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 -3exception 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.
We report on the results of a number of infrared spectra (0.8-2.5, 2.1-4.6, and 3-14 m) 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 H 2 O, CO, AlO, TiO, SiO, SO 2 , OH, VO, and SH, as well as a complex of emission near 10 m reminiscent of silicate emission, with a central absorbing feature at 10:3 m. 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 object's late stages. The derived shell mass and distance are 0.04 M and 9.2 kpc, respectively.
FS CMa stars form a group of objects with the B[e] phenomenon that were previously known as unclassified B[e] stars or B[e] stars with warm dust (B[e]WD) until recently. They exhibit strong emission-line spectra and strong IR excesses, most likely due to recently formed circumstellar dust. These properties have been suggested to be due to ongoing or recent rapid mass exchange in binary systems with hot primaries and various types of secondaries. The first paper of this series reported an analysis of the available information about previously known Galactic objects with the B[e] phenomenon, the initial selection of the FS CMa group objects, and a qualitative explanation of their properties. This paper reports the results of our new search for more FS CMa objects in the IRAS Point Source Catalog. We present new photometric criteria for identifying FS CMa stars as well as the first results of our observations of nine new FS CMa group members. With this addition, the FS CMa group has now 40 members, becoming the largest among the dust-forming hot star groups. We also present nine objects with no evidence for the B[e] phenomenon, but with newly discovered spectral line emission and/or strong IR excesses.
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 H 2 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 13 C/ 12 C ratio (0:83 AE 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 10830 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] 9911 and [Si vi] 19645, 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(BÀV Þ ¼ 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.
The AMPTE CCE satellite frequently observed transient (1 < r < 8 min) events marked by magnetic field strength increases and bipolar magnetic field signatures (peak-topeak amplitudes > 4 nT) while in the outer dayside magnetosphere. We report a survey of 59 prominant events observed from August to November 1984. The bipolar signatures and minimum variance analysis reveal that most events move poleward and antisunward, except in the immediate vicinity of local noon. Here the motion of the events appears to be better governed by the spiral/orthospiral interplanetary magnetic field (IMF) orientation than magnetic curvature forces associated with IMF By. The IMF orientation appears to have little or no influence on event occurrence or orientation. We interpret the events in terms of solar wind/foreshock pressure pulse induced ripples on the magnetopause surface. Our results can be reconciled with those obtained in previous studies which made use of ISEE 1/2, AMPTE IRM, and AMPTE UKS observations if pressure pulses produce large-amplitude events and bursty merging (or reconnection) produces small-amplitude events.Paper number 95JA03063. 0148-0227/96/95JA-03063505.00 vidual events and their statistical occurrence patterns (see review by Sibeck [1994]). Distinguishing between these two mechanisms is important. Under the merging interpretation, the prevalence of transient events implies that bursty merging is a major contributor to the solar wind-magnetosphere interaction [e.g., Cowley, 1982]. Under the pressure pulse interpretation, the prevalence of transient events implies that the magnetosphere is constantly being buffeted by highly variable solar wind parameters, perhaps with preconditioning in the foreshock [Sibeck, 1990].Since the bursty merging and pressure pulse models predict differing patterns for event occurrence, orientation, and direction of motion as a function of IMF orientation, local time, and latitude, statistical studies should help determine the relative significance of each model in the production of transient events. We begin by outlining the contrasting predictions of the two models concerning event occurrence patterns as a function of IMF orientation. We then present a case study and a statistical survey of transient events observed by the AMPTE CCE satellite in the outer dayside magnetosphere, and conclude with an attempt to reconcile the new results with those obtained in previous studies. Predictions of the Bursty Merging ModelMerging occurs when any one of several current layer instabilities [e.g., Schindler, 1974;Huba et al., 1977;Lui et al., 1991] enhances resistivities, disrupts currents, and allows magnetosheath and magnetospheric magnetic field lines to interconnect across the magnetopause. Since instability growth rates increase with increasing current density and there is no clear dependence of the current layer thickness upon the magnetosheath magnetic field orientation [Berchem and Russell, 1982], we expect growth rates to increase as the shear between the magnetosheath and magnetosp...
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