Close-up of primary and secondary asteroseismic CoRoT targets and the ground-based follow-up observations

نویسنده

  • K Uytterhoeven
چکیده

To optimise the science results of the asteroseismic part of the CoRoT satellite mission a complementary simultaneous ground-based observational campaign is organised for selected CoRoT targets. The observations include both high-resolution spectroscopic and multicolour photometric data. We present the preliminary results of the analysis of the ground-based observations of three targets. A line-profile analysis of 216 high-resolution FEROS spectra of the δ Sct star HD 50844 reveals more than ten pulsation frequencies in the frequency range 5–18 d, including possibly one radial fundamental mode (6.92 d). Based on more than 600 multi-colour photometric datapoints of the β Cep star HD 180642, spanning about three years and obtained with different telescopes and different instruments, we confirm the presence of a dominant radial mode ν1 = 5.48695 d , and detect also its first two harmonics. We find evidence for a second mode ν2 = 0.3017 d , possibly a g-mode, and indications for two more frequencies in the 7–8 d domain. From Strömgren photometry we find evidence for the hybrid δ Sct/γ Dor character of the F0 star HD 44195, as frequencies near 3 d and 21 d are detected simultaneously in the different filters. 1. The simultaneous ground-based follow-up observations of CoRoT targets The asteroseismic window of the CoRoT satellite mission aims at the monitoring of several types of pulsators along the Main Sequence. To optimise the science results, its targets are carefully chosen and selected. Preparatory observations from the ground have been a key stone in the selection process [1]. With the CoRoT satellite successfully launched (December 2006), simultaneous ground-based observations are very important and are complementary to the space data. Multi-colour photometry provides colour information, which allows identification of the degree l by means of amplitude ratios and phase shifts, while high-resolution spectroscopy allows the detection of high-degree modes and the identification of both the degree l and the azimuthal order m. In this framework a simultaneous ground-based observing campaign was organised by the CoRoT/SWG Ground-based Observations Working Group. Large Programme proposals (i.e. guaranteed observing time during four observing seasons) were submitted and accepted at ESO La Silla (FEROS/2.2m; 10+5 nights per semester), Observatoire de Haute Provence (SOPHIE/1.92m; 10+5 nights per semester) and Calar Alto Astronomical Observatory (FOCES/2.2m; 10+10 nights per semester), with the aim to obtain multi-site time-series of high-resolution spectra of a selection of δ Sct, γ Dor, β Cep and Be CoRoT primary and secondary targets. The first two observing seasons (winter 2006-2007 and summer 2007), (nearly) coinciding with CoRoT’s IR01 (first Initial Run) and LRc1 (first Long Run in the center direction), have been succesfully completed. Table 1 gives an overview of the targets and the amount of spectra obtained. In addition to the Large Programmes, and in continuation of a project started three years ago, observing time has been awarded at smaller telescopes with multi-colour photometric instruments (Strömgren photometry: 90cm@Sierra Nevada Observatory (SNO), 1.5m@San Pedro Mártir Observatory (SPMO); Geneva photometry: 1.2m Mercator@Observatorio Roque de los Muchachos (ORM); Johnson photometry: Konkoly Observatory (KO)). In particular, 18 and 14 consecutive nights have been awarded with the uvbyβ photometers at SPMO and SNO, respectively, in Nov-Dec 2006 and 2007. Table 1. Logbook of the spectroscopic observations, obtained in Jan-Feb 2007 and May-Jul 2007 with the FEROS, SOPHIE and FOCES instruments, dedicated to a selection of targets of the CoRoT IR01 (Feb-Apr 2007), LRc1 (summer 2007) and LRa1 (winter 2007-2008) runs. The different columns give the target name, its V magnitude, spectral type, variable type, name of the CoRoT run, amount of spectra obtained with FEROS, SOPHIE and FOCES, respectively. The targets indicated in boldface are discussed in this paper. target V Sp.T. type CoRoT run FEROS SOPHIE FOCES HD 50844 9.09 A2 δ Sct IR01 216 HD 50747 5.45 A4 SB2 IR01 17 14 6 HD 51106 7.35 A3m SB2 IR01 15 14 4 HD 50846 8.43 B5 EB IR01 16 12 4 HD 49434 5.74 F1V γ Dor LRa1 71 444 75 HD 50209 8.36 B9Ve Be star LRa1 68 HD 181555 7.52 A5 δ Sct LRc1 343 66 285 HD 174966 7.72 A3 δ Sct LRc1 119 HD 180642 8.27 B1.5III β Cep LRc1 213 35 HD 181231 9.69 B9.0V Be star LRc1 72 SB2=double-lined spectroscopic binary; EB=eclipsing binary In the next sections we report on the preliminary results of the analysis of the ground-based datasets of the δ Sct star HD 50844 and the β Cep star HD 180642, and give an outlook towards an interesting candidate CoRoT target, the hybrid δ Sct/γ Dor star HD 44195. Figure 1. A set of combined profiles of HD 50844, constructed using the LSD method. The individual profiles are slightly shifted in flux level for visibility reasons. Figure 2. Some lightcurves (Strömgren v) of HD 180642 obtained at SPMO (top) and SNO (middle and bottom). A nonsinusoidal behaviour, and a ’stillstand’, is clearly present. The Julian date is given with respect to JD 2453196. 2. The δ Sct star HD 50844 The δ Sct star HD 50844 was observed as secondary target around the solar-like oscillator HD 49933 during CoRoT’s IR01 (Feb-Apr 2007). Being a faint target (V = 9.09), obtaining ground-based high-resolution spectra with sufficiently high signal-to-noise ratio (S/N>100) proved to be a challenging task. In total 216 échelle spectra were obtained with FEROS/2.2m at ESO, La Silla, during the nights of 1–10 and 24–28 January 2007 (see Table 1). Two different teams (INAF-Brera and KULeuven), analysed the spectra independently, using different cross-correlation methods to increase the S/N. One team used the LSD deconvolution method [2], whereby taking into account more than 1000 lines (Figure 1), while the other constructed combined profiles from 9 carefully selected lines. Frequencies were sought for in the line-profile moments [3] and in the pixel-to-pixel intensity variations [4] of the combined profiles using Fourier analysis and least-squares fitting methods. The frequency spectrum (Figure 3) shows strong aliasing and looks particularly difficult to unravel due to closely spaced modes. In the variations of the first moment, more sensitive to the detection of low-degree modes (l < 4), at least nine modes in the range 6–15 d are found, with dominating frequencies near 6.9 and 12.8 d. The pixel-to-pixel variations, which allow also the detection of high-degree modes, show the presence of at least thirteen frequencies in the range 5–18 d, with dominating frequency peaks between 14 and 15 d. Only one mode detected in the moment variations was not detected in the pixel-to-pixel variations and therefore can be considered as a ’bona fide’ low-degree mode. The detected modes are represented in Figure 4. Preliminary identification of the associated modes, using the phase and amplitude variations 4 8 12 16 Frequency c/d N or m al iz ed p ow er

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تاریخ انتشار 2008