A&A 509, A70 (2010) Astronomy DOI: 10.1051/0004-6361/200913299 & c ESO 2010 Astrophysics The distance to the Andromeda galaxy from eclipsing binaries,, F. Vilardell1,2,I.Ribas3, C. Jordi2, E. L. Fitzpatrick4,andE.F.Guinan4 1 Departament de Física, Enginyeria de Sistemes i Teoria del Senyal, Universitat d’Alacant, Apartat 99, 03080 Alacant, Spain e-mail: [email protected] 2 Departament d’Astronomia i Meteorologia (ICC-IEEC), Universitat de Barcelona, c/ Martí i Franquès, 1, 08028 Barcelona, Spain e-mail: [email protected] 3 Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, Facultat de Ciències, Torre C5, parell, 2a pl., 08193 Bellaterra, Spain e-mail: [email protected] 4 Department of Astronomy and Astrophysics, Villanova University, 800 Lancaster Avenue, Villanova, PA 19085, USA e-mail: [edward.guinan;edward.fitzpatrick]@villanova.edu Received 15 September 2009 / Accepted 6 November 2009 ABSTRACT The cosmic distance scale largely depends on distance determinations to galaxies of the Local Group. In this sense, the Andromeda galaxy (M 31) is a key rung to better constrain the cosmic distance ladder. A project was started in 1999 to firmly establish a direct and accurate distance to M 31 using eclipsing binaries (EBs). After the determination of the first direct distance to M 31 from EBs, the second direct distance to an EB system is presented: M31V J00443610+4129194. Light and radial velocity curves were obtained and fitted to derive the masses and radii of the components. The acquired spectra were combined and disentangled to determine the temperature of the components. The analysis of the studied EB resulted in a distance determination to M 31 of (m − M)0 = 24.30 ± 0.11 mag. This result, when combined with the previous distance determination to M 31, results in a distance modulus of (m − M)0 = 24.36 ± 0.08 mag (744 ± 33 kpc), fully compatible with other distance determinations to M 31. With an error of only 4%, the obtained value firmly establishes the distance to this important galaxy and represents the fulfillment of the main goal of our project. Key words. binaries: eclipsing – stars: fundamental parameters – stars: distances – distance scale – galaxies: individual: M 31 1. Introduction The direct determination of the radii of the components of EB systems made several authors (e.g., Lacy 1977; Giménez Eclipsing binaries (EBs) have always been an important tool et al. 1994) suggest the possibility of using EBs for deriving to test and determine the physical properties of stars (Popper distances. The only additional requirement to determine the ab- 1967; Guinan 1993; Torres et al. 2009). They are composed of solute luminosity of an EB system and hence the distance is the two stars which, when orbiting around each other, produce pe- surface brightness or, equivalently, the effective temperature of riodic eclipses. The great potential of EBs is that their orbital the components. motion, inferred from the radial velocity curves, and the shape The potential of using EBs to derive distances encouraged of the eclipses, obtained from the light curves, can be entirely ex- several projects to obtain direct distance determinations either plained by the gravitation laws and the geometry of the system within the Milky Way (e.g., Munari et al. 2004), the Magellanic (see Hilditch 2001, for details). Clouds (e.g., Fitzpatrick et al. 2003, and references therein), or M31/M 33 (DIRECT project, Kaluzny et al. 1998). In 1999, a new project was started to obtain a direct distance determination Based on observations made with the Isaac Newton Telescope op- to the Andromeda galaxy (M 31) from EBs (Ribas & Jordi 2003; erated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Ribas et al. 2004), providing the first direct distance determina- Astrofísica de Canarias. tion in Ribas et al. (2005, hereafter Paper I). Based on observations obtained at the Gemini Observatory, which The main interest for an accurate distance determination to is operated by the Association of Universities for Research in As- M 31 lies in the potential of this galaxy to be a first-class dis- tronomy, Inc., under a cooperative agreement with the NSF on be- tance calibrator (e.g., Clementini et al. 2001). The reasons for half of the Gemini partnership: the National Science Foundation (U- this are: (1) Contrary to the Magellanic Clouds, the distance to nited States), the Science and Technology Facilities Council (United M 31 is large enough so that its geometry does not introduce any Kingdom), the National Research Council (Canada), CONICYT (Chil- systematics in the final distance determination; (2) typically with e), the Australian Research Council (Australia), Ministério da Ciência e − = ± Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación a moderate reddening value (E(B V) 0.16 0.01, Massey Productiva (Argentina) et al. 1995), it is close enough to enable the individual identifica- Original data are only available in electronic form at the CDS via tion of stars suitable for distance determination (such as EBs or anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via Cepheids); (3) an Sb I–II giant spiral galaxy (like M 31) provides http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/509/A70 an appropriate local counterpart for the galaxies commonly used Article published by EDP Sciences Page 1 of 7 A&A 509, A70 (2010) Table 1. Log of spectroscopic observations and heliocentric radial velocity determinations (when available). Date Time Exp. time S/N HJD Phase Primary Secondary [UT] [s] [km s−1][kms−1] 2004 Nov. 7 10:14 4100 22 2 453 316.931 0.272 –370.1 ± 8.2 143.5 ± 30.3 2004 Sep. 15 13:29 4100 11 2 453 264.066 0.467 . 2005 Feb. 12 5:39 3240 8 2 453 413.734 0.524 . 2004 Nov. 12 7:35 4100 15 2 453 321.820 0.659 14.3 ± 13.9 –414.1 ± 25.5 2004 Nov. 12 8:49 4100 16 2 453 321.872 0.684 57.3 ± 13.5 –448.2 ± 24.8 2004 Nov. 10 9:00 4100 24 2 453 319.879 0.711 30.6 ± 9.9 –455.5 ± 17.4 2004 Sep. 14 8:53 4100 20 2 453 262.874 0.885 . 2004 Sep. 12 9:45 4100 13 2 453 260.910 0.927 –69.7 ± 18.1 –309.2 ± 19.4 2004 Oct. 17 6:31 4100 14 2 453 295.776 0.946 –58.1 ± 24.2 –271.7 ± 22.9 for distance determination (e.g., Freedman et al. 2001); and (4) M31V J00443610+4129194. The first target was already ana- M 31 can also provide an absolute calibration of the Tully-Fisher lyzed in Paper I and the analysis of the second target is presented relationship, enabling the calibration of the furthest distance de- here. The three remaining EBs are unsuitable for distance deter- termination methods. Therefore, the characteristics of this spiral mination. One of them is a single-line EB and the absolute prop- galaxy make it an important step of the cosmic distance scale. erties of the stars cannot be obtained. Another EB is too faint As mentioned above, our project already provided the first for an accurate analysis of its spectra. The third one is a triple- direct distance determination to M 31 (Paper I). However, the line system, where the important contribution of the third light excellent dataset obtained (Sect. 2) allowed the determination of prevents a precise distance determination. additional distances in order to further constrain the distance to M 31. Therefore, in the present work, we are presenting the sec- ond direct distance determination to M 31 from an EB system 3. Radial velocities (Sects. 3–6). This result (which is the last one until further spec- The determination of radial velocities (RVs) was performed, as troscopic observations can be secured) enables us to engage in a in the case of Paper I, with TODCOR (Zucker & Mazeh 1994) critical discussion on the distance to M 31 (Sect. 7). and the ATLAS91 and TLUSTY2 (Lanz & Hubeny 2003, 2007) synthetic models. In this case, the best pair of synthetic spec- tra was determined with the following iterative approach. As a 2. Observations first step, the preliminary Wilson & Devinney (1971, hereafter W&D) fit (Sect. 4) was used to define an initial list with pairs of B V Photometric time series (in and passbands) were acquired models having a temperature ratio, gravity ratio and rotational with the Wide Field Camera at the 2.5 m Isaac Newton Telescope × velocities compatible with the W&D parameters. All the model at La Palma (Spain). The field of observation covers 34 34 pairs in the initial list were then used to determine RVs, and a at the north-eastern part of M 31. Around 260 images were ob- simple RV curve model was fitted to the obtained values. The tained in each passband during five campaigns between 1999 and ff free parameters in the fit were the semi-major axis (a), the sys- 2003. The Di erence Image Analysis technique (DIA, Wozniak temic velocity (γ) and the mass ratio (q), while the period and 2000) was used to perform the photometric data reduction (thor- reference time was fixed from the W&D solution. From all the oughly explained in Vilardell et al. 2006, hereafter Paper II), pro- derived solutions, the models having a lower dispersion around viding light curves for more than 3964 variable stars with 437 the fitted RV curve were selected, and all the neighboring mod- V being identified as EBs.