The effect of pulsation on the near-ultraviolet spectrum of AGB stars

Abstract

Pulsating and non-pulsating AGB stars exhibit a variable near-UV spectrum, which suggest that mechanisms other than pulsation may affect their near-UV spectrum. In this work we analyse the near-UV spectra of two groups of AGB stars: (1) regularly pulsating and (2) irregular, small-amplitude stars. Near-UV and blue spectra were obtained for 27 stars distributed into these two groups with the INT telescope (La Palma, Spain). Additional near-UV spectra were taken from the iue library. The occurrence of Fe ii lines depends mainly on the intensity of the stellar continuum: as it increases, Fe ii lines are gradually hampered. Balmer emission lines are pulsation driven, as they appear only among the large-amplitude pulsating stars of our sample, between −0.10 < φ < 0.50. Among the regularly pulsating stars, the intensity of the Mg iiλ2800 doublet is driven by pulsation, with its maximum between 0.20 < φ < 0.35. On the other hand, this feature is also highly variable among small-amplitude, irregularly pulsating stars. This suggests that, besides pulsation, other mechanisms may participate in the formation of this line. The spectral slope between 3000 < λ(Å)<3200 is approximately constant among small-amplitude stars, but it shows a strong correlation with the phase of the pulsation in the other group, which indicates that the origin of the continuum is chromospheric. Different phase lags between the Mg iiλ2800 and the slope of the continuum suggests that this line and its neighbouring continuum might be formed in distinct places in the chromosphere or its surroundings.

Comprehensive Analysis of a Symbiotic Candidate V503 Her

“…Montez et al (2017) found that NUV emission of some AGB stars is correlated with optical and near-infrared light curves, suggesting that it is connected to the star itself. Moreover, Guerrero & Ortiz (2020) also found a correlation between pulsations of large amplitude and UV characteristics.…”

Investigating the UV-excess in Star Clusters with N-body Simulations: Predictions for Future CSST Observations*

“…Old, hot and low-mass stars have been a popular choice. These include, for example, post-asymptotic giant branch (AGB) stars (Greggio & Renzini 1990) and regular AGB stars (Guerrero & Ortiz 2020); extreme horizontal branch stars (EHB, Yi et al 1997;Yoon et al 2004;Ree et al 2007); and young white dwarfs (WDs) that are hot and blue (Mestel 1952;Rauch et al 2014;Torres et al 2014;Pang et al 2016). On the other hand, interacting binary systems containing subdwarfs have also been a proposed source of the UV-upturn (Zhang et al 2005;Han et al 2007Han et al , 2010.…”

UV bright red-sequence galaxies: a comparative study between UV upturn and UV weak systems Maria

“…Greggio & Renzini, 1990a;Brown et al, 1998;Deharveng, Boselli, & Donas, 2002;Han, Podsiadlowski, & Lynas-Gray, 2007;Chavez & Bertone, 2011, and references therein); even in 'regular' AGB stars, UV excess has been detected (e.g. Ortiz, Guerrero, & Costa, 2019;Guerrero & Ortiz, 2020). Maraston (2005) and Bruzual (2007) highlight the importance of considering rare evolutionary stellar populations in models, which are frequently overlooked, with special remarks on thermally pulsating-AGB (or TP-AGB) stars (top right regions of Figs.…”