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Optical Flare Events on the RS Canum Venaticorum Star UX Arietis

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Optical Flare Events on the RS Canum Venaticorum Star UX Arietis RAA 2017 Vol. 17 No. 6, 55 (12pp) doi: 10.1088/1674–4527/17/6/55 R c 2017 National Astronomical Observatories, CAS and IOP Publishing Ltd. esearch in Astronomy and http://www.raa-journal.org http://iopscience.iop.org/raa Astrophysics Optical flare events on the RS Canum Venaticorum star UX Arietis Dong-Tao Cao and Sheng-Hong Gu Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China; [email protected], [email protected] Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China University of Chinese Academy of Sciences, Beijing 100049, China Received 2017 January 23; accepted 2017 February 25 Abstract Based on long-term high-resolution spectroscopic observations obtained during five observ- ing runs from 2001 to 2004, we study optical flare events and chromospheric activity variability of the very activeRS CVn star UX Ari. By means of the spectral subtraction technique, several optical chromo- spheric activity indicators (including the He I D3, Na I D1,D2 doublet, Hα and Ca II IRT lines) covered in our echelle spectra were analyzed. Four large optical flare events were detected on UX Ari during our observations, which show prominent He I D3 line emission together with great enhancement in emis- sion of the Hα and Ca II IRT lines and strong filled-in or emission reversal features in the Na I D1,D2 doublet lines. The newly detected flares are much more energetic than previous discoveries, especially for the flare identified during the 2002 December observing run. Optical flare events on UX Ari are more likely to be observed around two quadratures of the system, except for our optical flares detected during the 2004 November observing run. Moreover, we have found rotational modulation of chromo- spheric activity in the Hα and Ca II IRT lines, which suggests the presence of chromospherically active longitudes over the surface of UX Ari. The change in chromospherically active longitudes among our observing runs, as well as the variation in chromospheric activity level from 2001 to 2004, indicates a long-term evolution of active regions. Key words: stars: activity — stars: chromospheres — stars: binaries: spectroscopic — stars: flare — stars: individual (UX Ari) 1 INTRODUCTION Zwaan 2000), including both young and evolved stars, singles and members of close binary systems. A broad range of solar-type active phenomena, such UX Ari (HD 21242, BD+28◦532) is a non-eclipsing as starspots, plages, prominences and flares, has been spectroscopic system, consisting of a K0 IV primary and widely observed in cool stars. It is believed that all of a G5 V secondary, in an almost circular orbit with a pe- these phenomena arise from a powerful magnetic dy- riod of about 6.44d (Carlos & Popper 1971). According namo process generated by the coupling between convec- to previously published radial velocities together with tion and differential rotation. Among these active phe- their highly accurate ones, however,Duemmler & Aarum nomena, flares are very violent and abrupt events in (2001) concluded that a third star just happens to lie in the atmosphere, which are generally thought to result the same line of sight and therefore contributes a set of from the release of massive magnetic energy stored in weak lines in the spectra of UX Ari. Due to the bright- the corona through reconnection (Haisch et al. 1991; ness of the system (V = 6.37, Ducati 2002) and the Schrijver & Zwaan 2000). Flares have been observed K0 IV component showing a very high level of magnetic in many types of cool stars at almost all wavelengths activity, UX Ari has attracted much attention in nearly all (Pettersen 1989; Garcia Alvarez 2000; Schrijver & wavelength regions during recent years. 55–2 D.-T. Cao & S.-H. Gu: Optical Flare Events on UX Ari As a member of RS CVn-type stars, UX Ari al- et al. (1996). Aarum Ulv˚as & Engvold (2003) attributed ways shows significant photometric variability caused by stronger core emission in the Ca II λ8662 line to an op- starspot activity (e.g. Raveendran & Mohin 1995; Aarum tical flare-like event. Moreover, a simultaneous Hα and Ulv˚as & Henry 2003; Rosario et al. 2007, 2008; Ekmekci radio flare on UX Ari was reported by Catalano et al. 2010). Long-term photometric observations have been (2003). analyzed by Aarum Ulv˚as & Henry (2003), Rosario et al. In this paper, we present the study of optical flares (2007) and Ekmekci (2010). Using a Doppler imaging and the variation of chromospheric activity on UX Ari, technique, Vogt & Hatzes (1991) and Gu et al. (2004, based on long-term high-resolution spectroscopic obser- 2005) derived the spot distribution on the surface of vations of the Ca II IRT, Hα, Na I D1, D2 doublet and UX Ari and discussed possible spot evolution. He I D3 lines from 2001 to 2004. The details of our Strong chromospheric activity of UX Ari has been observations and data reduction are given in Section 2, exhibited by continuous Hα line emission above the con- and the procedure for the spectral subtraction of chro- tinuum, as well as Ca II H and K and Ca II IRT line core mospheric activity indicators and results are described in emission in the optical spectral range (Carlos & Popper Section 3. In Section 4, optical flares detected during our 1971; Bopp & Talcott 1978; Nations & Ramsey 1986; observations and the variation of chromospheric activity Huenemoerder et al. 1989; Frasca & Catalano 1994; are discussed in detail. Finally, we give a summary of the Raveendran & Mohin 1995; Montes et al. 1995a,b,c, present study in Section 5. 1996, 2000; Gu et al. 2002; Aarum Ulv˚as & Engvold 2 OBSERVATIONS AND DATA REDUCTION 2003). It is well accepted that the chromospheric emis- sion is mainly attributed to the K0 IV primary star of the The observations of UX Ari were carried out with the system. Bopp & Talcott (1978) found that the equivalent Coud´eechelle spectrograph (Zhao & Li 2001) mounted width (EW) of Hα was correlated to the orbital phase. on the 2.16-m telescope at Xinglong Station, adminis- Moreover, an orbital phase modulation of chromospheric tered by National Astronomical Observatories, Chinese emission in the Hα and Ca II IRT lines was also found by Academy of Sciences, during five observing runs from Gu et al. (2002). Aarum Ulv˚as & Engvold (2003) applied 2001 to 2004. The echelle spectra were recorded on a a technique for separation of individual components in 1024×1024-pixelTektronix CCD detector, and the spec- composite spectra of UX Ari, and concluded that the ac- ◦ trograph has a resolving power of about 37000. The re- tive longitudes seem to be separated by about 180 on ciprocal dispersions are 0.082 A/pixel˚ for the Na I D1, the surface of the primary star and the secondary is also D2 doublet and He I D3 spectral region, 0.091 A/pixel˚ subject to some chromospheric activity. A small amount for the Hα spectral region, 0.119 A/pixel˚ for the Ca II of chromospheric activity from the G5 V secondary was λλ8498, 8542 spectral region and 0.120 A/pixel˚ for the also found by Huenemoerder et al. (1989). Ca II λ8662 spectral region. The corresponding spectral Flares on UX Ari have been reported several times resolution determined as the full width at half-maximum over a wide range of wavelength regions from X-ray (FWHM) of the arc comparison lines is 0.152, 0.167, to radio wavelengths (e.g. Simon et al. 1980; Elias 0.211 and 0.216 A,˚ respectively. We acquired a total of et al. 1995; Montes et al. 1996; Franciosini et al. 2001; 51 spectra of UX Ari during our observations. As well as Gu et al. 2002; Richards et al. 2003; Aarum Ulv˚as the target star, some rapidly rotating early-type stars and & Engvold 2003; Catalano et al. 2003). According to slowly rotating inactive stars with the same spectral type International Ultraviolet Explorer (IUE) spectra, for ex- and luminosity class as each component of UX Ari were ample, Simon et al. (1980) found a very large ultraviolet also observed. The spectra of early-type stars were used (UV) flare on UX Ari and proposed a scenario to explain as telluric templates whereas the inactive stars were used activity in major long-lived RS CVn flares. In the optical as reference stars in the spectral subtraction technique. spectral lines, Montes et al. (1996) detected a strong flare We give the observing log of UX Ari in Table 1, through the presence of prominent He I D3 line emission which includes observing date, heliocentric Julian date together with a great enhancement of the Hα line emis- (HJD), orbital phase and exposure time. The orbital sion and larger filled-in cores of the Na I D1,D2 doublet phases are calculated with the ephemeris lines. Another flare-like event was observed with several HJD = 2450640.39272+ 6.4372703d × E, (1) frequently used optical chromospheric activity indicators (including the He I D3, Na I D1, D2 doublet, Hα and from Duemmler & Aarum (2001), where the epoch cor- Ca II IRT lines) by Gu et al. (2002), which happened at a responds to a conjunction with the K0 IV primary com- very similar orbital phase as the flare detected by Montes ponent in front. D.-T. Cao & S.-H. Gu: Optical Flare Events on UX Ari 55–3 Table 1 Observing Log of UX Ari Date HJD Phase Exp.time Date HJD Phase Exp.time (2 455 000+) (s) (2 455 000+) (s) 2001 Nov 23 2237.0773 0.0375 1800 2004 Feb 06 3042.0838 0.0915 900 2001 Nov 24 2238.0644 0.1909 2700 2004 Feb 06 3042.0946 0.0932 900 2001 Nov 24 2238.0988 0.1962 2700 2004 Feb 07 3043.1082 0.2507 900 2001 Nov 25 2239.0654 0.3464 2700 2004 Feb 07 3043.1189 0.2523 900 2001 Nov 25 2239.2790
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