arXiv:astro-ph/9904077v1 6 Apr 1999 A&A manuscript no. (will be inserted by hand later) Your thesaurus codes are: 04.03.1; 05.01.1 ASTRONOMY AND ASTROPHYSICS Extension of Tycho catalog for low-extinction windows in the galactic bulge T.P. Dominici, R. Teixeira, J.E. Horvath, G.A. Medina Tanco and P. Benevides-Soares Instituto Astronomico e Geof´ ısico, Universidade de S˜ao Paulo, Av. Miguel St´ efano 4200, S˜ao Paulo, SP, 04301-904, Brazil Received / accepted Abstract. We present in this work secondary catalogs 1 up to m V al ∼ 13 based on the Tycho reference frame (ESA, 1997) for 12 selected low-extinction fields towards the galactic bulge. The observations have been performed with the Askania-Zeiss Meridian Circle equiped with a CCD camera, located at the Abrah˜ao de Moraes Obser- vatory (Valinhos, Brazil) and operated by the Institute of Astronomy and Geophysics, S˜ ao Paulo University. The presented catalog, though not complete, has been designed to help in intensive search programmes (e.g. microlensing and variable searches) and therefore the selected standards have a high astrometric and photometric (V band, ap- proximately) quality. The mean precisions obtained were 0.0018 s in α, 0.013” in δ, 0.030 for the standard deviation in magnitude and 0.0042 for the magnitude when weighted with the error bars in each night (in the mean, 42 stars for the catalog of each window). Key words: Tycho catalogue – galactic bulge – low ex- tinction windows 1. Introduction The large amount of dust and gas in the direction of the galactic bulge is known to be responsible for the large extinction which makes very difficult the observation of the bulge in several bands. However, it has long been known that the irregular distribution of dust and gas presents some ”holes” where the extinction is consider- ably lower. The best known of these so-called windows has been found by Baade (1951) and extensively stud- ied since then. Generally speaking, these low-extinction windows offer a unique opportunity to obtain information about the bulge in the optical range. Several groups (MA- CHO, OGLE, for example) have been recently monitoring Send offprint requests to : T.P. Dominici Correspondence to : tania@orion.iagusp.usp.br 1 Tables B.1 to B.12 are also available in eletronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html the bulge in search of microlensing events (see Paczy´ nski (1996) for a review). Since April 1997 we have been conducting a small monitoring project of 12 selected low-extinction windows (see Table 1), taken from Blanco & Terndrup (1989) and Blanco (1988), using the recently refurbished Meridian Circle of the Abrah˜ao de Moraes Observatory(see below). The main objective of the project is the discovery and clas- sification of variable stars. The final goal will be to have an on-line, real time processing data to stimulate the study of potentially interesting events by other observing facilities. Meanwhile, we have faced the problem that the total number of reference stars of the usual catalogues inside the observed low-extinction fields (13’ in declination and between 3 and 6 minutes in right ascension) is small. This posed a serious difficulty for the reduction of data and the search for variable objects. As a first step towards a com- prenhensive study of the windows, we have attempted to construct dense (secondary) reference catalogues intended to be of general use. The publication of the Tycho Cat- alog (ESA, 1997) has added an additional motivation to our task, since the refinement of the latter beyond the completeness limit (V ∼ 11) was already foreseen as an useful tool for future observations. We shall describe in the next Section the selected fields. Section III is devoted to a presentation of the instrumental facilities and data reduction. The criteria of the selection of the set of standards is discussed in Section IV. A general discussion in Section V, the light curves in Appendix A and the new catalogues in Appendix B closes the present work. 2. Selection of the fields Table I summarizes the list of low-extinction windows in which we have defined a relatively dense set of standards for further use. Due to specific needs of our project, but also in view of more general applications, we have pro- ceeded to select standard stars up to m V al = 13 (we will use ”m V al ” to designate the magnitudes obtained with the Valinhos system (V V filter) and ”V ” for magnitudes in the standard Jonhson system (V J filter)). We should empha-