Recent observational and theoretical studies of classical Be stars have established the utility of polarization color diagrams (PCD) in helping to constrain the time-dependent mass decretion rates of these systems. We expand on our pilot observationa
l study of this phenomenon, and report the detailed analysis of a long-term (1989-2004) spectropolarimetric survey of 9 additional classical Be stars, including systems exhibiting evidence of partial disk-loss/disk-growth episodes as well as systems exhibiting long-term stable disks. After carefully characterizing and removing the interstellar polarization along the line of sight to each of these targets, we analyze their intrinsic polarization behavior. We find that many steady-state Be disks pause at the top of the PCD, as predicted by theory. We also observe sharp declines in the Balmer jump polarization for later spectral type, near edge-on steady-state disks, again as recently predicted by theory, likely caused when the base density of the disk is very high, and the outer region of the edge-on disk starts to self absorb a significant number of Balmer jump photons. The intrinsic $V$-band polarization and polarization position angle of $gamma$ Cas exhibits variations that seem to phase with the orbital period of a known one-armed density structure in this disk, similar to the theoretical predictions of Halonen & Jones. We also observe stochastic jumps in the intrinsic polarization across the Balmer jump of several known Be+sdO systems, and speculate that the thermal inflation of part of the outer region of these disks could be responsible for producing this observational phenomenon. Finally, we estimate the base densities of this sample of stars to be between $approx 8times 10^{-11}$ to $approx 4 times 10^{-12},rm g cm^{-3}$ during quasi steady state periods given there maximum observed polarization.
We present near-IR (J,H,Ks) photometry for 27 of the 28 candidate Herbig Ae/Be stars in the Small and Large Magellanic Clouds identified via the EROS1 and EROS2 surveys as well as near-contemporaneous optical (H-alpha) spectroscopy for 21 of these 28
candidates. Our observations extend previous efforts to determine the evolutionary status of these objects. We compare the IR brightness and colors of a subset of our sample with archival ground-based IR data and find evidence of statistically significant photometric differences for ELHC 5, 7, 12, 18, and 21 in one or more filter. In all cases, these near-IR photometric variations exhibit a grey color as compared to earlier epoch data. The ~1 magnitude IR brightening and minimal change in the H-alpha emission strength we observe in ELHC 7 is consistent with previous claims that it is a UX Ori type HAe/Be star, which is occasionally obscurred by dust clouds. We also detect a ~1 magnitude IR brightening of ELHC 12, but find little evidence of a similar large-scale change in its H-alpha line strength, suggesting that its behavior could also be caused by a UX Ori-like event. The ~0.5 magnitude IR variability we observe for ELHC 21, which also exhibited little evidence of a change in its H-alpha emission strength, could conceivably be caused by a major recent enhancement in the density of the inner disk region of a classical Be star. We also report the first near-IR photometry for two ESHC stars and the first H-alpha spectroscopy for one ELHC and five ESHC stars. Although H-alpha emission is detected in all of these new observations, they do not exhibit a strong near-IR excess. It is therefore possible that many of these objects may be classical Be stars rather than Herbig Ae/Be stars.
In Wisniewski et al. 2010, paper I, we analyzed 15 years of spectroscopic and spectropolarimetric data from the Ritter and Pine Bluff Observatories of 2 Be stars, 60 Cygni and {pi} Aquarii, when a transition from Be to B star occurred. Here we anayli
ze the intrinsic polarization, where we observe loop-like structures caused by the rise and fall of the polarization Balmer Jump and continuum V-band polarization being mismatched temporally with polarimetric outbursts. We also see polarization angle deviations from the mean, reported in paper I, which may be indicative of warps in the disk, blobs injected at an inclined orbit, or spiral density waves. We show our ongoing efforts to model time dependent behavior of the disk to constrain the phenomena, using 3D Monte Carlo radiative transfer codes.
(Abridged) Classical Be stars occasionally transition from having a gaseous circumstellar disk (Be phase) to a state in which all observational evidence for the presence of these disks disappears (normal B-star phase). We present one of the most comp
rehensive spectropolarimetric views to date of such a transition for two Be stars, pi Aquarii and 60 Cygni. 60 Cygs disk loss episode was characterized by a monotonic decrease in emission strength over a time-scale of 1000 days, consistent with the viscous time-scale of the disk, assuming alpha is 0.14. pi Aqrs disk loss was episodic in nature and occurred over a time-scale of 2440 days. An observed time lag between the behavior of the polarization and H-alpha in both stars indicates the disk clearing proceeded in an inside-out manner. We determine the position angle of the intrinsic polarization to be 166.7 +/- 0.1 degrees for pi Aqr and 107.7 +/- 0.4 degrees for 60 Cyg, and model the observed polarization during the quiescent diskless phase of each star to determine the interstellar polarization along the line of sight. Minor outbursts observed during the quiescent phase of each star shared similar lifetimes as those previously reported for mu Cen, suggesting that the outbursts represent the injection and subsequent viscous dissipation of individual blobs of material into the inner circumstellar environments of these stars. We also observe deviations from the mean intrinsic polarization position angle during polarization outbursts in each star, indicating deviations from axisymmetry. We propose that these deviations might be indicative of the injection (and subsequent circularization) of new blobs into the inner disk, either in the plane of the bulk of the disk material or in a slightly inclined (non-coplanar) orbit.
We report high spatial resolution 11.2 and 18.1 micron imaging of the eruptive variable V838 Monocerotis, obtained with Gemini Observatorys Michelle in 2007 March. The 2007 flux density of the unresolved stellar core is roughly 2 times brighter than
that observed in 2004. We interpret these data as evidence that V838 Mon has experienced a new circumstellar dust creation event. We also report a gap of spatially extended thermal emission over radial distances of 1860-93000 AU from the central source, which suggests that no prior significant circumstellar dust production events have occurred within the past 900-1500 years.
(Abridged version) We present the first detailed imaging polarization observations of six SMC and six LMC clusters, known to have large populations of B-type stars which exhibit excess H-alpha emission, to constrain the evolutionary status of these s
tars and hence better establish links between the onset of disk formation in classical Be stars and cluster age and/or metallicity. The wavelength dependence of our intrinsic polarization data provides a diagnostic of the dominant and any secondary polarigenic agents present, enabling us to discriminate pure gas disk systems, i.e. classical Be stars, from composite gas plus dust disk systems, i.e. Herbig Ae/Be or B[e] stars. Our intrinsic polarization results, along with available near-IR color information, strongly supports the suggestion of Wisniewski et al. that classical Be stars are present in clusters of age 5-8 Myr, and contradict assertions that the Be phenomenon only develops in the second half of a B stars main sequence lifetime, i.e. no earlier than 10 Myr. Comparing the polarimetric properties of our dataset to a similar survey of Galactic classical Be stars, we find that the prevalence of polarimetric Balmer jump signatures decreases with metallicity. We speculate that these results might indicate that either it is more difficult to form large disk systems in low metallicity environments, or that the average disk temperature is higher in these low metallicity environments. We have characterized the polarimetric signatures of all candidate Be stars in our data sample and find ~25% are unlikely to arise from true classical Be star-disk systems.
