Critical study of the distribution of rotational velocities of Be stars; II: Differential rotation and some hidden effects interfering with the interpretation of the Vsin i parameter

We assume that stars may undergo surface differential rotation to study its impact on the interpretation of $V\!\sin i$ and on the observed distribution $\Phi(u)$ of ratios of true rotational velocities u=V/V_\rm c (V_\rm c is the equatorial critical velocity). We discuss some phenomena affecting the formation of spectral lines and their broadening, which can obliterate the information carried by $V\!\sin i$ concerning the actual stellar rotation. We studied the line broadening produced by several differential rotational laws, but adopted Maunder's expression $\Omega(\theta)=\Omega_o(1+\alpha\cos^2\theta)$ as an attempt to account for all of these laws with the lowest possible number of free parameters. We studied the effect of the differential rotation parameter $\alpha$ on the measured $V\!\sin i$ parameter and on the distribution $\Phi(u)$ of ratios u=V/V_\rm c. We conclude that the inferred $V\!\sin i$ is smaller than implied by the actual equatorial linear rotation velocity V_\rm eq if the stars rotate with $\alpha0$. For a given $|\alpha|$ the deviations of $V\!\sin i$ are larger when $\alpha<0$. If the studied Be stars have on average $\alpha<0$, the number of rotators with V_\rm eq\simeq0.9V_\rm c is larger than expected from the observed distribution $\Phi(u)$; if these stars have on average $\alpha>0$, this number is lower than expected. We discuss seven phenomena that contribute either to narrow or broaden spectral lines, which blur the information on the rotation carried by $V\!\sin i$ and, in particular, to decide whether the Be phenomenon mostly rely on the critical rotation. We show that two-dimensional radiation transfer calculations are needed in rapid rotators to diagnose the stellar rotation more reliably.Comment: To appear in A&

Simultaneous J-, H-, K- and L-band spectroscopic observations of galactic Be stars. I. IR atlas

It is already accepted that Be stars are surrounded by circumstellar envelopes, which are mostly compatible with a disc geometry in Keplerian rotation. We aim to obtain a more complete characterisation of the properties of the circumstellar environment of Be stars that helps to constrain the theoretical models of the Be phenomenon. We present near-infrared, medium-resolution spectra of a sample of galactic Be stars with different spectral subtypes and luminosity classes. We measure different parameters of the hydrogen recombination lines from the Paschen, Brackett, Pfund, and Humphreys series, and use them to diagnose physical conditions in the circumstellar environment. We analysed the equivalent-width (EW) ratio between Br$\alpha$ and Br$\gamma$ lines and different diagrams of flux ratios. We also identify lines from He I, C I, N I, O I, Na I, Mg I, Mg II, Si I, Fe I, and Fe II. Analysing the EW measurements of particular He I, Mg II, Fe I, Fe II and O I lines, we find that for some lines they correlate with the spectral type of the star. Particularly, the emission of the O I~$\lambda\,1.3168\,\mu$m line decreases towards the later spectral types. We present an atlas of 22 Be stars, that covers a wide infrared (IR) spectral range with quasi-simultaneous observations. From a detailed analysis, we define new complementary criteria to Mennickent's classification of Be stars according to their disc opacity. Some objects in our sample present compact thick envelopes, while in others the envelope is extended and optically thin. The correlation between the full widths at half maximum (FWHM) and the peak separation ($\Delta \mathrm{V}$) versus $V\,\sin\,i$ for the Br10, Br$\delta$, and Hu14 lines reveals that the broadening mechanism is rotational. The Ly$\beta$ fluorescence is a key mechanism to explain the intensity of the emission of Mg II and O I lines.Comment: 27 pages, 14 figures. Accepted for publication in A&

BCD spectrophotometry for massive stars in transition phases

Context. Stars in transition phases, like those showing the B[e] phenomenon and luminous blue variables (LBVs), undergo strong, often irregular mass ejection events. The prediction of these phases in stellar evolution models is therefore extremely difficult if not impossible. As a result, their effective temperatures, their luminosities and even their true nature are not fully known. Aims. A suitable procedure to derive the stellar parameters of these types of objects is to use the BCD spectrophotometric classification system, based on the analysis of the Balmer discontinuity. The BCD parameters ({\lambda}_1, D) are independent of interstellar extinction and circumstellar contributions. Methods. We obtained low-resolution spectra for 14 stars with the B[e] phenomenon and LBVs. Using the BCD method, we derived the stellar and physical parameters. The study was complemented with the information provided by the JHK colour-colour diagram. Results. For each star, the BCD system gives a complete set of fundamental parameters and related quantities such as luminosity and distance. We confirmed HK Ori, HD 323771 and HD 52721 as pre-main sequence HAe/B[e], AS 202 and HD 85567 as FS CMa-type, and HD 62623 as sgB[e] stars. We classified Hen 3-847, CD-24 5721, and HD 53367 as young B[e] stars or FS CMa-type candidates, and HD 58647 as a slightly evolved B[e] star. In addition, Hen 3-1398 is an sgB[e] and MWC 877, CPD-59 2854 and LHA 120-S 65 are LBV candidates. The stellar parameters of the latter two LBVs are determined for the first time. Conclusions. Our results emphasise that the BCD system is a highly valuable tool to derive stellar parameters and physical properties of B-type stars in transition phases. This method can be combined with near-IR colour-colour diagrams to determine or confirm the evolutionary stage of emission-line stars with dust disks

Be and Bn stars: Balmer discontinuity and stellar-class relationship

International audienceContext. A significant number of Be stars show a second Balmer discontinuity (sBD) attributed to an extended circumstellar envelope (CE). The fast rotational velocity of Be stars undoubtedly plays a significant role in the formation of the CE. However, Bn stars, which are also B-type rapidly rotating stars, do not all present clear evidence of being surrounded by circumstellar material. Aims: We aim to characterize the populations of Be and Bn stars, and discuss the appearance of the sBD as a function of the stellar parameters. We expect to find new indices characterizing the properties of CEs in Be stars and properties relating Be and Bn stars. Methods: We obtained low- and high-resolution spectra of a sample of Be and Bn stars, derived stellar parameters, characterized the sBD, and measured the emission in the Hα line. Results: Correlations of the aspect and intensity of the sBD and the emission in the Hα line with the stellar parameters and the V sin i are presented. Some Bn stars exhibit the sBD in absorption, which may indicate the presence of rather dense CEs. Six Bn stars show emission in the Hα line, so they are reclassified as Be stars. The sBD in emission appears in Be stars with V sin i ≲ 250 km s-1, and in absorption in both Be and Bn stars with V sin i ≳ 50 km s-1. Low-mass Be and Bn stars share the same region in the Hertzsprung-Russell diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to the current stellar evolutionary stage are similar, while distributions inferred for the zero-age main sequence have different skewness. Conclusions: We found emission in the Hα line and signs of a CE in some Bn stars, which motivated us to think that Bn and Be stars probably belong to the same population. It should be noted that some of the most massive Bn stars could display the Be phenomenon at any time. The similarities found among Be and Bn stars deserve to be more deeply pursued. Based on observations obtained at the Complejo Astronómico El Leoncito (CASLEO), operated under an agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina, the Secretaría de Ciencia y Tecnología de la Nación and the National Universities of La Plata, Córdoba and San Juan, Argentina

BCD spectrophotometry for massive stars in transition phases

Context. Stars in transition phases, such as those showing the B[e] phenomenon and luminous blue variables (LBVs), undergo strong, often irregular mass-ejection events. The prediction of these phases in stellar evolution models is therefore extremely difficult, if not impossible. As a result, their effective temperatures, their luminosities, and even their true nature are not fully known. Aims. A suitable procedure to derive the stellar parameters of these types of objects is to use the BCD spectrophotometric classification system, which is based on the analysis of the Balmer discontinuity (BD). The BCD parameters (λ1, D) have the advantage that they are independent of interstellar extinction and circumstellar contributions. Methods. We obtained low-resolution spectra for a sample of 14 stars with the B[e] phenomenon and LBVs. Using the BCD classification system, we derived the stellar and physical parameters. The study was complemented with the information provided by the JHK colour-colour diagram. Results. For each star under investigation, the BCD system gives a complete set of fundamental parameters and related quantities such as luminosity and distance. Among the 14 studied stars, we confirmed the classification of HK Ori, HD 323771, and HD 52721 as pre-main sequence HAeB[e] stars, AS 202 and HD 85567 as FS CMa-type stars, and HD 62623 as sgB[e]. We also classified Hen 3- 847, CD-245721, and HD 53367 as young B[e] stars or FS CMa-type candidates, and HD58647 as a slightly evolved B[e] star. In addition, Hen 3–1398 is an sgB[e] and MWC 877, CPD–59 2854, and LHA 120-S 65 are LBV candidates. The stellar parameters of the latter two LBVs are determined for the first time. We also used the size-luminosity relation to estimate the inner radius of the dust disc around the pre-main sequence and main sequence B[e] stars. Conclusions. Our results emphasise that the BCD system is an important and highly valuable tool with which to derive the stellar parameters and physical properties of B-type stars in transition phases. This method can be combined with near-infrared (NIR) colour-colour diagrams to determine or confirm the evolutionary stage of emission-line stars with dust discs

Critical study of the distribution of rotational velocities of Be stars

Aims. We assume that stars may undergo surface differential rotation to study its impact on the interpretation of Vsini and on the observed distribution Φ(u) of ratios of true rotational velocities u = V/Vc (Vc is the equatorial critical velocity). We discuss some phenomena affecting the formation of spectral lines and their broadening, which can obliterate the information carried by Vsini concerning the actual stellar rotation. Methods. We studied the line broadening produced by several differential rotational laws, but adopted Maunder’s expression Ω(θ) = Ω0(1 + αcos2θ) as an attempt to account for all of these laws with the lowest possible number of free parameters. We studied the effect of the differential rotation parameter α on the measured Vsini parameter and on the distribution Φ(u) of ratios u = V/Vc. Results. We conclude that the inferred Vsini is smaller than implied by the actual equatorial linear rotation velocity Veq if the stars rotate with α 0. For a given | α | the deviations of Vsini are larger when α 0, this number is lower than expected. We discuss seven phenomena that contribute either to narrow or broaden spectral lines, which blur the information on the rotation carried by Vsini and, in particular, to decide whether the Be phenomenon mostly rely on the critical rotation. We show that two-dimensional radiation transfer calculations are needed in rapid rotators to diagnose the stellar rotation more reliably

Traumatic pasts and the historical imagination: Symptoms of loss, postcolonial suffering, and counter-memories among African migrants

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