H α line as an indicator of envelope presence around the Cepheid Polaris Aa ( α UMi )
I.A. Usenko, A.S. Miroshnichenko, V.G. Klochkova, N.S. Tavolzhanskaya
aa r X i v : . [ a s t r o - ph . S R ] O c t H α LINE AS AN INDICATOR OF ENVELOPE PRESENCEAROUND THE CEPHEID POLARIS Aa ( α U M i ). I.A. Usenko , A.S. Miroshnichenko , V.G. Klochkova , N.S. Tavolzhanskaya Astronomical Observatory, Odessa National UniversityT.G. Shevchenko Park, Odessa 65014, Ukraine [email protected] Department of Physics and Astronomy, University of North Carolina at Greensboro,Greensboro, NC 27402-6170, USA a [email protected] Special Astrophysical Observatory, Russian Academy of SciencesNizhnij Arkhyz, Karachaevo-Cherkessia, 369167 Russia [email protected]; [email protected]
ABSTRACT.We present the results of the radial velocity ( RV )measurements of metallic lines as well as H α (H β )obtained in 55 high-resolution spectra of the Cepheid α UMi (Polaris Aa) in 1994–2010. While the RV amplitudes of these lines are roughly equal, theirmean RV begin to differ essentially with growth ofthe Polaris Aa pulsational activity. This differenceis accompanied by the H α line core asymmetrieson the red side mainly (so-called knife-like profiles)and reaches 8–12 km s − in 2003 with a subsequentdecrease to 1.5–2 km s − . We interpret a so unusualbehaviour of the H α line core as dynamical changes inthe envelope around Polaris Aa. Key words : - Stars: Cepheids - Stars: radial ve-locities - Stars: H α absorption line - Stars: envelopes- Stars: individual - α UMi (Polaris A)
1. Introduction
Detection of an extended envelope around theCepheid Polaris (hereafter Polaris Aa) using a near-infrared interferometer (M´erand et al. 2006) suggestedan idea to check its presence spectroscopically. Usenkoet al. (2013, 2014ab), Usenko and Klochkova (2015)revealed that the H α absorption line could be usedas an indicator of the envelope presence not only inlong-period Cepheids but also in short-period ones. Asa rule, Cepheids with pulsational periods longer than7–10 d demonstrate a pronounced appearance of a sec-ondary variable absorption in the H α line cores, whileshort-period ones exhibit a smoother, so called knife-like shape. Besides that, a slight change in the RV of the H α line core with pulsational phase comparedto that determined from the metallic lines is anotherindicator of the envelope presence in Cepheids. Hence the main goal of this work is to measure the RV s of Polaris Aa in different pulsational phases usingthe metallic lines and H α (in some cases H β ) line coresand to estimate visually the shape of the latter ones.
2. Observations
Observations of Polaris Aa have been obtained usingthe following facilities:1. 1 m telescope of the Ritter Observatory, Universityof Toledo (Ohio, USA) - fiberfed echelle spectro-graph 1150 × λλ ×
400 pixelCCD ( λλ λλ RV measurementswere done using the DECH20 software (Galazutdinov1992). In Table 1 we present these RV data from thespectra obtained in 2005–2010. This table containsthe measurements derived from the metallic lines, H α ,and H β , respectively.
2. Radial velocity measurement analysis andthe H α line core behaviour As seen in Table 1 and Fig. 1, the difference betweenthe measurements obtained from the metallic lines and1
Odessa Astronomical Publications, vol. (2015)Table 1: Radial velocity data of Polaris Aa in 1994–2010
Spect- HJD Tel. Metallic lines H α H β rum ID 2400000+ RV σ NL RV RV H α (and one from H β ) for each spectrum does not ex-ceed 1.5 km s − in 1994. As seen from Fig. 2, the H α core does not demonstrate any visible asymmetries.Since 1999 (HJD 2451240–2451361) this differencebegins to increase (Fig. 1) and a slight asymmetryon the red side of the H α core are visible (Fig. 2).Two years later this difference becomes larger (from 1km s − to 2 km s − ), and the asymmetries on the redside of the core get quite visible (Fig. 3) during twoyears (2001–2002).During 2003 one can see the most interesting eventwhen the difference between the measurements reaches8–12 km s − (see Table 1 and Fig. 1) and the H α coreshows asymmetries on the red side as well as on theblue side (see Fig. 4).Since 2004 this difference decreases to 2–2.5 km s − (HJD 2453015–2453367), and the H α core exhibits Figure 1: Radial velocity measurements in the spectraof Polaris Aa during 1994–2010. Six-point stars – fromthe metallic lines, open five-point stars – from the H α line, and open circles – from the H β line. The solidline shows a second degree polynomial approximationfor the metallic lines.asymmetries on the red side only (see Fig. 1 and 5).During 2005–2006 (HJD 245689–2454073) thedifference is less than 1 km s − and the asymmetriesare less visible (Fig. 6). The same phenomena onecan see in other results obtained during 2008–2010(HJD 2454077–2454934) (Fig. 7). It should be notedthat the differences between the H α and H β linemeasurements are negligible.
3. Conclusions
We summarize the results of our investigation as fol-lows.1. As seen from the results in Table 1 and Fig.1,amplitudes of the RV curve from H α and H β arevery small and close to those determined from themetallic lines.2. First H α line core asymmetries on the red sidearise with an increase of the RV curve amplitudeafter the historical minimum of the Polaris Aa pul-sational activity in the beginning of the 1990’s.3. During 2003 the difference between the metallicline RV and those from the H α line core reaches8–12 km s − . This event is accompanied by thepronounced asymmetries of the H α core on the redside as well as on the blue side.4. Since 2004 the H α line core asymmetries are ob-served on the red side only and nearly disappearafter 2005, when the RV amplitude grows to a newminimum.5. H α core asymmetries (so-called a knife-like pro-file) in the atmosphere of Polaris Aa show thatthis absorption line could be an indicator of theenvelope presence in yellow pulsating supergiantswith short periods and small amplitudes. dessa Astronomical Publications, vol. (2015) 36. So unusual behaviour of the H α core during 2003could be explained by dynamical changes in theenvelope around Polaris Aa.[2mm]
3. Acknowledgments
This study was financially supported by theSCOPES Swiss National Science Foundation (projectno. IZ73Z0152485).
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