1 The Optical Gravitational Lensing Experiment. Planetary and Low-Luminosity Object Transits in the Fields of Galactic Disk. Results of the 2003 OGLE Observing Campaigns∗ A. Udalski1, M.K. Szyma´nski1, M. Kubiak1, G. Pietrzy´nski1,2, I. Soszy´nski1,2, K. Z˙ e b r u ´n1, O. Szewczyk1, andL. Wyrzykowski1 1Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland e-mail: (udalski,msz,mk,pietrzyn,soszynsk,zebrun,szewczyk,wyrzykow)@astrouw.edu.pl 2 Universidad de Concepci´on, Departamento de Fisica, Casilla 160–C, Concepci´on, Chile ABSTRACT We present results of two observing campaigns conducted by the OGLE-III survey in the 2003 observing season aiming at the detection of new objects with planetary transiting companions. Six fields of 35′ × 35′ each located in the Galactic disk were monitored with high frequency for several weeks in February–July 2003. Additional observations of three of these fields were also collected in the 2004 season. Altogether about 800 and 1500 epochs were collected for the fields of both campaigns, respectively. The search for low depth transits was conducted on about 230 000 stars with photometry better than 15 mmag. It was focused on detection of planetary companions, thus clear non- planetary cases were not included in the final list of selected objects. Altogether we discovered 40 stars with shallow (≤ 0.05 mag) flat-bottomed transits. In each case several individual transits were observed allowing determination of photometric elements. Additionally, the lower limits on radii of the primary and companion were calculated. From the photometric point of view the new OGLE sample contains many very good candidates for extrasolar transiting planets. However, only the future spectroscopic follow-up observations of the OGLE sample – determination of the amplitude of radial velocity and exclusion of blending possibilities – may allow to confirm their planetary status. In general, the transiting objects may be extrasolar planets, brown dwarfs, M-type dwarfs or fake transits arXiv:astro-ph/0411543v2 7 Jan 2005 caused by blending. All photometric data of objects with transiting companions discovered during the 2003 campaigns are available to the astronomical community from the OGLE Internet archive. 1. Introduction Detection of planetary transits in the spectroscopically discovered extrasolar planetary system HD209458 (Henry et al. 1999, Charbonneau et al. 1999) em- pirically proved that the photometric method of detection of extrasolar planets, called the “transit method”, can be a potentially important tool in planetary ∗Based on observations obtained with the 1.3 m Warsaw telescope at the Las Campanas Observatory of the Carnegie Institution of Washington. 2 searches. However, in spite of many efforts no extrasolar planet was detected in this manner during the next few years. The transit method offers many advan- tages compared to the traditional spectroscopic method. When the photometric observations are combined with precise spectroscopy all important parameters of a planet like its size and mass can be derived providing extremely important data to studying in detail the structure of planets and evolution of extrasolar planetary systems. In 2002 the Optical Gravitational Lensing Experiment (OGLE) survey announ- ced results of two photometric campaigns aimed at photometric discovery of low depth transits caused by extrasolar planets or other sort of “low-luminosity” objects (Udalski et al. 2002abc, 2003). About 140 stars such stars were found among about 150 000 stars in two lines-of-sight in the Galactic disk – toward the Galactic center and the constellation of Carina. It was obvious from the very beginning that the significant part of the discovered transits may be caused by stellar size companions, part can be an effect of blending of eclipsing stars with bright unresolvable neighbors while in the small number of objects the transits could be of planetary origin. Unfortunately, the photometry, which allows de- termination of size, cannot unambiguously distinguish planetary or brown dwarf from late M-type dwarf stellar companions as all of them may have the radii of the order of 0.1–0.2 R⊙ (1–2 RJup). The OGLE transiting objects were subject of many follow up studies in the past two years. The most important follow up observations of the OGLE ob- jects were high resolution spectroscopic observations for radial velocity changes. In the past two years the planetary status of four of the OGLE transiting com- panions was confirmed by measuring small amplitude radial velocity variation with the same period and in appropriate phase as resulting from the OGLE photometric orbits: OGLE-TR-56 (Konacki et al. 2003a, Torres et al. 2004a, Bouchy et al. 2004b), OGLE-TR-113 (Bouchy et al. 2004a, Konacki et al. 2004), OGLE-TR-132 (Bouchy et al. 2004a) and OGLE-TR-111 (Pont et al. 2004). An- other OGLE object – OGLE-TR-10 – also very likely hosts an extrasolar planet although additional radial velocity observations are necessary to fully confirm its planetary status (Konacki et al. 2003b, Bouchy et al. 2004b). Additionally, the high resolution spectroscopy allowed to rule out possible blending scenarios making the confirmations very reliable and sound. It is worth noting that except for OGLE-TR-132, which was at the edge of detection, all the remaining OGLE planets were on the OGLE short lists of the most likely planetary candidates based on their photometric properties. The sixth known extrasolar transiting planet, TrES-1, has recently been found by the small telescope wide field TrES survey (Alonso et al. 2004). Detection of the first transiting planets allowed to significantly increase the sample of extrasolar planets with precisely known masses and radii, thus pro- viding crucial empirical data for testing the planetary models etc. Surprisingly the first three OGLE planets (OGLE-TR-56, OGLE-TR-113 and OGLE-TR- 132) turned out to form a new class of “very hot Jupiters” – giant planets orbiting their hosts stars with periods shorter than 2 days. Such systems have been unknown so far in the solar neighborhood. It is clear that the detection 3 of additional transiting planets is of high importance for extrasolar planetary science. In 2003 the OGLE survey conducted two additional observational campaigns aimed at the detection of additional transiting exoplanets. Altogether six new fields located in the Galactic disk in the constellations of Carina, Centaurus and Musca were monitored regularly with high time resolution for a few weeks each. The search for objects with transiting companions was performed in similar manner as in the past campaigns. However, it was more focused on planetary transits. Hence, many clear detections of low mass stellar transiting companions were removed from the final sample of objects with transiting companions. Similarly to our previous transit samples (Udalski et al. 2002abc, 2003) the photometric data of our new objects with transiting companions from the 2003 fields can be found in the OGLE Internet archive. Thus the follow-up spectroscopic observations and confirmation of their planetary or non-planetary status can be made in short time scale. Details and pointers to the OGLE Internet archive can be found at the end of this paper. 2. Observational Data Observations presented in this paper were collected with the 1.3-m Warsaw telescope at the Las Campanas Observatory, Chile (operated by the Carnegie Institution of Washington), equipped with a wide field CCD mosaic camera. The camera consists of eight 2048 × 4096 pixel SITe ST002A detectors. The pixel size of each of the detectors is 15 µm giving the 0.26 arcsec/pixel scale at the focus of the Warsaw telescope. Full field of view of the camera is about 35′ × 35′. The gain of each chip is adjusted to be about 1.3 e−/ADU with the readout noise of about 6 to 9 e−, depending on the chip. More details on the instrumental setup can be found in Udalski (2003). Two observing campaigns were conducted in 2003. The first of them – OGLE campaign #3 – started on February 12, 2003 and lasted up to March 26, 2003. The photometric data were collected during 39 nights spanning 43 days. Three fields of the Galactic disk were observed continuously with the time resolution of about 15 minutes. Acronyms and equatorial coordinates of these fields are provided in Table 1. The second campaign – OGLE campaign #4 – started on March 25, 2003. While the main observing material was collected to the middle of May 2003, the fields were also observed from time to time until July, 25, 2003. Moreover, additional but less extensive photometric monitoring of the campaign #4 fields was conducted during the 2004 observing season – from March 14, 2004 up to July 30, 2004. As in the previous campaigns three fields of the Galactic disk were continuously monitored with the time resolution of about 15 minutes. Details on the location of these fields and their acronyms can also be found in Table 1. All observations were made through the I-band filter. The exposure time of each image was set to 180 seconds. Altogether about 820 images were collected for each field during our OGLE campaign #3 and 1050 images in 2003 plus 460 4 Table1 Equatorial coordinates of the observed fields Field RA (J2000) DEC (J2000) Campaign #3 CAR106 11h03m00s −61◦50′00′′ CEN106 11h32m30s −60◦50′00′′ CEN107 11h54m00s −62◦00′00′′ Campaign #4 CEN108 13h33m00s −64◦15′00′′ MUS100 13h15m00s −64◦51′00′′ MUS101 13h25m00s −64◦58′00′′ images in 2004 for the campaign #4 fields. The median seeing of images in both campaigns was about 1.′′2. 3. Data Reductions All collected images were preprocessed (de-biasing and flat-fielding) in real time with the standard OGLE-III data pipeline (Udalski 2003). Photometric reductions of the 2003 season images were performed off-line af- ter the end of both campaigns.