The RR Lyrae Distance to the Draco Dwarf Spheroidal Galaxy

a r X i v :a s t r o -p h /0310477v 2 30 O c t 2003The RR Lyrae Distance to the Draco Dwarf Spheroidal Galaxy

A.Z.Bonanos,K.Z.Stanek,A.H.Szentgyorgyi,D.D.Sasselov,G.′A.

Bakos Harvard-Smithsonian Center for Astrophysics,60Garden St.,Cambridge,MA 02138

e-mail:abonanos@https://www.sodocs.net/doc/c55915605.html,,kstanek@https://www.sodocs.net/doc/c55915605.html,,aszentgyorgyi@https://www.sodocs.net/doc/c55915605.html,,sasselov@https://www.sodocs.net/doc/c55915605.html,,gbakos@https://www.sodocs.net/doc/c55915605.html, ABSTRACT We present the ?rst CCD variability study of the Draco dwarf spheroidal galaxy.The data were obtained with the FLWO 1.2m telescope on 22nights,over a period of 10months,covering a 22′×22′?eld centered at α=17:19:57.5,δ=57:50:05,J2000.0.The analysis of the BV I images produced 163variable stars,146of which were RR Lyrae:123RRab,16RRc,6RRd and one RR12.The other variables include a SX Phe star,four anomalous Cepheids and a ?eld eclipsing https://www.sodocs.net/doc/c55915605.html,ing the short distance scale statistical parallax calibration of Gould &Popowski and 94RRab stars from our ?eld,we obtain a distance modulus of (m ?M)0=19.40±0.02(stat)±0.15(syst)mag for Draco,corresponding to a distance of 75.8±0.7(stat)±5.4(syst)kpc.By comparing the spread in magnitudes of RRab stars in B,V and I ,we ?nd no evidence for internal dust in the Draco dwarf spheroidal galaxy.The catalog of all variables,as well as their photometry and ?nding charts,is available electronically via anonymous ftp and the World Wide Web .The complete set of the CCD frames is available upon request.

Subject headings:Local Group —distance scale —galaxies:dwarf —galaxies:individual (Draco dSph,UGC 10822)

1.Introduction

Dwarf spheroidal (dSph)galaxies are probably the most common types of galaxies in the present-day Universe.They are metal poor galaxies with a metallicity Z <0.001(Mateo 1998),which resembles that found in galactic globular clusters.Most dSph galaxies show evidence for multiple star-formation episodes,having populations of di?erent ages.There

are very few,namely Tucana,Draco and possibly Ursa Minor,that host a single stellar population older than10Gyr(see Dall’Ora et al.2003,and references therein).

The Draco dSph galaxy,a companion to the Milky Way,was discovered by Wilson(1955) and was?rst observed by Baade&Swope(1961)for variables.They found261variables in their24′×24′?eld,but only measured137for magnitudes.Of these,133were RR Lyrae variables,which they used to derive the distance to the galaxy.There have not been any recent variability studies of Draco,except for the survey by Kinemuchi et al.(2002)which is currently underway.The lack of high quality CCD observations of the RR Lyrae in Draco dSph motivated us to do this project.

However,several studies of Draco’s stellar population have been conducted and for these CCD photometry has been obtained.Grillmair et al.(1998)present the CMD diagram obtained from observations with the Hubble Space Telescope(HST)and con?rm that star formation in Draco was primarily single-epoch and that Draco is very similar to the globular clusters M68and M92,but1.6Gyr older.It has a luminosity of2×105L⊙,which places it among the least luminous galaxies known.Bellazzini et al.(2002)have done a comparative study of the Draco and Ursa Minor dSph galaxies with new V,I photometry.Recently,Rave et al.(2003)have released a catalog of photometry of～5,600stars in Draco.They?nd 142candidate variables from their colors,using photometry from?ve catalogs.However, a uniform dataset taken with the same instrument would be more reliable for?nding RR Lyrae and obtaining accurate photometry and periods for them.

In this paper,we present a catalog of variable stars found in Draco dSph.The paper is organized as follows:§2provides a description of the observations;the data reduction procedure,calibration and astrometry is outlined in§3;the catalog of variable stars is presented in§4.In§5we determine the distance to Draco and in§6we summarize our results.

2.Observations

The observations of the Draco dSph were made with the1.2m telescope at the Fred Lawrence Whipple Observatory on Mount Hopkins,Arizona,between August19th,1998 and June20th,1999,over22nights.We used the“4Shooter”camera(Szentgyorgyi et al. 2003),with4thinned and AR-coated Loral20482pixel CCDs.The pixels are15microns in size and map to0.33′′per pixel on the focal plane,making each image11′on the side.The camera was centered atα=17:19:57.5,δ=57:50:05,J2000.0.The data consists of148×600 s exposures in the V?lter,44×900s exposures in the B?lter and47×600s exposures in

the I?lter.The median value of the seeing in V was2.0′′.The?eld was observed through airmasses ranging from1.12to1.55,with the median being1.19.The completeness of our photometry starts to drop rapidly at about22.5in I and23mag in V and B.The CCDs saturate for stars brighter than14in I,15in V and15.5mag in B.On one photometric night of the run,several images of standard Landolt(1992)?elds were taken.

3.Data Reduction,Calibration and Astrometry

Preliminary processing of the data was performed with standard routines in the IRAF1 CCDPROC package.The di?erential photometry for the variable stars was extracted using the ISIS image subtraction package(Alard&Lupton1998;Alard2000)from the V-band data.The DAOPHOT/ALLSTAR package(Stetson1987)was used for the conversion into magnitudes.Mochejska et al.(2001)describe the procedure in detail.

On August31st,1998,we observed2sets of3Landolt(1992)?elds in the BV I?lters at air masses ranging from1.18to1.99.The transformation from the instrumental to the standard system was derived for each chip in the following form:

b=B+χb+ξb·(B?V)+κb·X

v=V+χv1+ξv1·(B?V)+κv1·X

v=V+χv2+ξv2·(V?I)+κv2·X

i=I+χi+ξi·(V?I)+κi·X

where lowercase letters correspond to the instrumental magnitudes,uppercase letters to standard magnitudes,X is the airmass,χis the zeropoint,ξthe color andκthe airmass coe?cient.Since most of the color coe?cients are small,we used B?V=V?I=1when transforming the magnitudes of our stars.Note that the B-band coe?cients are larger, therefore our B magnitudes for red stars may be o?by0.1mag or0.2mag(in chip2),in the worst case.

Stetson(2000)has calibrated～400stars in the Draco dSph as secondary standards. In chips3and4,where our overlap was large,we normalized to his photometry using the

brightest80stars(to19.5mag)and66stars(to20th mag)respectively to determine the o?sets in V.The di?erence between his photometry and ours in these chips was0.04and 0.02mag.In chips1and2the overlap was too small for a meaningful comparison,thus we kept our own photometry.We then compared our normalized V photometry in chips3and4 with the photometry of Grillmair et al.(1998)from the HST.For100stars down to19mag, the di?erences between their photometry and ours were0.06and0.03mag.Bellazzini et al. (2002)have obtained(V,I)photometry of the?eld and the agreement with our photometry is good,the largest o?set being0.07mag in chip3,V-band.

Equatorial coordinates were determined for the V star list.The transformation from rectangular to equatorial coordinates was derived using for chips1-4:174,146,400and 282transformation stars respectively with V<20from the USNO-A2.0(Monet1996) catalog.The median di?erence between the catalog and the computed coordinates for the transformation stars was0.′′3in RA and0.′′3in Dec.We also compared the astrometry to Stetson’s catalog and found18,4,272and151matches for chips1-4,having a median o?set <0.′′2.We use these derived J2000.0equatorial coordinates to name the variables in the format:Draco hhmmss.s+ddmmss.s.The?rst three?elds(hhmmss.s)correspond to RA expressed in hours,the last three(ddmmss.s)to Dec,expressed in degrees,separated by the declination sign.

4.Results

Our search for variables in our?eld in Draco produced163stars,136of which were previously identi?ed by Baade&Swope(1961).The remaining27are new discoveries.Of these163stars,146are RR Lyrae,4are anomalous Cepheids and the remaining13are other long period or non-periodic variables.We found a new?eld eclipsing binary and a SX Phe star among these.Of the146RR Lyrae,123are fundamental mode pulsators(RRab),16are ?rst overtone pulsators(RRc),6are double-mode pulsators(RRd)and one is pulsating in the ?rst and second overtone(RR12).Figures1,2and3show typical light curves of RR Lyrae and other variables found in Draco.Tables1and2present coordinates,periods,intensity averaged BV I magnitudes,V-band amplitudes,the type of variable and the corresponding name given in Baade&Swope(1961).Stars exhibiting the Blazhko e?ect are also marked. The catalog of all variables,as well as their BV I photometry and V?nding charts,is available electronically via anonymous ftp2and the World Wide Web3.The complete set of

the CCD frames is available upon request.

We used the multiharmonic analysis of variance technique(Schwarzenberg-Czerny1996) to search the light curves for periodicity.Additionally,Fourier series were?t to the RR Lyrae light curves phased to the period determined earlier and parameters such as the amplitude of the variation and amplitude and phase of each harmonic were calculated.We searched for multiperiod variables by subtracting the?rst three harmonics of the Fourier series from the phased light curves and then repeating the period search.We then redetermined secondary periods for the6double mode(RRd)stars found by Nemec(1985)in the data of Baade& Swope(1961),by searching the periodogram where the second period was expected.

Baade&Swope(1961)do not?nd any red irregular or long period variables in their data,with the exception of BS-203,a bright blue variable with a period of～3years.We observed all of their“special variables”except for BS-138.The only signi?cant di?erence in these is that the period we?nd for BS-134is0.592days,not1.458days,which agrees with Nemec(1985)who realanyzed the data of Baade&Swope(1961).The periods we calculated for variables also found by Baade&Swope(1961)are very similar in most cases. The cases that di?er are marked with an asterisk in Table1.As a result some stars are classi?ed di?erently from Baade&Swope(1961).BS-97,BS-121,BS-173and BS-145are all RRc stars and BS-190,BS-169,BS-143,BS-72,BS-11,BS-112are RRd stars.We did not ?nd variables BS-10,BS-31,BS-111and BS-195due to the proximity of highly saturated stars.

We present a histogram of the139RRab and RRc Lyrae in Draco,with0.02day bins in Figure4.Both components of the double-mode stars are also plotted(in black).The median period for RRab stars is0.617days and for RRc stars is0.392days,which places the Draco dSph between Oosterho?type I(～0.55days)and type II(～0.65days)clusters,similarly to other dSph(Dall’Ora et al.2003).In Figure5we present a color magnitude diagram(CMD) of stars in Draco.Circles represent RR Lyrae,squares are anomalous Cepheids and triangles are other variables.Among these other variables is the long period blue variable BS-203, a foreground0.24day eclipsing binary,and a multimode SX Phe star which is pulsating in three modes,with periods0.068,0.073and0.079days.The period-amplitude diagram for the146RR Lyrae in Draco is shown in Figure6.Circles represent RRab stars,triangles RRc stars and squares RRd stars,for which both periods are plotted.

5.Distance to Draco dSph

The distance to the Draco dSph galaxy has been estimated by several authors.Baade &Swope(1961)obtained a distance of d=99kpc assuming an absolute magnitude of M B=0.5mag for RR Lyrae;Nemec(1985)obtained d=84±12kpc using RRd stars found by reanalyzing the data of Baade&Swope(1961);Aparicio et al.(2001)obtained d=80±7 kpc by the magnitude of the horizontal branch at the RR Lyrae instability strip;Bellazzini et al.(2002)obtained d=92.9±6kpc by?tting template cluster horizontal branches.

We use the short distance scale statistical parallax calibration of Gould&Popowski (1998),which is a robust method of measuring the absolute magnitude of RR Lyrae stars. They?nd

M V=0.77±0.13,(1) at [Fe/H] =?1.60,for a sample of147halo RR Lyrae stars with high-quality proper motions from the Hipparcos(European Space Agency1997)and Lick NPM1(Klemola, Hanson&Jones1993)surveys.Shetrone et al.(2001)?nd [Fe/H] =?2.00±0.21in Draco dSph from high resolution spectroscopy of6red giants in the galaxy.Lehnert et al. (1992)also found a metallicity of [Fe/H] =?1.9,σ=0.4,from spectra of14giants.The abundances seem to fall into two groups,one with an average[Ca,Mg/H]near?1.6±0.2 and the other?2.3±0.2.We adopt the value [Fe/H] =?2.00(also quoted in Mateo1998) for our distance determination.The metallicity correction is derived from the slope of the luminosity-metallicity relation for RR Lyrae,which lies between0.2(Chaboyer1999)and 0.3(Sandage1993).Using0.2for the value of the slope and0.4dex for the di?erence in metallicity of Draco dSph from galactic RR Lyrae,we?nd M V=0.69for the RR Lyrae in Draco dSph.

Draco dSph is located at Galactic coordinates l=86?.37,b=34?.72.To remove the e?ects of the Galactic interstellar extinction we used the reddening map of Schlegel et al. (1998)which yields E(B?V)=0.027mag.This corresponds to expected values of Galactic extinction of A I=0.053,A V=0.091,A B=0.118mag,using the extinction corrections of Cardelli et al.(1989)as prescribed in Schlegel et al.(1998).

For the distance determination we only used the RRab stars found in chips3and4, which are normalized to the photometry of Stetson(2000).There are94such stars in our data.The remaining RRab stars from chip1and2are not included in this list.We?t a Gaussian to a histogram of these94stars,using0.03mag bins and found m V =20.18±0.02,σ=0.08,as shown in Figure7.This value is in agreement with the value of Aparicio et

al.(2001)for the horizontal branch at the RR Lyrae instability strip,20.2±0.1mag,and with the value of Bellazzini et al.(2002)of20.30±0.12mag,obtained by?tting to the template cluster M68.Our measurement implies a distance modulus of m V?M V=19.49 mag.Correcting for extinction gives a true distance modulus of(m?M)0=19.40mag and a distance of75.8kpc to Draco dSph.

The systematic errors are0.06mag in A V,0.03mag in photometry,0.13mag in the calibration method and0.04mag in metallicity.The error in the reddening from Schlegel et al.(1998)is0.02mag,which corresponds to0.06mag in A V,and the error in metallicity is calculated from a conservative error of0.1in the slope of the luminosity-metallicity relation times0.4dex,the metallicity di?erence.Adding the systematic errors in quadrature gives a conservative total estimate of0.15mag,which is dominated by the calibration error.We consider the e?ects of internal extinction to be negligible from a comparison of the spread in magnitudes of RRab stars in di?erent?lters.Similarly to Figure7for V withσ=0.08,the spread in magnitudes of RRab stars in B and I areσ=0.10and0.12,respectively.Thus we?nd no evidence for internal dust in the Draco dSph galaxy.

The statistical error is0.02mag,which leads to a true distance modulus of(m?M)0= 19.40±0.02(stat)±0.15(syst)mag,corresponding to a distance of75.8±0.7(stat)±5.4(syst) kpc.

6.Conclusions

We have presented the results of the?rst CCD variability study in the Draco dSph galaxy since Baade&Swope(1961).Our search produced163variable stars,146of which are RR Lyrae,4are anomalous Cepheids,1is a?eld eclipsing binary,1a SX Phe star and 11are other types of variables.We have used the short distance scale statistical parallax calibration of Gould&Popowski(1998)for94RRab in our?eld and obtained a distance modulus of(m?M)0=19.40±0.02(stat)±0.15(syst)mag.By comparing the spread in magnitudes of RRab stars in di?erent?lters,we?nd no evidence for internal dust in the Draco dSph galaxy.

The catalog of all variables,as well as their photometry and?nding charts,is available electronically via anonymous ftp and the World Wide Web.The complete set of the CCD frames is available upon request.

We thank Grzegorz Pojma′n ski and Wojtek Pych for their LC and Fourier series pro-grams and Barbara Mochejska for her help.We also thank Janusz Kaluzny,Piotr Popowski,

John Huchra and the referee for their careful reading of and comments on the manuscript. We?nally thank Mike Pahre,Emilio Falco,Lucas Macri and Saurabh Jha,for helping ob-tain the observations.Guest User,Canadian Astronomy Data Centre,which is operated by the Dominion Astrophysical Observatory for the National Research Council of Canada’s Herzberg Institute of Astrophysics.

Fig. 1.—Sample light curves of RR Lyrae variables found in Draco dSph,representing typical quality data over a range of periods.

Fig.2.—Light curves of selected other periodic variables found in Draco dSph.An eclipsing binary and3anomalous Cepheids are shown.

Fig.3.—Sample light curves of other variables found in Draco dSph.

Fig.4.—Period distribution of146RR Lyrae in Draco.The median period for RRab stars is0.617and for RRc stars0.392.Both components of the double-mode stars are also plotted

in black.

Fig.5.—CMD for variables and nonvariable stars.Circles represent RR Lyrae,squares are

anomalous Cepheids and triangles are other variables.

Fig. 6.—Period-amplitude relation for146RR Lyrae in Draco dSph.Circles represent

RRab stars,triangles RRc stars and squares RRd stars,for which both periods are plotted.

Fig.7.—Histogram of94RRab magnitudes and the Gaussian?t,centered at m V = 20.18±0.02mag.

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Table1.RR Lyrae in Draco Dwarf Galaxy

P V I B Amp V

Name(days)(mag)(mag)(mag)(mag)Type Comments a

Table1—Continued

P V I B Amp V

Name(days)(mag)(mag)(mag)(mag)Type Comments a

Table1—Continued

P V I B Amp V

Name(days)(mag)(mag)(mag)(mag)Type Comments a

Table1—Continued

P V I B Amp V

Name(days)(mag)(mag)(mag)(mag)Type Comments a