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Issue
A&A
Volume 539, March 2012
Article Number A14
Number of page(s) 16
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/201116934
Published online 20 February 2012

© ESO, 2012

1. Introduction

The CoRoT space mission was launched in December 2006 with the dual objectives of searching for extrasolar transiting planets and a detailed characterization of stellar variability (Baglin et al. 2006). Since its launch CoRoT has been monitoring over 100   000 stars in two different regions of the sky, the center direction targeting the constellation of Aquila (Galactic longitude 40°), and the anti-center direction toward the constellation of Monoceros (Galactic longitude 210°). Owing to the orbital constraints of the satellite, target fields in these two directions can be monitored for about 150 days. These CoRoT long runs are fully devoted to stellar seismology and planet search. In addition, CoRoT short runs with a duration of around 30 days are performed in-between. Besides serving the same purpose as the long runs, these observations are also devoted to the CoRoT additional program (Weiss et al. 2004).

The mission CoRoT acquires photometric lightcurves by measuring the flux either in white light or in three colors from each target. Before being analyzed, the data are cleaned of cosmic ray hits, then corrected for spacecraft jitter and other instrumental effects. Thereafter, data access is exclusive to the CoRoT co-investigators for one year, after which it is placed in the archive of the CoRoT Data Center for general access1. The search for planetary transit candidates and the performed ground-based follow-up observations are summarized in this type of paper on a field-by-field basis and in a sequential order upon conclusion of the follow-up effort. The purpose is twofold. First, to make the detection yield and ground-based observational data generally available to the scientific community to allow future analysis of individual targets. Moreover, the results presented can be used to study extrasolar-planet detection statistics in general (Mayor et al. 2009; Borucki et al. 2011; Howard et al. 2011), both in comparisons to other transit surveys as well as for variations in the CoRoT fields monitored. In particular, the candidate yield in individual CoRoT fields might be correlated with the existing differences in their stellar populations.

The first results of the planetary candidate detection and confirmation of the CoRoT initial run (IRa01) and the first long run in the center direction (LRc01) were previously presented by Carpano et al. (2009), Moutou et al. (2009), Cabrera et al. (2009), respectively.

In the present paper, we report on the analysis and subsequent follow-up observations of transit candidates in the first short run in the center direction to be observed by CoRoT (SRc01 coordinates: 18h58m22.42s;   3°04′48′′). The 3.05° by 2.8° field was observed for a total of 26 days between 13 April and 9 May 2007.

In the following, we describe the characteristics of the SRc01 field, the detection process to find planetary transit candidates from the CoRoT lightcurves, as well as the subsequent ground-based follow-up observations performed for a number of those candidates to determine their true character. The final yield of that process was that among the 6974 SRc01 targets observed by CoRoT, we identified 51 transit candidates and 139 eclipsing binaries. Follow-up observations were started for the candidates and a non-planetary nature has so far been established in seven cases. The remaining candidates are unresolved pending future observations.

2. Field characterization

The CoRoT SRc01 pointing was primarily chosen to cover stars within the seismology core program and has not been particularly optimized for the planet-finding programme of CoRoT. The target stars were selected using the information from ground-based surveys available in the ExoDat database (Meunier et al. 2007; Deleuil et al. 2009). Compared to CoRoT LRc01, the first long run field in the same direction of the sky (Cabrera et al. 2009), the SRc01 field contains fewer target stars and has a more inhomogeneous distribution. The number of stars observed by CoRoT is 11 408 and 6974 for the respective fields. Moreover, to increase the planet detection probability we have to consider not only the total number of stars in the field. It is equally crucial to select the field for which the fraction of dwarf stars among the target stars is optimized (Brown 2003; Batalha et al. 2010). Figure 1 shows a J − K versus J color–magnitude diagram from the 2MASS survey (Skrutskie et al. 2006) for all the stars observed by CoRoT in the SRc01. Dwarfs and giants can be distinguished with the help of magnitude-limited samples in these colour-magnitude diagrams (see Deleuil et al. 2006, for its application to CoRoT data). On the basis of the division shown in the figure, the fraction of dwarf stars in the SRc01 field can be estimated to be around 65%, which is in agreement with the 68% fraction provided by the stellar classification in ExoDat (Deleuil et al. 2009). This is considerably higher than the corresponding number (42%) for the previously reported LRc01 field (Cabrera et al. 2009).

More detailed studies of the dwarf and giant populations in other CoRoT runs have appeared in the literature (Gazzano et al. 2010; Hekker et al. 2009, 2010; Miglio et al. 2009a,b; Mosser et al. 2010). The fraction of giant stars determined in previous papers are consistent with the estimates made from the color-magnitude diagrams (Aigrain et al. 2009).

thumbnail Fig. 1

J − K versus J color–magnitude diagram of the 6974 stars in SRc01 field. Dwarf and giants stars are most likely to be found to the left and right, respectively.

3. Data acquisition and reduction

The SRc01 field was observed by CoRoT for a total of 26 days between Julian dates 2 454 193.94 and 2 454 229.81. A complete description of the satellite operations can be found in Boisnard & Auvergne (2006); Barge et al. (2008b); Auvergne et al. (2009). For the exoplanet channel of CoRoT, the light is dispersed by a bi-prism to help separate true planetary transits, which are nearly achromatic, from stellar variability. Depending on the brightness of the star, the lightcurve is either measured in chromatic mode (CHR) consisting of three separate channels (red, green, and blue), or in monochromatic mode (MON). Out of 6974 SRc01 targets, 1269 were observed in chromatic and 5705 in monochromatic mode.

During subsequent data treatment the main systematic error sources (jitter, hot pixels, outliers) are corrected or flagged (see Drummond et al. 2008; Auvergne et al. 2009; Pinheiro da Silva et al. 2008). The resulting CoRoT lightcurves are then provided for further analysis at the CoRoT N2 data level (Baudin et al. 2006). A discussion of systematic noise sources still present and how to address them, is found in Aigrain et al. (2009); Carpano et al. (2009); Cabrera et al. (2009). The data used for the analysis presented in this paper is the version 1.3 released on 1 April 2008.

4. Data analysis

The analysis of the SRc01 data set was performed by the different teams listed in Cabrera et al. (2009). All of them applied different methods for filtering and detrending the lightcurves, and searched for transit-like signals (Alapini & Aigrain 2008; Bordé et al. 2007; Carpano & Fridlund 2008; Defaÿ et al. 2001; Mazeh et al. 2009; Mislis et al. 2010; Moutou et al. 2005, 2007; Ofir et al. 2010; Régulo et al. 2007; Renner et al. 2008). The main advantage of this approach is that different methods have different types of false alarms and that a combined analysis tends to minimize their occurrence in the final candidate list (Moutou et al. 2005, 2007). In a second step the candidates found by the different teams are compared and ranked according to the quality of the signal and the planetary likelihood as described in detail in Carpano et al. (2009) and Cabrera et al. (2009); for the rate and nature of false positives, we refer to Almenara et al. (2009). The particular aim here is to identify as many as possible of the candidates for which the transit-like event can be attributed to either eclipsing binaries, contaminating binaries, or stellar activity. The outcome of this process is a ranking consisting of four separate Classes: priority 1 consists of very promising planetary candidates (see Fig. 2 for an example), priority 2 and 3 are candidates with indications of a non-planetary nature, but where there is not enough information to reject them from a pure photometric analysis, and finally priority 4 consists of candidates for which the transit signal is most likely not due to a planet. In addition, a large number of eclipsing binary systems are found during the detection process.

thumbnail Fig. 2

Lightcurve of CoRoT SRc01 E1 0936. The variability of the star has a period of 3.93 days. Also, a transiting candidate was found with a period of 5.15 days and a depth of 0.25%.

4.1. Detected planetary transit candidates

The 51 transit candidates found in the SRc01 field are listed in Table 10. For each case, this table indicates the priority, CoRoT identification (CoRoT-ID and CoRoT win-ID, i.e. the identification of the target window in the specific field), coordinates, B and R magnitudes from ExoDat, orbital period, epoch of the detected event, depth and length of the transit, and the outcome of ground-based follow-up observations (see Sect. 5). Table 10 indicates three priority 1 candidates, 15 priority 2 candidates, 21 priority 3 candidates, and 12 priority 4 candidates. From these, single transit events were detected for nine cases. Two of the latter are discussed below in detail.

The lightcurve of SRc01 E2 1066 (Fig. 3) shows a single event with a depth of 4% and a duration of 66h (slightly more than double the expected duration of the transit of Jupiter around the Sun). The J − K color of the star (1.32) and its position in the color-magnitude diagram (Fig. 1) indicate that the target is probably a giant star, although there is no spectral confirmation that could rule out the possibility of a late dwarf star. However, from the detection point of view, the detection of this signal is similar to the challenge of detecting the transit of a Jupiter around a solar-like star with a semi-major axis of several AU. Moreover, the shape of the transit shows the characteristic signal of the passage of a transiting object across an active region of a star (see e.g. the case of TrEs-1b, Charbonneau et al. 2007; for which an alternative explanation was proposed by Rabus et al. 2009).

The lightcurve of SRc01 E1 3314 shows two events of different depth and duration. The first one occurs in HJD 2   454   211.4, and has a duration of 4.5 h and a depth of 1%; the second one is centered on HJD 2   454   216.1, and has a duration of 4.1 h and a depth of 0.7%. The J − K color of the star is 0.6, which is compatible with a dwarf. There are no more events of similar amplitude in the 13 days that remained until the end of the observations. These observations are compatible with a system of two Jupiter-sized transiting planets with orbital periods longer than 13 days or with the primary and secondary eclipses of a highly eccentric eclipsing binary (as in the case of LRc01 E2 0379, Cabrera et al. 2009).

The R magnitudes of these two targets at 15.7 and 15.4, respectively, are too faint for efficient ground-based follow-up observations, which were not performed in any case.

4.2. Detected eclipsing binaries

The 139 eclipsing binaries identified in the SRc01 data set during the detection process are presented in Table 11. The CoRoT mask of SRc01 E1 1760 integrates the light of two eclipsing binaries with periods of 12.7 and 5.3 days. The field is relatively crowded and from the CoRoT measurements alone we cannot determine which of the stars in the PSF are the eclipsing binaries. This is also true for SRc01 E2 1728, which contains two binaries having periods of 1.8 and 5.3 days. SRc01 E1 3171 is probably a very eccentric binary where only one secondary eclipse was observed, although it is unclear whether the event at HJD 2   454   219.9 occurs in the same target as the primary event. The targets SRc01 E1 1827, E2 1388, E1 2373, E2 2240, E1 3240, E1 1819, E2 2115, E1 3091, E2 1418, and E1 0770 are eccentric binaries with a secondary eclipse at a phase that is different from 0.5. The star SRc01 E1 1193 (a peculiar eclipsing binary, probably with a disk), E1 2695 (with a pulsating component), E1 0836 (with giant components), and E2 1760 (classified as a delta Scuti binary by Debosscher et al. 2009) show interesting lightcurves.

More detailed analysis of individual eclipsing binary stars observed with CoRoT have been published in the literature (Damiani et al. 2010; Desmet et al. 2010; Dolez et al. 2009; Maceroni et al. 2009, 2010), but so far none of these analyses correspond to an object in this field. For an analysis of the variability of the stars in the CoRoT fields, we refer to the general works of Debosscher et al. (2009) and Sarro et al. (2009). There have also been specialized analyses of O stars (Degroote et al. 2010), B stars (Charpinet et al. 2010; Degroote et al. 2009a; Diago et al. 2009; Lefever et al. 2010; Huat et al. 2009; Neiner et al. 2009; Gutiérrez-Soto et al. 2009), solar-like stars (Appourchaux et al. 2008; Barban et al. 2009; Belkacem et al. 2009; Deheuvels et al. 2010; Deheuvels & Michel 2010; García et al. 2009, 2010; Michel et al. 2008; Mosser et al. 2009; Samadi et al. 2010), giants stars (Carrier et al. 2010; de Ridder et al. 2009), beta Cephei (Degroote et al. 2009b), delta Scuti (Poretti et al. 2009), and in particular HD 174936, observed in the asteroseismology field of SRc01 (García Hernández et al. 2009), RR Lyrae stars (Chadid et al. 2010), or HgMn stars (Alecian et al. 2009) to cite some examples.

thumbnail Fig. 3

The single detected transit-like event of CoRoT SRc01 E2 1066. Notable is the 0.5% local maximum at mid-transit.

Table 1

Instruments and methods used in the follow-up observations of the SRc01 candidates.

5. Detailed follow-up of candidates

5.1. Ground based observations

The CoRoT mission has proved its ability to detect very small planets such as CoRoT-7b (Léger et al. 2009; Queloz et al. 2009), as well as planets with long orbital periods such as CoRoT-9b (Deeg et al. 2010). A ground-based follow-up observation constitutes an integrated part of the detection process, both to reject scenarios that can mimic planetary transits among the candidates found by the detection teams, as well as to confirm and characterize the CoRoT planets. Details of this process can be found in Deeg et al. (2009) for the photometric follow-up and in Moutou et al. (2009) for the spectroscopic observations. The instruments used during the ground-based follow-up of the SRc01 transiting candidates are listed in Table 1.

5.1.1. Photometric follow-up

High-precision photometric follow-up is needed to discard contaminating eclipsing binaries located inside or in the neighborhood of the CoRoT photometric mask. The objective of such an observation is to re-observe a transit with a high spatial resolution (seeing limited) and check the flux variations of all nearby stars. We note that for photometric follow-up, a precise prediction of expected transit times is needed. Owing to the relatively short observing span of SRc01 of 26 days, the precision of candidate periods is significantly lower than those of the CoRoT long runs. This leads to a larger error in transit predictions that implying that short run candidates need to be re-observed from the ground as quickly as possible. For typical Hot-Jupiter transit candidates, ground-based transit observations become unfeasible about one year after the CoRoT-observations, when the prediction errors exceed a few hours and consequently, transit events can no longer be reliably observed during a single night. For the SRc01 fields, photometric follow-up was performed with the 0.8 m telescope in Observatorio del Teide (Spain) from the Instituto de Astrofsica de Canarias (hereafter IAC80), with the 1.2 m Euler telescope in La Silla Observatory (Chile) from the Geneva Observatory and with the 1m telescope at the Wise Observatory (Israel) from Tel-Aviv University (see Table 1).

5.1.2. Radial velocity follow-up

Radial velocity (RV) follow-up is needed to identify the nature of the transiting object and establish the mass of the planet and the eccentricity of its orbit. This can be done by measuring the RV variations of a star caused by the presence of a companion. Such an RV variation is directly linked to the mass ratio of the main star to its companion object. Radial velocities are obtained by a weighted cross-correlation function (hereafter CCF) between a numerical spectral mask (see Baranne et al. 1996; Pepe et al. 2002) and a stellar spectrum taken with the SOPHIE spectrograph, mounted on the 1.93 m telescope in Observatoire de Haute-Provence (France), with the HARPS spectrograph mounted on the ESO-3.6 m telescope in the ESO La Silla Observatory (Chile) and with the FLAMES/UVES spectrograph, mounted on the VLT UT2 telescope in the ESO Paranal Observatory (Chile) (see Table 1). Both SOPHIE and HARPS are fiber-fed echelle spectrographs with a resolution of between about 39   000 (SOPHIE, high efficiency mode) and 110   000 (HARPS, high accuracy mode) at 550 nm. Observations of SOPHIE and HARPS were obtained using the observing mode Obj_AB without acquisition of a simultaneous thorium lamp spectrum to monitor the Moon background light in the second fibre. The intrinsic stability of these spectrographs does not require the use of lamp calibration spectra, the instrumental drift during one exposure being in our case always smaller than the stellar RV photon noise uncertainties. The FLAMES spectrograph contains a multi fiber-link, which makes it possible to feed up to seven targets and a ThAr calibration lamp into the UVES echelle spectrograph with a resolution of about 47   000 at 550 nm (Loeillet et al. 2008; Bouchy et al. 2005).

In the magnitude range of the CoRoT targets, RV uncertainties are mostly dominated by photon noise limitation (Santerne et al. 2011). Thus, the RV follow-up with SOPHIE and HARPS was mainly concentrated on candidates with priority 1 and 2 and a V-magnitude brighter than 16. Taking advantage of the large aperture of ESO-VLT/FLAMES facilities, we followed up some priority 3 and 4 candidates or candidates fainter than the above magnitude limit in the FLAMES fields.

5.2. Results of the follow-up observations

Ground-based follow-up observations were performed for 26 of the 51 transit candidates in the SRc01 field (see Table 10), including all three priority 1 candidates and the eleven candidates in priority 2 brighter than mV ~ 16. On the basis of these observations, it was possible to conclude a non-planetary nature for seven of the candidates (four spectroscopic binaries, two contaminating or background eclipsing binaries, and one blended binary system). Furthermore, nine targets are found to be hot or rapidly rotating stars for which we not can obtain precise enough RV measurements to determine their nature. For five cases, it was possible to estimate an upper limit to the mass of a potential companion. The remaining candidates are still unresolved mainly owing to the photon noise limitation or ephemeris uncertainties mentioned above. For instance among the 18 high priority cases (class 1 and 2), eight remain unresolved and are good planetary candidates pending future detailed investigation.

We now discuss the follow-up results below. For each candidate we provide the CoRoT win-ID, acquisition mode (monochromatic (MON) or chromatic (CHR) band), and CoRoT-ID.

5.2.1. Priority 1 candidates

SRc01 E1 0936 – MON – 0211645912

For this target, the CoRoT data show a variable lightcurve with an amplitude of 0.7% and a period of 3.93 days; superimposed on this, there are transit-like events with a period of 5.1 days (see Fig. 2). On-off photometry with Euler in R-filter indicates a blend of three stars. From CoRoT photometry, one cannot determine which of the three stars is responsible for either the transit-like events or the variability; but given the amplitudes of the effects and the relative fluxes of the stars, one can calculate the expected on-target amplitudes that, blended, will be compatible with CoRoT measurements. If one assigns A, B, and C to these stars, the flux ratios are A/B ~ 1.5 and B/C ~ 19. Thus, if star A were to be responsible for the transits, one would observe transits 0.43% deep; for star B, the depth would be 0.64% and for star C the transit would be 12% deep. For the variability, if star A were responsible for the variability, the amplitude would be 1.2%, whereas it would be 1.8% in the case of star B, and 34% in the case of star C. Aperture photometry shows no evidence of any nearby eclipsing binary. The resulting light curves of the on-off photometric measurements indicate that A and B are constant to within the 1% level, whereas C is variable at the 20% level. There is maybe a hint of an eclipse event on A, but the photometry is too imprecise for us to draw any firm conclusions. Even the variability of C could be an artifact. Both stars A and B were followed up with HARPS: star A showed no peak in the CCF, which would be indicative of either a hot or fast-rotator star; star B does not show any significant RV variation in phase with the CoRoT ephemeris up to 80 m/s (see Table 2). The nature of this candidate remains unresolved. If the transit is on star A, the case cannot be solved because of the nature of the host star. If the transit occurs in star B, the upper limit to the mass of the hypothetical companion would be 0.6 MJup.

Table 2

Radial-velocity measurements of E1 0936 composant B. EGGS is the high efficiency mode of HARPS.

SRc01 E1 1773 – MON – 0211660040

CoRoT data reveal the presence of a 0.96% deep transit with a period of 16.2 days. Additional stellar activity is also seen with a characteristic period of 1.35 days. Subsequent ground-based follow-up observations were performed with SOPHIE and HARPS from June to September 2008 (see Table 3 and Fig. 4). For the observational data at hand, the cross-correlation function (CCF) has a single peak with a vsini of about 6.6 ± 1.0 km s-1. The mean errors in the radial velocity measurement are about 45 m/s. An analysis of the RV data indicates a drift of about 3.8 m/s per day over 60 days with a possible asymmetry in the stellar line profile. This implies that there is a binary system blended in the HARPS point spread function (PSF). On the other hand, we could not detect any significant RV variations in the residual in-phase with the transit ephemeris or activity. The DSS2 skychart from Aladin shows that at least four stars contribute to the flux measured in the CoRoT mask. Photometric follow-up of the transit event with the IAC80 was missed by a few hours owing to the large ephemeris uncertainties. The target was also observed with the Wise 1m telescope, with no filter, at  ~43 degrees from the full Moon. A partial lunar eclipse occurred during this observation. The transit and/or the stellar activity could be due to one of the contaminants. Thus, the exact nature of the candidate remains unresolved, pending future photometric observations.

thumbnail Fig. 4

Radial velocity measurements of E1 1773 with SOPHIE (black points) and HARPS – HAM (blue square) and HARPS – EGGS (blue triangle).

Table 3

Radial-velocity measurements of E1 1773.

SRc01 E1 3315 – MON – 0211662780

The candidate E1 3315 displays a 1.0% deep transit with a period of 4.551 days. FLAMES observations were acquired on 9 and 10 June 2008 and the star then displayed a variation of 15 km s-1 between phase 0.02 and 0.21. This result is indicative of a spectroscopic binary (SB1) with a secondary mass of about 130 MJup, assuming a solar-mass primary star.

5.2.2. Priority 2 candidates

SRc01 E1 2630 – MON – 0211630451

CoRoT detects 0.46% deep transits with a period of 1.5943 days for this candidate. The star was observed three times with HARPS from 20 to 23 August 2009 (see Table 4). With radial velocity uncertainties ranging from 40 to 80 m/s, we could not detect any significant RV variation at the CoRoT ephemeris. If on target, this result places an upper limit on the mass of the companion of 0.5 MJup, assuming a solar mass star. Thus, the nature of this candidate remains unresolved.

Table 4

Radial-velocity measurements of E1 2630.

SRc01 E1 3860 – MON – 0211635507

For the candidate E1 3860 with a 2.5% deep and a period of 21.238 days, one HARPS spectrum was acquired on 25 July 2008. The cross-correlation function (see Fig. 5) indicates a spectroscopic binary of type 3 (SB3), with components separated by 50 and 60 km s-1. Thus, one exposure was sufficient to discard this candidate as an eclipsing binary diluted by a third star. That the eclipses measured by CoRoT, which are already 2.5% deep, are diluted by the light of two stars makes the planet hypothesis unlikely.

thumbnail Fig. 5

Result of the CCF between the HARPS spectrum of E1 3860 and a G2 mask. The three peaks indicate the presence of three stars in this system.

SRc01 E2 1822 – MON – 0211642286

This priority 2 candidate shows shallow (0.25%) eclipses with a period of 0.94 days. IAC80 photometric follow-up data found no evidence of any nearby eclipsing binary, but the ephemeris at the time of the observations had large errors (estimated 2 h). With a mR of 15.8, this candidate is too faint for precise enough RV measurements. The nature of the candidate remains unclear.

SRc01 E1 2700 – MON – 0211650167

Ground-based follow-up observations for E1 2700 were performed with SOPHIE on 27 June 2008. No peak in the CCF was detected and the spectrum contains only Balmer lines. The target is also a hot star or a very fast rotator making accurate radial velocity measurements difficult. The nature of the 0.37% deep transit event with a 2.857 days period found by CoRoT remains unresolved.

SRc01 E1 3691 – MON – 0211651069

The candidate E1 3691, with a 0.35% transit and a 2.763 days period was observed with HARPS on 23 August 2009. No peak in the CCF was detected. The target is also a hot star or a very fast rotator. The cause of the transit could not be determined by RV measurements.

SRc01 E1 2322 – MON – 0211657825

The candidate E1 2322 has a 0.35% deep and 2.246 days period and was observed with HARPS on 21 and 24 June 2009. A first analysis of the CCF shows a single peak caused by Moon background light contamination. After correcting this contamination, there is no longer a peak in the CCF, which indicates that the target is also a hot star or a very fast rotator, thereby making precise RV measurements difficult. Unfortunately, a later reanalysis of the photometric CoRoT lightcurve revealed that the light from the bright eclipsing binary SRc01 E1 0198 contaminated the nearby masks of the candidates E1 2322 (at 27′′) and E1 4746 (at 16′′), because both share the ephemeris of the eclipsing binary. Therefore, these candidates were resolved with the help of CoRoT on-off photometry. This is one of the consequences of the severe crowding of this particular area in the SRc01 field and was not realized at an earlier stage of the mission because of an underestimate of the importance of the contamination. The eclipsing binary contributes only 0.7% of the flux within the mask of E1 4746 and only 0.3% in the case of E1 2322, but this small contribution is still enough to produce a false detection, because of the high precision of the measurements.

SRc01 E2 2046 – MON – 0211660744

This transit candidate has a 0.28% deep and a 0.821 days period. Photometric follow-up observations were done with the IAC80 telescope in April 2009. Two close neighbors (separation  ≤ 3′′) could be excluded as possible contaminants and no deep eclipses were found in any nearby stars. Given the faintness of the target (mR = 15.6) and the extremely shallow transit found in the CoRoT data, it was impossible to resolve this case with the performed observations.

SRc01 E2 0338 – CHR – 0211662131

In the CoRoT data of E2 0338, a transit event with 0.26% depth and a period of 6.098 days was found. The candidate was observed with FLAMES on 10 June 2008. Two peaks are present in the CCF that indicate a clear SB2. One exposure was thus sufficient to identify this candidate as an eclipsing binary. We did not attempt to constrain the secondary mass with a second measurement, nor to check whether the velocity variation was in-phase with the CoRoT ephemeris. The lightcurve shows an additional transit-like event at the epoch 2   454   221.2 with a depth of 0.7% in the blue channel, which was invisible in the red or green channels, which indicates that this event occurs in a background object, probably a long-period eclipsing binary.

SRc01 E1 3835 – MON – 0211666030

From CoRoT, the candidate E1 3835 has a 0.51% deep transit event with a period of 2.261 days. The candidate was observed with HARPS on 22 June 2008 showing a vsini of about 2.5  ±  1.0 km s-1 (see Table 5). Later photometric on-off observations with the IAC80 revealed that a 1.1 mag fainter star located 10′′ NW of the CoRoT target had a 5% deep eclipse at the CoRoT ephemerides. These observations are consistent with the observed transit depth found by CoRoT (Deeg et al. 2009), so this candidate was discarded as a background eclipsing binary.

Table 5

Radial-velocity measurements of E1 3835.

SRc01 E1 2059 – MON – 0211670576

From the CoRoT observations, a 0.20% deep transit with a period of 2.732 days was found for E1 2059. The candidate was observed with both SOPHIE and HARPS in July and August 2008 (see Table 6). With radial velocity uncertainties ranging from 40 to 90 m/s, we could not detect any significant RV variation in the CoRoT ephemeris implying that the upper limit to the mass of an hypothetical companion is 0.4 MJup. From HARPS CCF, the vsini estimate is about 2.2  ±  1.0 km s-1. Thus, the nature of this target remains unclear.

Table 6

Radial-velocity measurements of E1 2059.

SRc01 E1 4522 – MON – 0211676514

This candidate was observed with HARPS on 26 and 28 June 2008. We found different amplitudes in the radial velocity signal depending on the cross-correlation mask (according to Bouchy et al. 2009). The largest value was determined for the F0 mask, implying an early-type primary star in a blended binary system. The velocity variation in the blended binary is in-phase with the CoRoT ephemeris. Thus, the diluted eclipses of this double star are the origin of the 0.38% deep photometric signal with a period of 1.391 days found by CoRoT.

Table 7

Radial-velocity measurements of E1 0780.

5.2.3. Priority 3 candidates

SRc01 E1 0346 – MON – 0211620782

CoRoT observations of the candidate E1 0346 found a 0.45% deep event with a period of 14.499 days. Subsequent follow-up observations were performed with HARPS on 21 August 2008 without detecting any peak in the CCF. The target is either a hot star or a very fast rotator and precise radial velocity measurements are impossible. The cause of this event remains unclear and no planetary mass could be characterized for this target.

SRc01 E1 0507 – CHR – 0211626061

In the same way as for the previous candidate, HARPS observations took place on 21 August 2008 for this 0.25% deep candidate with a period of 6.409 days. No peak in the CCF was detected for the target and the case remains unresolved. The star is also either a hot star or a very fast rotator for which we not can measure a planetary mass companion.

SRc01 E1 3584 – MON – 0211643311

Follow-up observations of the candidate E1 3584, with a depth of 0.64% and a period of 4.239 days were performed with FLAMES on 10 June 2008. No peak in the CCF was detected in the data. The target is either a hot star or a very fast rotator, hence precise RV measurements are not possible. The true nature of the candidate remains unclear.

SRc01 E1 3468 – MON – 0211647986

The candidate E1 3468 was also observed with FLAMES on 10 June 2008 but no peak was detected in the CCF. As in the previous case, this is either a hot star or a very fast rotator thereby limiting the precision of the radial velocity measurements. The cause of the 1.2% deep event with a period of 2.037 days found in the CoRoT data could not be resolved.

SRc01 E2 1288 – MON – 0211650063

CoRoT observed a single transit event with a depth of 1.7% for this candidate with a minimal period of about 18 days. Follow-up observations were performed with FLAMES on 10 and 12 June 2008. The data obtained shows a variation of 5 km s-1 indicating a SB1 system with a minimum mass for the invisible companion of 200 MJup.

SRc01 E2 0713 – MON – 0211657608

This candidate, with a depth of 0.11% and a period of 2.003 days was also observed with FLAMES on 10 and 12 June 2008 (phases close to 0.94 and 0.86, respectively) and shows a variation of 2.2 km s-1 but not in phase with the CoRoT ephemeris. These two measurements are insufficient to conclude anything about this target’s nature. Thus, the nature of this candidate remains unclear pending new RV measurements.

SRc01 E2 1283 – MON – 0211663583

This candidate with a depth of 0.3% and a period of 2.458 days was also observed with FLAMES on 10 and 12 June 2008 (phases close to 0.35 and 0.1, respectively) and shows a variation of 1.4 km s-1 but not in phase with the CoRoT ephemeris. From the two measurements, it was impossible to determine on the cause of the transit event. Thus, the candidate remains unresolved pending new RV measurements.

SRc01 E1 2420 – MON – 0211670372

FLAMES observations for E1 2420 were acquired on 9 June and no peak in the CCF was detected. In addition, the target is either a hot or a very fast rotating star making precise radial velocity measurements challenging. Thus, the cause of the detected 0.6% event with a period of 2.161 days remains unclear.

Table 8

Radial-velocity measurement of E1 4599.

Table 9

Radial-velocity measurements of E1 2291.

thumbnail Fig. 6

Transit signal vs. R magnitude for the objects described in this paper. The asterisks represent the position of the planets as well as two brown-dwarfs discovered by CoRoT (CoRoT-7b is in the bottom left corner). For comparison the horizontal dashed lines represent (from top to bottom) the expected signal produced by a Jupiter-size planet, a Neptune-size planet, and a 2 Earth-radii planet, respectively orbiting a solar-like star at a short period.

5.2.4. Priority 4 candidates

SRc01 E1 0780 – CHR – 0211628697

The target E1 0780 was observed with both SOPHIE and HARPS during summer 2008 (see Table 7) to investigate the nature of this relatively bright candidate (mR = 14.1), which shows a 0.14% deep transit with a period of 2.434 days in the CoRoT data. From the HARPS data, we found a vsini of about 5.9  ±  1.0 km s-1 and RV uncertainties of about 20 m/s for three observations. No significant variation was found in phase with the CoRoT ephemeris, corresponding to an upper limit in mass of 0.3 MJup for a companion. The cause of the events hence remains unresolved.

SRc01 E1 1078 – MON – 0211647475

CoRoT detected a single 0.65% transit with a total duration of 25 h. This mono-transit candidate was observed in the field of FLAMES/GIRAFFE observations for stellar characterization of the CoRoT exoplanet fields (Gazzano et al. 2010). The acquired spectrum is compatible with a hot star but no precise radial velocities could be obtained. The cause of this single transit cannot be established with the instrumentation used.

thumbnail Fig. 7

Histogram with a comparison of the depths of the detections found in the SRc01 and LRc01 runs.

thumbnail Fig. 8

Periods vs. depths for the objects found in the SRc01 field. The asterisks represent the position of the planets and two brown-dwarfs discovered by CoRoT, except CoRoT-9b.

Table 10

Planetary transit candidates in the CoRoT SRc01 field.

Table 11

Eclipsing binaries found in SRc01.

SRc01 E1 4599 – MON – 0211663968

CoRoT detected a 0.26% transit event with a period of 2.450 days for E1 4599. A subsequent ground based follow-up observation was performed with HARPS on 19 August 2009 (see Table 8). The CCF shows a noisy peak with a vsini of about 6.5 ± 1.0 km s-1 and a contrast of 4.8%, which gave a radial velocity uncertainty of about 0.7 km s-1. The faint CCF area suggests that the observed spectrum is diluted by a brighter hot or fast-rotating star. A second observation is planned with lower priority owing to the low precision measurements available with HARPS. At present, the cause of the detected events remains unclear.

SRc01 E1 2291 – MON – 0211664319

The candidate E1 2291 with a 0.10% deep event and a period of 2.667 days was observed with HARPS three times in July and August 2008 (see Table 9). With radial velocity uncertainties ranging from 40 to 60 m/s, no significant RV variation at the CoRoT ephemeris could be detected, which is compatible with a upper limit in mass of about 0.2 MJup for a companion. The cause of the event observed by CoRoT is unclear.

6. Discussion

Among the 6974 SRc01 targets observed by CoRoT, we have found 51 transit candidates and 139 eclipsing binaries. As summarized in Table 10, ground-based follow-up observations were initiated for 26 of the candidates, including a large part of the high priority candidates. Nevertheless, until now, none of them have been confirmed as a planet. For seven of the candidates, the detected signal could be attributed to a stellar origin, but the nature of the remaining candidates needs to be investigated further with future observations. In particular, eight priority 1 and 2 cases in the field remain good planetary candidates. The large observational effort needed to determine the nature of the CoRoT transit candidates was highlighted already in previous publications (see e.g. Moutou et al. 2009). An additional complicating factor for the SRc01 data is that the short duration of the observations (26 days) causes less accurately determined transit parameters than the previously observed CoRoT fields, thereby making subsequent follow-up observations more challenging. This is particulary so when the follow-up is not performed in the same season as the CoRoT observations.

The detection statistics of the CoRoT observations can be used as a performance estimate as outlined in Aigrain et al. (2009); Cabrera et al. (2009); Carpano et al. (2009); Moutou et al. (2009). Figure 6 shows the R magnitude versus the transit signal (the product of the transit depth and the square root of the number of in-transit measurements, as described in Pont et al. 2006) for all the objects described in this paper. The CoRoT planets considered include CoRoT-1b (Barge et al. 2008a); CoRoT-2b (Alonso et al. 2008; Bouchy et al. 2008); CoRoT-4b (Aigrain et al. 2008; Moutou et al. 2008); CoRoT-5b (Rauer et al. 2009); CoRoT-6b (Fridlund et al. 2010); CoRoT-7b (Léger et al. 2009; Queloz et al. 2009); CoRoT-8b (Bordé et al. 2010); CoRoT-9b (Deeg et al. 2010); CoRoT-10b (Bonomo et al. 2010); CoRoT-11b (Gandolfi et al. 2010); CoRoT-12b (Gillon et al. 2010); CoRoT-13b (Cabrera et al. 2010); CoRoT-14b (Tingley et al. 2011); CoRoT-16b (Ollivier, in prep.); CoRoT-17b (Csizmadia et al. 2011); CoRoT-18b (Hébrard et al. 2011); CoRoT-19b (Guenther et al. 2012); CoRoT-20b (Deleuil et al. 2012); CoRoT-21b (Paetzold, in prep.); CoRoT-22b (Moutou et al., in prep.); CoRoT-23b (Rouan et al. 2012); CoRoT-24b and c (Alonso et al., in prep.), and as well as two brown-dwarfs CoRoT-3b (Deleuil et al. 2008); and CoRoT-15b (Bouchy et al. 2011). We see a similar trend to the one found in IRa01 (Moutou et al. 2009) and LRc01 (Cabrera et al. 2009): Jupiter and Neptune-sized planets are within the photometric reach of CoRoT over the whole magnitude range, whereas small, terrestrial-sized planets are only reachable at the bright end. If we compare the depths of the detections in both runs (see Fig. 7), LRc01 performs slightly better than SRc01, an expected result, since the length of the observations (145 days instead of 26) allows one to integrate more transits and achieve a higher signal-to-noise ratio. Another expected difference is shown in Fig. 8, which compares the depth of the detections as a function of period; this figure implies that the detection limit is not reached for periods longer than three days. In LRc01, the limiting period was around ten days.

It is clear that the absolute number of detections (eclipsing binaries and planetary candidates) is not very different in both runs: 187 in SRc01 and 200 in LRc01. According to the discussion in Cabrera et al. (2009), we expect around a 50% relative detection yield in a 26 day run when compared to a long run; however, we find 90% while observing only 60% of the targets (6974 in SRc01 and 11408 in LRc01). We understand that this difference is caused by the different stellar populations observed. Although the SRc01 and the LRc01 fields are separated by only a few degrees in the sky, the stellar populations observed are very different (65% dwarf stars in SRc01 and 42% in LRc01). On the other hand, the extinction in the regions close to the plane of the Galaxy can be very inhomogeneous from one pointing to the next (see, for example, Schlegel et al. 1998). The shape of the color–magnitude diagrams of both runs suggests that the fields have different extinction rates and therefore study different parts of the Galaxy, although the impact of this effect is not fully understood. A more detailed study of the CoRoT detection statistics, taking more observed fields into account is ongoing, and will be the subject of a forthcoming paper.

Acknowledgments

The team at IAC acknowledges support by grant ESP2007-65480-C02-02 of the Spanish Ministerio de Ciencia e Inovación. The German CoRoT Team (TLS and the University of Cologne) acknowledges DLR grants 50OW0204, 50OW0603 and 50QM1004. Some of the data published in this article were acquired with the IAC80 telescope operated by the Instituto de Astrofísica de Canarias in the Observatorio del Teide, and special thanks is given to its staff for performing a large fraction of these observations. This research has made use of the Exo-Dat database, operated at LAM-OAMP, Marseille, France, on behalf of the CoRoT/Exoplanet program, whose input catalogue was made possible thanks to observations collected for years at the Isaac Newton Telescope (INT), operated on the island of La Palma by the Isaac Newton group in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias. This publication made use of NASA’s Astrophysics Data System Bibliographic Services and 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.

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All Tables

Table 1

Instruments and methods used in the follow-up observations of the SRc01 candidates.

In the text
Table 2

Radial-velocity measurements of E1 0936 composant B. EGGS is the high efficiency mode of HARPS.

In the text
Table 3

Radial-velocity measurements of E1 1773.

In the text
Table 4

Radial-velocity measurements of E1 2630.

In the text
Table 5

Radial-velocity measurements of E1 3835.

In the text
Table 6

Radial-velocity measurements of E1 2059.

In the text
Table 7

Radial-velocity measurements of E1 0780.

In the text
Table 8

Radial-velocity measurement of E1 4599.

In the text
Table 9

Radial-velocity measurements of E1 2291.

In the text
Table 10

Planetary transit candidates in the CoRoT SRc01 field.

In the text
Table 11

Eclipsing binaries found in SRc01.

In the text

All Figures

thumbnail Fig. 1

J − K versus J color–magnitude diagram of the 6974 stars in SRc01 field. Dwarf and giants stars are most likely to be found to the left and right, respectively.
In the text
thumbnail Fig. 2

Lightcurve of CoRoT SRc01 E1 0936. The variability of the star has a period of 3.93 days. Also, a transiting candidate was found with a period of 5.15 days and a depth of 0.25%.
In the text
thumbnail Fig. 3

The single detected transit-like event of CoRoT SRc01 E2 1066. Notable is the 0.5% local maximum at mid-transit.
In the text
thumbnail Fig. 4

Radial velocity measurements of E1 1773 with SOPHIE (black points) and HARPS – HAM (blue square) and HARPS – EGGS (blue triangle).
In the text
thumbnail Fig. 5

Result of the CCF between the HARPS spectrum of E1 3860 and a G2 mask. The three peaks indicate the presence of three stars in this system.
In the text
thumbnail Fig. 6

Transit signal vs. R magnitude for the objects described in this paper. The asterisks represent the position of the planets as well as two brown-dwarfs discovered by CoRoT (CoRoT-7b is in the bottom left corner). For comparison the horizontal dashed lines represent (from top to bottom) the expected signal produced by a Jupiter-size planet, a Neptune-size planet, and a 2 Earth-radii planet, respectively orbiting a solar-like star at a short period.
In the text
thumbnail Fig. 7

Histogram with a comparison of the depths of the detections found in the SRc01 and LRc01 runs.
In the text
thumbnail Fig. 8

Periods vs. depths for the objects found in the SRc01 field. The asterisks represent the position of the planets and two brown-dwarfs discovered by CoRoT, except CoRoT-9b.
In the text

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