802/12 enabled us to isolate and follow for several hours small structures (emission subpeaks) associated with density enhancements within the wind of the Wolf-Rayet star. Cross-correlating the variability patterns seen in different lines, we find a weak but significant correlation between the varability in emission lines with different ionization potential, i.e. in lines formed at different distances from the WR stellar core. Adopting a β wind-velocity law, from the motion of individual subpeaks we find β ∼ 5, which is significantly larger than the canonical value β ≃ 1 found in O-star winds.

The spectral variability of HD 192639 and its implications for the star's wind structure

We report the analysis of an extensive set of spectroscopic data of the O(f) supergiant HD 192639. A Fourier analysis of our time-series reveals a recurrent variability with a “period” of roughly 4.8 days which is most prominent in the absorption components of the He ii λ 4686 and Hα P-Cygni profiles. The same periodicity is also detected in the blue wing of several absorption lines (e.g. Hβ). The variations of the absorption components correspond most probably to a cyclical modulation of the amount of stellar wind material along the line of sight towards the star. The 4.8-day period affects also the morphology of the double-peaked He ii λ 4686 and Hα emission components, although these emission components display also variations on other (mainly longer) time scales. The most likely explanation for the 4.8-day modulation is that this cycle reflects the stellar rotational period (or half this period). We find that the most important observational properties can be explained – at least qualitatively – by a corotating interaction region or a tilted confined corotating wind.