Stephan Mazuk

Variability of Disk Emission in Pre-main-sequence and Related Stars. I. HD 31648 and HD 163296: Isolated Herbig Ae Stars Driving Herbig-Haro Flows

Infrared photometry and spectroscopy covering a time span of a quarter-century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are isolated Herbig Ae stars that exhibit signs of active accretion, including driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was found to be relatively quiescent photometrically in its inner disk region, with the exception of a major increase in emitted flux in a broad wavelength region centered near 3 μm in 2002. In contrast, HD 31648 has exhibited sporadic changes in the entire 3-13 μm region throughout this span of time. In both stars, the changes in the 1-5 μm flux indicate structural changes in the region of the disk near the dust sublimation zone, possibly causing its distance from the star to vary with time. Repeated thermal cycling through this region will result in the preferential survival of large grains, and an increase in the degree of crystallinity. The variability observed in these objects has important consequences for the interpretation of other types of observations. For example, source variability will compromise models based on interferometry measurements unless the interferometry observations are accompanied by nearly simultaneous photometric data.

0.8-2.5 Micron Spectroscopy of Nova Ophiuchi 1998*

We report 0.8-2.5 μm spectroscopy of the very fast Nova Ophiuchi 1998 from June 19 and 22 to October 2.15 1998 UT. On the first night the Paschen and Brackett emission lines dominated the spectrum, although He I 1.0830 μm was the strongest single line present and He II was very weak. There were also broad, symmetric emission wings underlying the H I and He I lines with FWHM of about 10,000 km s-1. Three nights later the rapid evolution of the nova to higher excitation conditions was evident from the much stronger He II lines which had increased by a factor of 4 relative to the hydrogen lines. The C III line at 0.9710 μm also appeared the second night, a line that we have heretofore seen only in Wolf-Rayet stars and which almost certainly indicates an overabundance of carbon. The broad emission wings were also present. About 110 days later on October 2.15 UT, the emission lines were very weak (5%-15% of the continuum) and only a few Paschen features and lines of He II were present. At no time during our observations was there any evidence of a long-wavelength upturn indicative of thermal emission from dust. The continuum magnitudes on the dates of observation at J (1.25 microns) were 9.4, 10.9 to 15.0, respectively. The optical decline of 0.37 mag day-1 makes this nova one of the fastest ever seen. Model fits of the broad lines profiles suggest that they originate in an optically thin, spherically expanding shell. The line ratios from the narrow components deviate significantly from case B values and seem to come from an optically thick (τ = 10-100 at line center in Paα), high-density (ne ~ 1011 cm-3) gas at around 10,000 K.

0.8-13 Micron Spectroscopy of V838 Monocerotis and a Model for Its Emission

Abstract : We report on the results of a number of infrared spectra (0.8-2.5, 2.1-4.6, and 3-14 microns) of V838 Monocerotis, taken from a short time after discovery in 2002 January to about 14 months later, in early 2003. The spectrum evolved dramatically, changing from a quasi-photospheric stellar spectrum with weak atomic emission lines (some with P Cygni profiles) to one showing a wide range of deep absorption features indicative of a cool, extended atmosphere with a circumstellar dust shell. The early spectra showed lines of s-process elements, such as Sr II and Ba I. The later spectra showed absorption by gaseous H2O, CO, AlO, TiO, SiO, SO2, OH, VO, and SH, as well as a complex of emission near 10 microns reminiscent of silicate emission, with a central absorbing feature at 10:3 microns. Thus, V838 Mon appears to be oxygen-rich. A simple, spherically symmetric model of the system involving a central star with a two-component expanding circumstellar shell is presented that is able to explain the major molecular features and spectral energy distribution in the objects late stages. The derived shell mass and distance are 0.04 M(circle dot solar) and 9.2 kpc, respectively.

Near-Infrared Spectrophotometry and the CO Emission in V2274 Cygni (Nova Cygni 2001 No. 1)

Infrared spectroscopy of V2274 Cygni (=Nova Cygni 2001 No. 1) is presented for two widely separated epochs, 17 and 370 days after discovery. In addition to the Paschen and Brackett series of H I, the early-time spectrum shows strong emission lines of C I and N I, fluorescently excited lines of O I, and emission from the first overtone of carbon monoxide. Because the initial data were probably acquired no more than 18 days after outburst, CO molecule formation occurred remarkably quickly in the dense, cool, carbon and oxygen rich ejecta. Rapid formation was also seen in NQ Vul, V842 Cen, and V705 Cas, three other novae in which first-overtone CO emission has been detected. Formation of the CO molecule may occur chemically in a process that requires H2 as a precursor or directly through radiative association. The overtone emission of V2274 Cyg indicates a temperature of ~2500 K. The vibrational levels show no obvious departures from thermal equilibrium, which may indicate high optical depths in the fundamental. A large 13C/12C ratio (0.83 ± 0.3) is also indicated by the observations, consistent with the fast CNO burning expected in novae explosions. By the time of the second epoch observations, the emission lines of the neutral C, N, and O had disappeared. He I λ10830 was the dominant emission feature in the spectrum. In addition to the hydrogen lines, recombination features of He II were also strong. The common nebular lines of [S III] were seen but only two coronal line, [S VIII] λ9911 and [Si VI] λ19645, were detected. The CO emission had disappeared, but a strong thermal dust component was present. The interstellar reddening for the system was found to be E(B-V) = 1.3. This extinction, together with the absolute magnitude derived from the rate of decline of the light curve, suggest a distance of ~10.8 kpc. This places V2274 Cyg well out of the Galactic plane. The small number of novae with spectroscopic detections of carbon monoxide all have prominent C I lines, moderate speed classes and ejection velocities, exhibit marked dust formation events, and result from an explosion on a CO-type white dwarf. Based on these similarities, the spectrum of V2274 is proposed as a likely near-infrared spectral template for other novae that display carbon monoxide emission.

Near-Infrared Emission Lines of V723 Cassiopeiae (Nova Cassiopeiae 1995)

Near-infrared spectroscopy (0.8-2.5 μm) from two epochs is presented for the very slow nova V723 Cassiopeiae (=Nova Cassiopeiae 1995). Its bright, comparatively narrow emission lines (FWHM ~ 500 km s-1) make it ideal for determining wavelengths of unidentified emission lines that appear in the nebular/coronal stages of novae. We present wavelengths for 16 previously unobserved or unidentified features and search for identifications by looking for coincidences with known atomic transitions. For a candidate transition, the likely abundance of the parent ion is considered and then a collisional, fluorescent, or recombination process capable of exciting the feature is sought. Four of the lines are identified in this manner, all of them collisionally excited, forbidden transitions. One of these, [Ti VII] λ22050, is a new coronal line that de-excites directly to the ground state. Three other lines are associated with transitions from collisionally excited, low-lying, metastable levels of Fe+5. Of the remaining unidentified features, five, with wavelengths of 8926, 11110, 11900, 15545, and 20996 A, have been observed in other novae. Comparing the time development of these features with the [S VIII] λ9913 coronal line indicates that all have parent ions of lower excitation (<280 eV) than S+7.

The Near-Infrared Spectrum of the Planetary Nebula IC 5117

Infrared spectroscopy from 0.8 to 2.5 μm is presented for the planetary nebula IC 5117. The emission lines of IC 5117 span a wide range of ionization that includes He II, [S III], [S II], [N I], and H2. The reddening measured from the hydrogen lines is E(B-V) = 0.79, most of which is probably interstellar in origin. The He/H abundance ratio is 0.113 ± 0.015, with approximately 10% of the helium being doubly ionized. Using our measurements of [S II] and [S III] lines and published observations of [S IV], we find a sulfur abundance, relative to hydrogen, of N(S)/N(H) = 7.8 × 10-6, or approximately half the solar value. Fluxes and flux limits for several lines of molecular hydrogen are presented. Measurements of 1–0 transitions, together with the limits on 2–1 transitions, indicate Tvib ~ Trot = 1900 K, suggesting a purely collisional excitation mechanism. The ortho-to-para ratio is ~3, a value that is also indicative of collisional excitation. The presence of [C I] λ9850 is consistent with previous studies of IC 5117 that indicated carbon is more abundant than oxygen. IC 5117 follows the trend of planetary nebulae that display bipolar outflows and H2 emission to be carbon-rich. We confirm the results of Zhang & Kwok, who reported infrared continuum emission substantially in excess of that produced by the ionized gas. This emission is most likely due to hot dust (T ~ 1300 K) and accounts for roughly half of the continuum between 1.5 and 2 μm.

Dust Emission Features in NGC 7023 between 0.35 and 2.5 Microns: Extended Red Emission (0.7 Microns) and Two New Emission Features (1.15 and 1.5 Microns)

We present 0.35-2.5 μm spectra of the south and northwest filaments in the reflection nebula NGC 7023. These spectra were used to test the theory of Seahra & Duley that carbon nanoparticles are responsible for extended red emission (ERE). Our spectra fail to show their predicted second emission band at 1.0 μm even though both filaments exhibit strong emission in the familiar 0.7 μm ERE band. The northwest filament spectrum does show one, and possibly two, new dust emission features in the near-infrared. We clearly detect a strong emission band at 1.5 μm which we tentatively attribute to β-FeSi2 grains. We tentatively detect a weaker emission band at 1.15 μm which coincides with the location expected for transitions from the conduction band to midgap defect states of silicon nanoparticles. This is added evidence that silicon nanoparticles are responsible for ERE as they already can explain the observed behavior of the main visible ERE band.

Paschen Lines and the Reddening of the Radio Galaxy 3C 109

This paper presents spectrophotometric observations from 0.8 to 2.5 μm of the broad emission line radio galaxy 3C 109. Three Paschen lines plus Hα are measured and combined with a published value for Hβ to estimate the reddening for the continuum source and the region producing the broad emission lines. A key factor in this determination is the foreground reddening due to the Galaxy. The recent COBE/IRAS maps of Schlegel, Finkbeiner, & Davis indicate a Galactic extinction of E(B-V) = 0.57, more than twice the value inferred from the H I measurements. If the former value is adopted, the in situ reddening is E(B-V) = 0.77. This value implies, per the analysis of Goodrich & Cohen, that 3C 109 is an object of extremes, displaying a dereddened UV-optical continuum that is extraordinarily blue (fν ~ ν1.5) and harboring dust grains that are more efficient polarizers than any found in the Milky Way. Both extremes hint at some additional factor or mechanism, and a few possibilities are discussed. Regardless, the significant amount of internal reddening indicated by infrared spectroscopy is consistent with results from previous optical polarimetry and spectropolarimetry, optical and X-ray spectroscopy, and infrared photometry, in demonstrating that 3C 109 is an obscured quasar—a luminous object whose nuclear radiations from the soft X-ray to the near-infrared are absorbed, scattered, polarized, and reradiated further into the infrared by a circumnuclear envelope of dust.

Early spectral evolution of Nova Sagittarii 2004 (V5114 Sagittarii)

Aims. We present optical and near-infrared spectral evolution of the Galactic nova V5114 Sgr (2004) during few months after the outburst. Methods. We use multi-band photometry and line intensities derived from spectroscopy to put constrains on the distance and the physical conditions of the ejecta of V5114 Sgr. Results. The nova showed a fast decline (t 2 ≃ 11 days) and spectral features of Fell spectroscopic class. It reached M V = -8.7 ± 0.2 mag at maximum light, from which we derive a distance of 7700 ± 700 kpc and a distance from the galactic plane of about 800 pc. Hydrogen and oxygen mass of the ejecta are measured from emission lines, leading to ∼10 -6 and 10 -7 M ⊙ , respectively. We compute the filling factor of the ejecta to be in the range 0.1-10 -3 . We found the value of the filling factor to decrease with time. The same is also observed in other novae, then giving support to the idea that nova shells are not homogeneously filled in, rather being the material clumped in relatively higher density blobs less affected by the general expanding motion of the ejecta.

Near-Infrared Spectroscopy of V1493 Aquilae and V4642 Sagittarii: Two Novae with Unusual Spectral Features

We present 0.8–2.5 μm spectroscopy of two novae with uncommon spectral features at different stages in their decline after outburst. Nova Aql 1999 No. 1 (V1493 Aql) was a very fast nova that initially exhibited a rapid decline in brightness that flattened by day 20 and actually reversed to show an unusual secondary peak. The near-infrared spectrum was observed during this secondary peak at 46 days after initial peak brightness. Blended low-excitation lines such as H I Brackett and Paschen and O I largely populated the spectrum, which also showed a strong continuum declining monotonically toward the red (1.4–2.5 μm). He II lines were just beginning to emerge. The overall spectral appearance was much more representative of the period a few days after outburst. A possible explanation for this, and the secondary peak in the visible light curve, is that the nova experienced a second period of mass loss, but of a more continuous rather than explosive nature. Nova Sgr 2000 (V4642 Sgr) was a moderately fast nova, observed 160 days after peak brightness. High-excitation lines, including coronal lines, were present in the spectrum. However, their profiles were distinctly different from those of the lower excitation lines. Also present were four emission lines that have only recently been detected in the spectra of novae and which remain unidentified.