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We test the viability of training machine learning algorithms with synthetic H alpha line profiles to determine the inclination angles of Be stars (the angle between the central B star's rotation axis and the observer's line of sight) from a single observed medium-resolution, moderate S/N, spectrum. The performance of three different machine learning algorithms were compared: neural networks tasked with regression, neural networks tasked with classification, and support vector regression. Of these three algorithms, neural networks tasked with regression consistently outperformed the other methods with a RMSE error of 7.6 degrees on an observational sample of 92 galactic Be stars with inclination angles known from direct H alpha profile fitting, from the spectroscopic signature of gravitational darkening, and, in a few cases, from interferometric observations that resolved the disk. The trained neural networks enable a quick and useful determination of the inclination angles of observed Be stars which can be used to search for correlated spin axes in young open clusters or to extract an equatorial rotation velocity from a measurement of v sin(i)., Comment: 14 pages, 7 figures

Using a sample of 92 Galactic Be stars, we compare inclination angles (the angle between a star's rotation axis and the line-of-sight) determined from H$\alpha$ emission line profile fitting to those determined by the spectroscopic signature of gravitational darkening. We find good agreement: 70% of the sample (64 out of 92 stars) is consistent with zero difference between the two methods using $1\sigma$ errors, and there is a strong linear correlation coefficient between the two methods of $r=+0.63\pm0.05$. There is some evidence that the H$\alpha$ profile fitting method overestimates the inclination angle for $i\lesssim 25^\circ$, perhaps due to the neglect of incoherent electron scattering on the H$\alpha$ line widths, while the gravitational darkening method underestimates the inclination angle for $i\gtrsim70^\circ$, perhaps due to the neglect of disk radiative transfer effects on the optical spectrum. Overall, it is demonstrated that a single H$\alpha$ spectrum of modest resolution and SNR can be used to extract a useful estimate for the inclination angle of an individual Be star. This allows equatorial rotation velocities for individual Be stars to be derived from $v \sin i $ measurements and will allow Be stars to be used to search for correlated spin axes in young, open clusters if unbiased (with respect to inclination) samples of Be stars are used., Comment: 21 pages, 18 figures

We demonstrate that the angle between star's rotation axis and the observer's line-of-sight, usually called the inclination angle, can be reliably determined for Be stars via H$\alpha$ emission-line profile fitting. We test our method on a sample of 11 Be stars with available long-baseline interferometric data from the Navy Precision Optical Interferometer~(NPOI). We fit the H$\alpha$ emission line profile of each star to obtain a spectroscopic inclination angle $i_{\rm H\alpha}$. We then obtain an independent inclination angle estimate, $i_{\rm V^2}$, by fitting the observed interferometric visibilities with model visibilities based on a purely geometric representation of the light distribution on the sky. The sample differences, $\Delta i \equiv i_{\rm H\alpha} - i_{\rm V^2}$, are normally distributed with a mean of zero and a standard deviation of $6.7$ degrees, and the linear correlation coefficient between $i_{\rm H\alpha}$ and $i_{\rm V^2}$ is $r=0.93$. As Be stars comprise upwards of one fifth of all main-sequence B-type stars, this H$\alpha$ line profile fitting technique has the potential to provide an efficient method for detecting correlated stellar spin axes in young open clusters. Furthermore, if the orientation of the Be star circumstellar disk on the plane of the sky can be constrained by polarization measurements, it is possible to determine the full 3D stellar rotation vector of each Be star., Comment: 19 pages, 23 figures, accepted for publication in The Astrophysical Journal

We present results from a 2-epoch HST H$\alpha$ emission line survey of the Andromeda Galaxy that overlaps the footprint of the Panchromatic Hubble Andromeda Treasury (PHAT) survey. We find 552 (542) classical Be stars and 8429 (8556) normal B-type stars in epoch # 1 (epoch # 2), yielding an overall fractional Be content of 6.15% $\pm$0.26% (5.96% $\pm$0.25%). The fractional Be content decreased with spectral sub-type from $\sim$23.6% $\pm$2.0% ($\sim$23.9% $\pm$2.0%) for B0-type stars to $\sim$3.1% $\pm$0.34% ($\sim$3.4% $\pm$0.35%) for B8-type stars in epoch # 1 (epoch # 2). We observe a clear population of cluster Be stars at early fractional main sequence lifetimes, indicating that a subset of Be stars emerge onto the ZAMS as rapid rotators. Be stars are 2.8x rarer in M31 for the earliest sub-types compared to the SMC, confirming that the fractional Be content decreases in significantly more metal rich environments (like the Milky Way and M31). However, M31 does not follow a clear trend of Be fraction decreasing with metallicity compared to the Milky Way, which may reflect that the Be phenomenon is enhanced with evolutionary age. The rate of disk-loss or disk-regeneration episodes we observe, 22% $\pm$ 2% yr$^{-1}$, is similar to that observed for seven other Galactic clusters reported in the literature, assuming these latter transient fractions scale by a linear rate. The similar number of disk-loss events (57) as disk-renewal events (43) was unexpected since disk dissipation time-scales can be $\sim$2x the typical time-scales for disk build-up phases., Comment: Accepted to AJ

Through the modelling of near-infrared photometry of star plus disk systems with the codes BEDISK/BERAY, we successfully describe the WISE photometric characteristics of Be stars in five young open clusters, NGC 663, NGC 869, NGC 884, NGC 3766 and NGC 4755, broadly studied in the literature. WISE photometry allows previously known Be stars to be detected and to find new Be candidates which could be confirmed spectroscopically. The location of Be stars in the WISE colour-magnitude diagram, separates them in two groups; active (Be stars hosting a developed circumstellar disk) and quiescent objects (Be stars in a diskless phase), and this way, we can explore how often stars are observed in these different stages. The variability observed in most active variable Be stars is compatible with a disk dissipation phase. We find that 50 percent of Be stars in the studied open clusters are in an active phase. We can interpret this as Be stars having a developed circumstellar disk half of the time. The location of Be stars with a developed disk in the CMD require mass loss rates in agreement with values recently reported in the literature. For these objects, we expect to have a tight relation between the equivalent width of the H{\alpha} line and the mass of the disk, if the inclination is known. Also, near-infrared photometry of Be stars in stellar clusters has the potential of being useful to test whether there is a preferential viewing angle., Comment: 25 pages, 14 figures, Accepted for publication in AJ

Photospheric nitrogen abundances for a sample of 26 Be stars and 16 normal B stars were found using high-resolution spectra from the Magnetism in Massive Stars (MiMeS) spectroscopic survey. Nitrogen abundances were obtained using non-LTE equivalent widths and line profiles, and Monte Carlo simulation was used to determine the error bounds of the measured nitrogen abundances due to uncertainties in the adopted stellar parameters, continuum normalization, and atomic data. In addition, the effects of the gravitational darkening and disk contamination on the measured Be star nitrogen abundances were investigated. About one third of the sample Be stars and half of the normal B-type stars have nitrogen enrichments that may be due to rotational mixing in these rapidly rotating objects. Corrections for gravitational darkening and disk contamination do not significantly change the overall results for the Be star sample. The average nitrogen abundance of the B and Be star samples coincide with the solar abundance, although the dispersion in the nitrogen abundances is much larger in the Be star sample. The Be star sample also has a significant fraction of sub-solar nitrogen abundance objects which are not present in significant numbers in the B star sample. This may point to yet unresolved systematic errors in the analysis of the Be stars, Comment: 24 pages; 27 figures, Accepted for publication in the MNRAS

We utilize a multi-step modelling process to produce synthetic interferometric and spectroscopic observables, which are then compared to their observed counterparts. Our extensive set of interferometric observations of the Be star 48 Per, totaling 291 data points, were obtained at the Navy Precision Optical Interferometer from 2006 Nov 07 to 2006 Nov 23. Our models were further constrained by comparison with contemporaneous H$\alpha$ line spectroscopy obtained at the John S. Hall Telescope at Lowell Observatory on 2006 Nov 1. Theoretical spectral energy distributions, SEDs, for 48~Per were confirmed by comparison with observations over a wavelength regime of 0.4 to 60 microns from Touhami et al. (2010) and Vieira et al. (2017). Our best-fitting combined model from H$\alpha$ spectroscopy, H$\alpha$ interferometry and SED fitting has a power law density fall off, $n$, of 2.3 and an initial density at the stellar surface of $\rho_0 = 1.0 \times 10^{-11} \rm{g\,cm}^{-3}$ with a inclination constrained by H$\alpha$ spectroscopy and interferometry of 45$^{\circ} \pm 5^{\circ}$. The position angle for the system, measured east from north, is 121$^{\circ}$ $\pm$ 1$^{\circ}$. Our best-fit model shows that the disk emission originates in a moderately large disk with a radius of 25 R$_{*}$ which is consistent with a disk mass of approximately 5 $\times$ 10$^{24}$ g or 3 $\times$ 10$^{-10}$ M$_{*}$. Finally, we compare our results with previous studies of 48~Per by Quirrenbach et al. (1997) with agreement but find, with our much larger data set, that our disk size contradicts the findings of Delaa et al. (2011)., Comment: 26 pages, 9 figures

The circumstellar disk density distributions for a sample of 63 Be southern stars from the BeSOS survey were found by modelling their H$\alpha$ emission line profiles. These disk densities were used to compute disk masses and disk angular momenta for the sample. Average values for the disk mass are 3.4$\times$10$^{-9}$ and 9.5$\times$10$^{-10}$ $M_{\star}$ for early (B0-B3) and late (B4-B9) spectral types, respectively. We also find that the range of disk angular momentum relative to the star are between 150-200 and 100-150 $J_{\star}/M_{\star}$, again for early and late-type Be stars respectively. The distributions of the disk mass and disk angular momentum are different between early and late-type Be stars at a 1\% level of significance. Finally, we construct the disk mass distribution for the BeSOS sample as a function of spectral type and compare it to the predictions of stellar evolutionary models with rapid rotation. The observed disk masses are typically larger than the theoretical predictions, although the observed spread in disk masses is typically large.

We investigate the physical properties of the inner gaseous disks of the three, hot, Herbig B2e stars, HD 76534, HD 114981 and HD 216629, by modelling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by the photospheric radiation from the central star in order to test if the optical and near-IR emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of HI (H$\alpha$ and H$\beta$), HeI, CaII and FeII. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as $\sim 10^{-10}\,(R_{*}/R)^3\, \rm g\,cm^{-3}$ in the equatorial plane of a 25 R$_{*}$ ($0.78$ AU) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the H$\alpha$-emitting portion of the inner gaseous disk of $\sim 10^{-9} M_*$. We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a $\approx\,1$ AU, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium., Comment: Accepted to be published in AAS Journals. 30 pages, 24 figures and 6 tables

We present eleven years of high-resolution, spectroscopic observations for the star EM~Cep. EM~Cep switches between B and Be star states, as revealed by the level of H$\alpha$ emission, but spends most of its time in the B~star state. EM~Cep has been considered to be an eclipsing, near contact binary of nearly equal-massed B stars in order to reproduce regular photometric variations; however, this model is problematic due to the lack of any observed Doppler shift in the spectrum. Our observations confirm that there are no apparent Doppler shifts in the wide spectral lines H$\alpha$ and HeI~6678 in either the B or Be star states.The profiles of HeI~6678 typically exhibited a filled-in absorption core, but we detected weak emission in this line during the highest Be state. Given the lack of observed Doppler shifts, we model EM~Cep as an isolated Be star with a variable circumstellar disk. We can reproduce the observed H$\alpha$ emission profiles over the eleven year period reasonably well with disk masses on the order of $3$ to $10\times\,10^{-11}\;M_*$ in the Be state with the circumstellar disk seen at an inclination of $78^{\circ}$ to the line of sight. From a disk ejection episode in 2014, we estimate a mass loss rate of $\approx\,3\times 10^{-9}\;\rm M_{\odot}\,yr^{-1}$. The derived disk density parameters are typical of those found for the classical Be stars. We therefore suggest that the EM~Cep is a classical Be star and that its photometric variations are the result of $\beta\;$Cep or non-radial pulsations., Comment: 15 pages, 14 figures

We present a study of FeII emission in the near-infrared region (NIR) for 25 active galactic nuclei (AGNs) to obtain information about the excitation mechanisms that power it and the location where it is formed. We employ a NIR FeII template derived in the literature and found that it successfully reproduces the observed FeII spectrum. The FeII bump at 9200 Angstroms detected in all objects studied confirms that Lyman-alpha fluorescence is always present in AGNs. The The correlation found between the flux of the 9200 Angstroms bump, the 1 micron lines and the optical FeII imply that Lyman-alpha fluorescence plays an important role in the FeII production. We determined that at least 18% of the optical FeII is due to this process while collisional excitation dominates the production of the observed FeII. The line profiles of FeII 10502, OI 11287, CaII 8664 and Paschen-beta were compared to gather information about the most likely location where they are emitted. We found that FeII, OI and CaII have similar widths and are, on average, 30% narrower than Paschen-beta. Assuming that the clouds emitting the lines are virialized, we show that the FeII is emitted in a region twice as far from the central source than Paschen-beta. The distance though strongly varies: from 8.5 light-days for NGC4051 to 198.2 light-days for Mrk509. Our results reinforce the importance of the FeII in the NIR to constrain critical parameters that drive its physics and the underlying AGN kinematics as well as more accurate models aimed at reproducing this complex emission., Comment: Accepted for publication in The Astrophysical Journal

We attempt to constrain the physical properties of the inner, gaseous disk of the Herbig Be star BD+65 1637 using non-LTE, circumstellar disk codes and observed spectra (3700 to 10,500 \r{A}) from the ESPaDOnS instrument on CFHT. The photoionizing radiation of the central star is assumed to be the sole source of input energy for the disk. We model optical and near-infrared emission lines that are thought to form in this region using standard techniques that have been successful in modeling the spectra of Classical Be stars. By comparing synthetic line profiles of hydrogen, helium, iron and calcium with the observed line profiles, we try to constrain the geometry, density structure, and kinematics of the gaseous disk. Reasonable matches have been found for all line profiles individually; however, no disk density model based on a single power-law for the equatorial density was able to simultaneously fit all of the observed emission lines. Amongst the emission lines, the metal lines, especially the Ca II IR triplet, seem to require higher disk densities than the other lines. Excluding the Ca II lines, a model in which the equatorial disk density falls as $10^{-10} (R_{*}/R)^3 g\,cm^{-3}$ seen at an inclination of 45{\deg} for a $50\,R_{*}$ disk provides reasonable matches to the overall line shapes and strengths. The Ca II lines seem to require a shallower drop off as $10^{-10} (R_{*}/R)^2 g\,cm^{-3}$ to match their strength. More complex disk density models are likely required to refine the match to the BD+65 1637 spectrum.

Non-LTE calculations are performed for NII in stellar atmospheric models appropriate to main sequence B-stars to produce new grids of equivalent widths for the strongest NII lines commonly used for abundance analysis. There is reasonable agreement between our calculations and previous results, although we find weaker non-LTE effects in the strongest optical NII transition. We also present a detailed estimation of the uncertainties in the equivalent widths due to inaccuracies in the atomic data via Monte Carlo simulation and investigate the completeness of our model atom in terms of included energy levels. Uncertainties in the basic NII atomic data limit the accuracy of abundance determinations to ~+/-0.10 dex at the peak of the NII optical spectrum near Teff~ 24,000 K., Comment: 18 pages; 11 figures

Omicron Aquarii is late-type, Be shell star with a stable and nearly symmetric H$\alpha$ emission line. We combine H$\alpha$ interferometric observations obtained with the Navy Precision Optical Interferometer (NPOI) covering 2007 through 2014 with H$\alpha$ spectroscopic observations over the same period and a 2008 observation of the system's near-infrared spectral energy distribution to constrain the properties of $o$~Aqr's circumstellar disk. All observations are consistent with a circumstellar disk seen at an inclination of $75\pm\,3^{\circ}$ with a position angle on the sky of $110\pm\,8^{\circ}$ measured E from N. From the best-fit disk density model, we find that 90\% of the H$\alpha$ emission arises from within $9.5$ stellar radii, and the mass associated with this H$\alpha$ disk is $\sim 1.8\times10^{-10}$ of the stellar mass and the associated angular momentum, assuming Keplerian rotation for the disk, is $\sim 1.6\times10^{-8}$ of the total stellar angular momentum. The occurrence of a central quasi-emission (CQE) feature in Mg\,{\sc ii} $\lambda\,4481$ is also predicted by this best-fit disk model and the computed profile compares successfully with observations from 1999. To obtain consistency between the H$\alpha$ line profile modelling and the other constraints, it was necessary in the profile fitting to weight the line core (emission peaks and central depression) more heavily than the line wings, which were not well reproduced by our models. This may reflect the limitation of assuming a single power-law for the disk's equatorial density variation. The best-fit disk density model for $o$~Aqr predicts that H$\alpha$ is near its maximum strength as a function of disk density, and hence the H$\alpha$ equivalent width and line profile change only weakly in response to large (factor of $\sim 5$) changes in the disk density., Comment: 16 pages; 16 figures

H\alpha\ equivalent widths and UBV magnitudes are calculated for Be star disk models that grow in size and/or density with time. We show that these simple models are consistent with the known Be star classes of positive and inverse correlations between long-term variations in H\alpha\ and V magnitude as identified by Harmanec. We support the conclusion of Harmanec that the distinction is controlled by the inclination of the disk to the line of sight. We demonstrate that the strength of these correlations, particularly those of an inverse correlation where the system becomes fainter as the H\alpha\ emission strength grows, is strongly influenced by the scale height of the inner Be star disk and the extent of the gravitational darkening of the central B star due to rapid rotation. This dependence may allow coordinated spectroscopic and photometric observations to better constrain these poorly known Be star parameters., Comment: Corrected misspelling in the abstract of the original version

The effects of gravitational darkening on the thermal structure of Be star disks of differing densities are systematically examined. Gravitational darkening is the decrease of the effective temperature near the equator and the corresponding increase near the poles of a star caused by rapid rotation. We also include the rotational distortion of the star using the Roche Model. Increasing the disk density increases the optical depths in the equatorial plane, resulting in the formation of an inner cool region near the equatorial plane of the disk. High rotation rates result in disks that have temperatures similar to those of a denser disk, namely cooler overall. However the effect of increasing rotation produces additional heating in the upper disk due to the hotter stellar pole. Cool regions in the equatorial plane normally associated with high density are seen in low density models at high rotation rates. Gravitational darkening increases the amount of very cool and very hot material in the disk and decreases the amount of disk material at moderate temperatures. We also present models which study the effect of gravitational darkening on hydrostatically-converged disks, in which the temperature structure is consistent with vertical hydrostatic equilibrium. Because the equatorial regions become cooler, hydrostatically converged models that include gravity darkening have smaller vertical scale heights, and $H/R$ is smaller by as much as 56% near $v_{\rm crit}$. Finally we explore differences in disk temperatures when alternate formulations of gravitational darkening, which lower the temperature difference between the pole and the equator, are used.

In AGN spectra, a series of FeII multiplets form a pseudo-continuum that extends from the ultraviolet to the near-infrared (NIR). This emission is believed to originate in the Broad Line Region (BLR), and it has been known for a long time that pure photoionization fails to reproduce it in the most extreme cases, as does the collisional-excitation alone. The most recent models by Sigut & Pradhan (2003) include details of the FeII ion microphysics and cover a wide range in ionization parameter log U_ion= (-3.0 -> -1.3) and density log n_H = (9.6 -> 12.6). With the aid of such models and a spectral synthesis approach, we study for the first time in detail the NIR emission of I Zw 1. The main goals are to confirm the role played by Ly\alpha-fluorescence mechanisms in the production of the FeII spectrum and to construct the first semi-empirical NIR FeII template that best represents this emission and can be used to subtract it in other sources. A good overall match between the observed FeII+MgII features with those predicted by the best fitted model is obtained, corroborating the Ly\alpha-fluorescence as a key process to understand the FeII spectrum. The best model is then adjusted by applying a deconvolution method on the observed FeII+MgII spectrum. The derived semi-empirical template is then fitted to the spectrum of Ark 564, suitably reproducing its observed FeII+MgII emission. Our approach extends the current set of available FeII templates into the NIR region., Comment: 47 pages, 5 tables, 12 figures. Accepted for publication in The Astrophysical Journal

The temperature structure of Be star circumstellar disks at the sub-solar metallicity appropriate to the Small Magellanic Cloud (SMC) is investigated. It is found that for central stars of the same spectral type, Be star disks in the SMC are systematically hotter by several thousand degrees compared to Milky Way (MW) disks with the same density structure. For early spectral types (B0e - B3e), this results in systematically smaller H{\alpha} equivalent widths for Be stars in the SMC. The implication of this result on Be star frequency comparisons between MW and SMC clusters is shown to be a 5 - 10% lowering of the detection efficiency of Be stars in SMC clusters. These calculations are also compared to the known H{\alpha} equivalent width distributions in the MW and SMC. For the MW, reasonable agreement is found; however, for the SMC, the match is not as good and systematically larger Be star disks may be required., Comment: 31 pages, 10 figures, Accepted for publication in the astrophysical Journal

The effect of gravitational darkening on models of the thermal structure of Be star disks is systematically studied for a wide range of Be star spectral types and rotation rates. Gravitational darkening causes a reduction of the stellar effective temperature towards the equator and a redirection of energy towards the poles. It is an important physical effect in these star-disk systems because the photoionizing radiation from the central B star is the main energy source for the disk. We have added gravitational darkening to the bedisk code to produce circumstellar disk models that include both the variation in the effective temperature with latitude and the non-spherical shape of the star in the calculation of the stellar photoionizing radiation field. The effect of gravitational darkening on global measures of disk temperature is generally significant for rotation rates above 80% of critical rotation. For example, a B0V model rotating at 95% of critical has a density-averaged disk temperature \approx 2500 K cooler than its nonrotating counterpart. However, detailed differences in the temperature structure of disks surrounding rotating and non-rotating stars reveal a complex pattern of heating and cooling. Spherical gravitational darkening, an approximation that ignores the changing shape of the star, gives good results for disk temperatures for rotation rates less than \approx 80% of critical. However for the highest rotation rates, the distortion of the stellar surface caused by rotation becomes important., Comment: 35 pages, 19 figures

The line profiles of the stars with v sin i below a few km/s can reveal direct signatures of local velocity fields (e.g. convection) in stellar atmospheres. This effect is well established in cool main sequence stars, and has been detected and studied in three A stars. This paper reports observations of main sequence B, A and F stars with two goals: (1) to identify additional stars having sufficiently low values of v sin i to search for spectral line profile signatures of local velocity fields, and (2) to explore how the signatures of the local velocity fields in the atmosphere depend on stellar parameters such as effective temperature T_eff and peculiarity type. For stars having T_eff below about 10000 K, we always detect local atmospheric velocity fields indirectly through a non-zero microturbulence parameter, but not for hotter stars. Among the A and F stars in our sample having the sharpest lines, direct tracers of atmospheric velocity fields are found in six new stars. The velocity field signatures identified include asymmetric excess line wing absorption, deeper in the blue line wing than in the red; line profiles of strong lines that are poorly fit by computed profiles; and strong lines that are broader than they should be for the v sin i values deduced from weak lines. These effects are found in both normal and Am stars, but seem stronger in Am stars. These data still have not been satisfactorily explained by models of atmospheric convection, including numerical simulations., Comment: Acepted for publication by Astronomy and Astrophysics

A popular model for the circumstellar disks of Be stars is that of a geometrically thin disk with a density in the equatorial plane that drops as a power law of distance from the star. It is usually assumed that the vertical structure of such a disk (in the direction parallel to the stellar rotation axis) is governed by the hydrostatic equilibrium set by the vertical component of the star's gravitational acceleration. Previous radiative equilibrium models for such disks have usually been computed assuming a fixed density structure. This introduces an inconsistency as the gas density is not allowed to respond to temperature changes and the resultant disk model is not in vertical, hydrostatic equilibrium. In this work, we modify the {\sc bedisk} code of \citet{sig07} so that it enforces a hydrostatic equilibrium consistent with the temperature solution. We compare the disk densities, temperatures, H$\alpha$ line profiles, and near-IR excesses predicted by such models with those computed from models with a fixed density structure. We find that the fixed models can differ substantially from the consistent hydrostatic models when the disk density is high enough that the circumstellar disk develops a cool ($T\lesssim10,000 $K) equatorial region close to the parent star. Based on these new hydrostatic disks, we also predict an approximate relation between the (global) density-averaged disk temperature and the $T_{\rm eff}$ of the central star, covering the full range of central Be star spectral types., Comment: 25 pages; 11 figures

We present a numerical model describing a circularly symmetric gaseous disk around the Be star chi Ophiuchi. The model is constrained by long-baseline interferometric observations that are sensitive to the H-alpha Balmer line emission from the disk. For the first time our interferometric observations spatially resolve the inner region of the circumstellar disk around chi Ophiuchi and we use these results to place a constraint on the physical extent of the H-alpha-emitting region. We demonstrate how this in turn results in very specific constraints on the parameters that describe the variation of the gas density as a function of radial distance from the central star., Comment: Accepted for publication in ApJ

We have computed theoretical models of circumstellar disks for the classical Be stars $\kappa$ Dra, $\beta$ Psc, and $\upsilon$ Cyg. Models were constructed using a non-LTE radiative transfer code developed by \citet{sig07} which incorporates a number of improvements over previous treatments of the disk thermal structure, including a realistic chemical composition. Our models are constrained by direct comparison with long baseline optical interferometric observations of the H$\alpha$ emitting regions and by contemporaneous H$\alpha$ line profiles. Detailed comparisons of our predictions with H$\alpha$ interferometry and spectroscopy place very tight constraints on the density distributions for these circumstellar disks., Comment: 10 figures,28 pages, accepted by ApJ

We present infrared photometric observations of the Be binary system delta Scorpii obtained in 2006. The J,H and K magnitudes are the same within the errors compared to observations taken 10 months earlier. We derive the infrared excess from the observation and compare this to the color excess predicted by a radiative equilibrium model of the primary star and its circumstellar disk. We use a non-LTE computational code to model the gaseous envelope concentrated in the star's equatorial plane and calculate the expected spectral energy distribution and Halpha emission profile of the star with its circumstellar disk. Using the observed infrared excess of delta Sco, as well as Halpha spectroscopy bracketing the IR observations in time, we place constraints on the radial density distribution in the circumstellar disk. Because the disk exhibits variability in its density distribution, this work will be helpful in understanding its dynamics., Comment: 12 pages, 14 figures, to be published in PASP May 2008

We have computed radiative equilibrium models for the gas in the circumstellar envelope surrounding the hot, classical Be star $\gamma $Cassiopeia. This calculation is performed using a code that incorporates a number of improvements over previous treatments of the disk's thermal structure by \citet{mil98} and \citet{jon04}; most importantly, heating and cooling rates are computed with atomic models for H, He, CNO, Mg, Si, Ca, & Fe and their relevant ions. Thus, for the first time, the thermal structure of a Be disk is computed for a gas with a solar chemical composition as opposed to assuming a pure hydrogen envelope. We compare the predicted average disk temperature, the total energy loss in H$\alpha$, and the near-IR excess with observations and find that all can be accounted for by a disk that is in vertical hydrostatic equilibrium with a density in the equatorial plane of $\rho(R)\approx 3$ to $5\cdot 10^{-11} (R/R_*)^{-2.5} \rm g cm^{-3}$. We also discuss the changes in the disk's thermal structure that result from the additional heating and cooling processes available to a gas with a solar chemical composition over those available to a pure hydrogen plasma., Comment: 11 pages, 8 figures high resolution figures available at http://inverse.astro.uwo.ca/sig_jon07.html

(Abridge) Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of the H2 and [FeII] gas in a sample of AGN. The H2 lines are unresolved in all objects in which they were detected while the [FeII] lines have widths implying gas velocities of up to 650 km/s. This suggests that, very likely, the H2 and [FeII] emission does not originate from the same parcel of gas. Molecular H2 were detected in 90% of the sample, including PG objects, indicating detectavel amounts of molecular material even in objects with low levels of circumnuclear starburst activity. The data favors thermal excitation for the H2 lines. Indeed, in NGC3227, Mrk766, NGC4051 and NGC4151, the molecular emission is found to be purely thermal. This result is also confirmed by the rather similar vibrational and rotational temperatures in the objects for which they were derived. [FeII] lines are detected in all of the AGN. The [FeII] 1.254mu/Pa-beta ratio is compatible with excitation of the [FeII] by the active nucleus, but in Mrk 766 it implies a stellar origin. A correlation between H2/Br-gamma and [FeII]/Pa-beta is found. We confirm that it is a useful diagnostic tool in the NIR to separate emitting line objects by their level of nuclear activity. X-ray excitation models are able to explain the observed H2 and part of the [FeII] emission. Most likely, a combination of X-ray heating, shocks driven by the radio jet, and circumnuclear star formation contributes, in different proportions, to the H2 and [FeII] emission. In most of our spectra, the [FeII] 1.257mu/1.644mu ratio is found to be 30% lower than the intrinsic value based on current atomic data. This implies either than the extinction towards the [FeII] emitting clouds is very similar in most objects or there are possible inaccuracies in the A-values in the [FeII] transitions., Comment: 18 pages, 6 figures, Accepted for publication in Astronomy & Astrophysics

We present theoretical iron emission line strengths for physical conditions typical of Active Galactic Nuclei with Broad-Line Regions. The non-local thermodynamic equilibrium (NLTE) models include a new and extensive treatment of radiative transfer in the FeIII ion, complementing the FeII emission line strengths predicted in our earlier works. We also briefly present preliminary results for the FeI emission from AGN using a reduced atom model. We can satisfactorily reproduce the empirical UV FeIII emission line template of Vestergaard & Wilkes (2001) for the prototypical narrow-line Seyfert 1 galaxy I Zw 1, both in terms of the general FeIII flux distribution and the relative strength of the FeIII and FeII emission. However, a number of detailed features are still not matched; the most prominent example is the strongest single FeIII feature observed in the I Zw 1 spectrum, UV47: it is predicted to be strong only in models suppressing Fe-H charge exchange reactions. We examine the role of variations in cloud turbulent velocity and iron abundance and carry out Monte Carlo simulations to demonstrate the effect of uncertainties in atomic data on the computed spectra., Comment: 30 pages, 11 eps figures Submitted to ApJ

We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured from 1998 February to 2002 January with the spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct brightness and magnetic surface topologies of the K1 primary subgiant of the system, at five different epochs. We confirm the presence of large, axisymmetric regions where the magnetic field is mainly azimuthal, providing further support to the hypothesis that dynamo processes may be distributed throughout the whole convective zone in this star. We study the short-term evolution of surface structures from a comparison of our images with observations secured at close-by epochs by Donati et al. (2003) at the Anglo-Australian Telescope. We conclude that the small-scale brightness and magnetic patterns undergo major changes within a timescale of 4 to 6 weeks, while the largest structures remain stable over several years. We report the detection of a weak surface differential rotation (both from brightness and magnetic tracers) indicating that the equator rotates faster than the pole with a difference in rotation rate between the pole and the equator about 4 times smaller than that of the Sun. This result suggests that tidal forces also impact the global dynamic equilibrium of convective zones in cool active stars., Comment: accepted by MNRAS

We present theoretical FeII emission line strengths for physical conditions typical of Active Galactic Nuclei with Broad-Line Regions. The FeII line strengths were computed with a precise treatment of radiative transfer using extensive and accurate atomic data from the Iron Project. Excitation mechanisms for the FeII emission included continuum fluorescence, collisional excitation, self-fluorescence amoung the FeII transitions, and fluorescent excitation by Lyman-alpha and Lyman-beta. A large FeII atomic model consisting of 827 fine structure levels (including states to E ~ 15 eV) was used to predict fluxes for approximately 23,000 FeII transitions, covering most of the UV, optical, and IR wavelengths of astrophysical interest. Spectral synthesis for wavelengths from 1600 Angstroms to 1.2 microns is presented. Applications of present theoretical templates to the analysis of observations are described. In particular, we discuss recent observations of near-IR FeII lines in the 8500 Angstrom -- 1 micron region which are predicted by the Lyman-alpha fluorescence mechanism. We also compare our UV spectral synthesis with an empirical iron template for the prototypical, narrow-line Seyfert galaxy I Zw 1. The theoretical FeII template presented in this work should also applicable to a variety of objects with FeII spectra formed under similar excitation conditions, such as supernovae and symbiotic stars., Comment: 33 pages, 15 postscript figures

Emission in the 4d - 4f transitions of MnII (multiplet 13, 6122-6132 Ang), in the 4f - 6g transitions of PII, and in 6149.5 Ang of HgII has been detected in the spectrum of the helium weak star 3 Centauri A (B5 III-IVp). Weaker emission from the same MnII multiplet is also seen in the hot, mild HgMn star 46 Aquila (B9 III).It is suggested that the emission is of photospheric origin and may be evidence for the stratification of manganese, phosphorus and mercury in the photosphere of 3 CenA, and of manganese in 46Aql., Comment: 16 pages, 3 figures

We have calculated FeII emission line strengths for Active Galactic Nuclei Broad-Line Regions using precise radiative transfer and Iron Project atomic data. We improve the treatment of all previously considered excitation mechanisms for the FeII emission, continuum fluorescence, collisional excitation, fluorescence by self-overlap among the iron lines, and fluorescent excitation by Lyman-alpha. We demonstrate that Lyman-alpha fluorescence is of fundamental importance in determining the strength of the FeII emission. In addition to enhancing the ultraviolet and optical FeII flux, Lyman-alpha fluorescence also results in significant near-infrared FeII emission in the 8500-9500 Angstrom wavelength range. New observations are suggested to probe this effect in strong FeII emitting quasars., Comment: 12 pages, 2 encapsulated Postscript figures. To be Published in The Astrophysical Journal Letters

Geometrical (uniform disk) and numerical models were calculated for a set of B-emission (Be) stars observed with the Palomar Testbed Interferometer (PTI). Physical extents have been estimated for the disks of a total of15 stars via uniform disk models. Our numerical non-LTE models used parameters for the B0, B2, B5, and B8spectral classes and following the framework laid by previous studies, we have compared them to infrared K-band interferometric observations taken at PTI. This is the first time such an extensive set of Be stars observed with long-baseline interferometry has been analyzed with self-consistent non-LTE numerical disk models.