A&A 469, 713-719 (2007)
B. J. Medhi1,2 - S. Messina3 - Padmakar S. Parihar4 - I. Pagano3 - S. Muneer5 - K. Duorah1
1 - Department of Physics, Gauhati University, Assam 781014, India
2 - Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263129, India
3 - INAF - Catania Astrophysical Observatory, via S. Sofia 78, 95123 Catania, Italy
4 - Indian Institute of Astrophysics, CREST, Block II, Koramangala, Bangalore 560034, India
5 - Indian Institute of Astrophysics, VBO, Bangalore, India
Received 8 September 2005 / Accepted 12 April 2007
Aims. In this paper we report the results of the spectroscopic survey we carried out to detect chromospheric activity of late-type stars in the fields of view of the CoRoT space mission.
Methods. We give an accurate MK classification of all targets, which is valuable information on both the main CoRoT project and additional science programs, by means of cross-correlation with MK standard stars, by using artificial neural networks. The presence or absence of excess H emission, determined using spectral subtraction technique, is used to characterize the chromospheric activity level.
Results. In most cases our MK classification agrees with the spectral classification reported in the SIMBAD database; however, there are a few stars that are found to have very different MK classes. Our survey reveals that 7% of late-type stars in our sample indeed possess a very active chromosphere. The measured H excess and the presence of the 6708 Å Li line allow us to confirm the membership of 5 targets in the young open cluster NGC 2264.
Key words: stars: late-type - stars: activity - stars: fundamental parameters
CoRoT (Convection Rotation and Planetary Transits) is a small optical space telescope dedicated to asteroseismology and to the search for extrasolar planets (Baglin et al. 2000) and put in orbit on Dec. 27, 2006. In the coming years CoRoT will provide ultra-high precision photometry for thousands of stars, with a temporal resolution from 8 min down to 1 s and up to 150 days long. The CoRoT observable skies are two regions of 10 deg radius located at the intersection of the galactic plane with the celestial equator: RA = 1850, Dec = 0 (in the direction of the galactic center) and RA = 0650, Dec = 0 (anti-center direction). The central program of asteroseismology aims at observing at least 6 different sets of target stars very precisely over a period of 150 days each, whereas under the exoplanet program, CoRoT will provide extremely long and uninterrupted sequences of photometric data on more than one hundred thousand stars of V magnitude between 12 and 16. In addition to asteroseismology and the search for extrasolar planets, the data provided by CoRoT are expected to be extremely useful for investigating chromospherically active stars, QSOs, X-ray binaries, white dwarfs, cataclysmic variables, pre-main sequence Ae-Be stars, and magnetic Ap stars (Gilmore 2001; Weiss et al. 2003). For instance, stellar magnetic activity, which is a potential source of noise in the detection of planetary transits (see e.g., Lanza et al. 2003,2005; Moutou et al. 2005), will be studied in the context of the CoRoT additional programs. In fact, the ultra-precise CoRoT photometric data can be used to study the convection, properties, and evolution of photospheric active regions (Aigrain et al. 2004; Lanza et al. 2003, 2004), stochastic variability such as micro-flaring, stellar rotation, and surface differential rotation of late-type stars.
To gather as much a priori information as possible on the potential target stars, i.e., fundamental stellar physical parameters and peculiarities such as photometric or spectroscopic variability, and to maximize the scientific outcome of CoRoT data, an ambitious ground-based observing program, of obtaining Strömgren photometry and high-resolution spectroscopy for more than 1500 objects was carried out by several teams within the framework of the CoRoT working groups for ground-based observations (Solano et al. 2005; Poretti et al. 2003,2005). The characterization of magnetic activity of CoRoT asteroseismology primary targets was carried out at Catania Observatory by monitoring the Ca II H&K lines (Pagano et al., in preparation). Here, we report the results of our spectroscopic observations of a sample of late-type stars in order to identify new chromospherically active stars to be proposed as potentially rewarding targets for the CoRoT Additional Programs. The paper is organized as follows: in Sect. 2 we discuss our target selection criteria and give brief information about the instruments, observation, and reduction technique. In Sect. 3, we discuss the analysis methods and give results on the spectral classification and H behavior. The conclusions are given in Sect. 4.
|Figure 1: The observed bright and weak stars in the galactic center and anti-center fields of view of the CoRoT space telescope.|
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Our main stellar sample, hereafter referred to as the bright stars sample (BS), was selected according to the following criteria: stars in the CoRoT fields of view, i.e. in circles having 10 radius and centered on RA = 1850, Dec = 0 (galactic center direction) and RA = 0650, Dec = 0 (galactic anticenter direction), respectively; mag, according to the observational constraints of the CoRoT asteroseismology program; and late-type stars ( ) for which some degree of magnetic activity can be expected. As determined from the Tycho 2.0 catalog (Hog et al. 2000), which is complete at 99% level for stars brighter than V=11.0, the BS sample turned out to have 2260 targets. We report here on a sub-sample of 150 bright stars (6.6% of the sample) randomly selected among the complete BS sample: the target identification, coordinate, V magnitude, and SIMBAD spectral type are given in Table 1, together with each target observation date. A second sample of targets, hereafter referred to as the weak stars sample (WS), has been selected to test the capability of our instrumental setup and analysis tools to detect the presence of chromospheric activity and to determine accurate MK spectral classes also on targets of interest to the CoRoT exoplanetary program. This sample is made of 30 targets selected according to the above and criteria but with mag (see Table 2 for information on WS targets). Among many candidate targets that satisfied these selection criteria, we preferentially selected those which were already known to be chromospherically active. The location of the observed BS and WS target stars is shown in Fig. 1 in the galactic coordinate system. Our sample is complete neither for the BS sample nor (and here is strongly incomplete) for stars in the range V = 12-16.
Our spectroscopic survey was carried out on a total of 11 nights during the years 2003-2004 using the 2 m HCT (Himalayan Chandra Telescope) of the Indian Astronomical Observatory, equipped with the HFOSC spectrograph. We made use of the H line as chromospheric activity diagnostic, rather than the Ca II H & K lines, the instrumental response being very poor in the blue spectral region. We used the red grism Gr8, which covers nearly a 4000 Å spectral window starting from 5400 Å and which gives a dispersion of about 1.3 Å/pixel. The spectrograph slit width was fixed at 67 micron ( ). The instrumental spectral resolution, as derived from the 2.5 pixels FWHM of sharp Fe and Ne lines, resulted in Å (). The optimum exposure time, ranging from a few tens to a few hundreds seconds, was chosen to typically achieve a 100-300 signal-to-noise ratio. During our observations, the seeing value varied between 1.2 and 2.5 arcsec. To estimate and to correct for atmospheric extinction effects, we observed several spectroscopic standard stars from the STELIB catalog (Le Borgne et al. 2003) each night, and then compared them with program star spectra. We also observed a large number of chromospherically inactive stars from Montes et al. (1995, 1997) and Montes et al. (1999) using the same instrumental setup as used for the program stars. On each night, observations of at least one spectrophotometric standard close to the CoRoT fields were also carried out with the purpose of flux calibration. The spectroscopic observations were reduced and analyzed with the CCD image reduction package ccdred and spectroscopic reduction package specred available within IRAF. After bias subtraction and flat field correction, stellar spectra were optimally extracted. A FeNe arc lamp was used for the wavelength calibration, and, finally, the flux calibration was done using spectro-photometric standard stars.
We used two different techniques to determine an accurate MK spectral type for the program stars, i.e. the cross-correlation between the program and MK standard star spectra, and the artificial neural network (ANN). A brief description of these two techniques and the results we obtained are given below.
This is a classical method used in spectral MK classification work. In this technique, the cross-correlation of an unknown program's stellar spectrum is done with a sample of MK standard stellar spectra covering a range of spectral types and luminosity classes. Since the instrumental profile in the spectrum of the program star may degrade the classification accuracy, we used spectra of program and standard stars obtained from the same instrumental setup. To have a complete sample of MK standards as possible, we decided to use two other sets of standard star spectra taken from the STELIB library of stellar spectra and the Atlas of Southern MK Standards (5800-10 200 Å) by Danks & Dennefeld (1994). Our final grid of standard stars includes almost all subtypes from F0V to M3V, from F6IV to K1IV, from F0III to M7III, from G5II to K3II, and from F0I to M2I. Imperfect flux calibration, interstellar reddening, and atmospheric dispersion can all distort the shape of the stellar continuum. Therefore, we decided to normalize the spectrum with respect to the continuum using the median and boxcar filtering procedure as described by Bailer-Jones (1997) and Bailer-Jones et al. (1998). In the following step, the spectral resolution of the program and standard spectra were matched by convolving one or both spectra with a Gaussian kernel. To remove from our analysis those regions severely affected by telluric lines (6850-7360, 7560-7730, 8100-8400 Å), as well for consistency with the spectral range used with the ANN method described in the next section, our classification was achieved by considering only the 5450-6800 Å spectral region. Both target and standard spectra were re-binned using quadratic spline interpolation, and the normalized flux was computed in the same wavelength bin. Finally, individual reference spectra from the catalogs of standards were cross-correlated with program stars. In order to also take the rotational velocity of the targets into account, each standard spectrum was rotationally broadened within a range of values from 0 to +50 with a step of 3 km s-1 (Frasca et al. 2003). The quality of the match was judged by considering the correlation coefficients between program and standard spectra with different rotational velocity broadenings. The standard spectrum giving the highest correlation coefficient was used to assign the MK class. In Cols. 2-3 of Table 3 we list the final MK class and correlation coefficient as derived from the cross-correlation technique. A comparison with the spectral classes provided either by SIMBAD and assigned by ANN for a sample of standard stars shows that the cross-correlation technique is capable of assigning the correct luminosity class to dwarfs and subgiants and with an accuracy of one class to giants and supergiants and of assigning the spectral type with an accuracy of about 2 sub-types.
Table 6: The 20 stars showing H excess.
|Figure 2: The observed spectrum of SW Mon (dots) with the template spectrum of the M5 III star HD 130144 (solid line) overplotted.|
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As shown in Table 3, cross-correlation and ANN methods assign
similar MK spectral types (within a difference of 2-3 sub-classes) to more than 90% of our targets and the same luminosity class to almost 65% of our targets. In Fig. 2 we show as an example the case of SW Mon
to which both methods assign the same spectral class. The
cross-correlation method tends to give higher correlation
coefficient values with respect to ANN. In order to confirm the MK spectral
class or to assign the correct one in those cases for which cross-correlation and ANN methods gave
we compared the target's absolute magnitude and B-V color
with a grid of theoretical evolutionary tracks (see Fig. 3) for a range of masses and
Z=0.0004-0.03), as taken from Girardi et al. (2000). Specifically, we used JHK
magnitudes from the 2MASS All Sky Survey (Cutri et al. 2003) and J-H vs. H-K
color diagrams in the Johnson-Glass system, as established by Bessel & Brett (1988), to determine the color excess, and the Hipparcos parallax (Perryman et al. 1998) to compute the target's absolute magnitude. For 73 out of 180 targets,
the parallax and the JHK magnitudes existed and were accurate enough to
constrain the spectral class. Absolute magnitudes and colors resulted in
agreement with the spectral class assigned by one or both mentioned methods.
|Figure 3: HR diagram of program stars (asterisks) with known parallax. Bullets indicate the H emission stars. The ZAMS and a sample of evolutionary tracks for Z=0.019 from Girardi et al. (2000) are also plotted.|
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Out of 180 target stars, only 40 stars were detected as X-ray sources during the ROSAT All Sky Survey (RASS). Count rates H and S in the hard (0.4-2.0 KeV) and soft (0.1-0.4 KeV) ROSAT-PSPC pulse height channels, respectively, the hardness ratio (HR), and associated errors have been retrieved from the ROSAT All-Sky Survey Faint Source Catalogue (Voges et al. 1999). In order to calculate the X-ray luminosity of these stars, we estimated a count rate-to-energy flux conversion factor (CF) as a function of the hardness ratio following the linear regression curve published by Schmitt et al. (1995) of CF as a function of HR. The uncertainty on the X-ray luminosity was estimated by taking into account the errors on the stellar parallax, count rate, hardness ratio, and intrinsic stellar color.
The X-ray luminosities of the WS sample in the field of the NGC 2264 open cluster are taken from the XMM-Newton EPIC survey (Dahm & Simon 2005). The bolometric correction to compute the bolometric luminosity was retrieved from Schmidt-Kaler (1982). The uncertainty on the ratio of the X-ray to bolometric luminosities was assumed to be the same as calculated for the X-ray luminosity, the error on bolometric luminosity being negligible compared to the error on X-ray luminosity. The derived / values are listed in Col. 7 of Table 5.
It is known that X-ray and H emissions are manifestations of magnetic activity at two different levels of the stellar atmosphere, corona and chromosphere and that they have their common origin in a hydromagnetic dynamo operating just beneath the external convection zone (e.g. Schüssler 1983). As expected from theoretical considerations, both X-ray and H emission levels are related to the dynamo efficiency and, therefore, are expected to be correlated to each other, as already observed for activity indicators at different atmospheric levels (e.g. Messina et al. 2003). For a small subsample of targets we have the measured H EW, as well as the computed ratio of X-ray to bolometric luminosities. The ROSAT X-ray luminosities plotted in Fig. 4 were corrected in order to take into account the difference in the energy bands measured by ROSAT with respect to XMM-Newton. We find that these quantities have a linear correlation coefficient r= 0.72 with a significance level: . With the exception of the subgiant HD 46122, which significantly deviates from the general trend showed by the other stars, in Fig. 4, IP Mon, IO Mon, MM Mon, MO Mon, and NGC 2264 Vas 92, all of which are in the field of the NGC2264 cluster, show a level of activity/emission that is generally higher than MS field stars.
|Figure 4: / vs. H EW for H emission stars. The solid line is a linear fit to the data. The linear correlation coefficient (r) and its significance level () are given in the label. A logarithmic scale is used for the X-axis.|
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|Figure 5: Top panels: normalized spectra of program stars with evidence of the Li 6708 Å line (continuous line) and non-active template spectra (small filled circles) used for SST. Bottom panels: the residual flux from the spectral subtraction technique.|
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|Figure 6: Results of the spectral subtraction.|
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In this paper we have described our exploration of the chromospheric activity in a sample of late-type stars. The important findings of the spectral subtraction technique used to characterize the H activity and the MK spectral classification of 180 stars in the COROT fields of view are as follows:
The Observations reported in this article were obtained using the 2-m Himalayan Chandra Telescope of the Indian Astronomical Observatory, Hanle, the high-altitude station of the Indian Institute of Astrophysics, Bangalore, India. We thank the staff at the IAO and Ramya R. and Jessi J. for their active support during the course of the observations. This work was also supported by the Italian Ministero degli Affari Esteri (MAE) and dell'Università e Ricerca (MUR) and by the Indian Ministery of Science and Technology. The extensive use of the SIMBAD and ADS databases operated by the CDS center, Strasbourg, France, is gratefully acknowledged. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.