A CARBON STAR SURVEY OF THE LOCAL GROUP DWARF GALAXIES. I. IC 1613 LOI C ALBERT1 AND SERGE DEMERS

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1 THE ASTRONOMICAL JOURNAL, 119:2780È2788, 2000 June ( The American Astronomical Society. All rights reserved. Printed in U.S.A. A CARBON STAR SURVEY OF THE LOCAL GROUP DWARF GALAXIES. I. IC 1613 LOI C ALBERT1 AND SERGE DEMERS De partement de Physique, Universite de Montre al, Montreal H3C 3J7, QC, Canada AND W. E. KUNKEL Observatories of the Carnegie Institution of Washington, Casilla 601, La Serena, Chile Received 1999 September 1; accepted 2000 February 17 ABSTRACT We present results of a multiðlter survey of the whole Magellanic-type galaxy IC Narrowband CN and TiO Ðlters are used to identify carbon stars among red giants. We have identiðed 195 carbon stars, extending up to 15@ from the center of the galaxy. We present well-calibrated R and I magnitudes for all stars. The large Ðeld surveyed allows a reliable foreground estimate of M stars, leading to a C/M ratio of 0.64, when giants as early as M0 are counted. Analysis of the photometric properties of the C star population reveals a narrow M distribution with a mean M of [4.69, with a dispersion of ^0.28. IC 1613 has, for its absolute magnitude, I a normal number of C stars. I Key words: galaxies: individual (IC 1613) È galaxies: stellar content È stars: carbon 1. INTRODUCTION The pioneering survey of a few Local Group (LG) dwarf galaxies by Cook, Aaronson, & Norris (1986), using a narrowband technique, has yielded very little: two 1@.5 ] 2@.5 Ðelds in NGC 6822 and IC 1613 and only one Ðeld in WLM. Such early study, using a small CCD, thus covering tiny areas, must be considered quite exploratory. Therefore, the statistics on asymptotic giant branch (AGB) populations in dwarf members of the LG recently compiled by Mateo (1998) truly represent lower limits and are quite difficult to interpret, as Groenewegen (1999) has demonstrated. It is obvious that large format CCDs, CCD mosaics, or a focal reducer are required to fully explore large nearby dwarf galaxies such as NGC 3109, NGC 6822, IC 1613, or WLM. Cool carbon stars, bright members of the intermediateage population, are located at the tip of the AGB. They are bright, very red, and quite easy to pinpoint from the colormagnitude diagram (CMD) of whole nearby galaxies. They are among the easiest stars to reach ÏÏ in external galaxies. The tricky part, which explains why so few lists of extragalactic C stars have been published, is to isolate C stars from M giants that have the same apparent magnitudes, almost the same color, and are quite abundant in the halo of galaxies. This is particularly obvious in the recent V, I survey of the Magellanic dwarf NGC 3109 by Minniti, Zijlstra, & Alonso (1999): there are many stars on the extended giant branch, thus presumably many carbon stars, but which ones? and how many? Nobody can tell without further photometric or spectroscopic data. Carbon stars can provide a robust standard candle. It is known (Aaronson et al. 1989) that the absolute magnitude (M or M ) distribution of C stars is narrow and independent V of the I metallicity of the parent galaxy (Brewer, Richer, & Crabtree 1995). FrappierÏs (1998) study of Fornax found SM T \[2.4 ^ 0.4, identical to the median magnitude of M31 V and Large Magellanic Cloud (LMC) C stars, thus ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Guest Investigator, Las Campanas Observatory giving one more evidence for the universality of the C star mean luminosity. In order to conðrm this trend, data on C stars in galaxies of various masses need to be collected. Our paper represents a Ðrst step to remedy to this situation. We have initiated, last year, a large-scale survey of C stars in LG dwarf irregular galaxies. To do so, we have adopted BrewerÏs et al. (1995) technique based on two narrowband Ðlters that allow the separation of C and M stars in a color-color plot. Our photometric system is described in 2. We initiate our survey with IC 1613, a galaxy of relatively large apparent size with negligible reddening and of low surface brightness. Its stellar population can easily be resolved. IC 1613 was discovered by Wolf (1906) and has long been accepted as a LG member. Baade (1935) was the Ðrst to determine its distance using Cepheids. The following year, it was included among the LG members by Hubble (1936). IC 1613 is a faint Magellanic-type dwarf irregular galaxy, with an absolute magnitude of M \[14.7. Thus, V IC 1613 is much fainter than the Large and Small Magellanic Clouds. It is comparable in brightness and presumably in mass with two other LG dwarfs: WLM and Sextans A (Mateo 1998). IC 1613 seems to be a neglected member of the LG because very few investigations of this galaxy have been published in the last decades, probably because of its relatively large distance to us (725 kpc) and also because of its lack of appeal due to its low surface brightness. The fact that IC 1613 does not contain any globular clusters makes it even less interesting to many. Lake & Skillman (1989) have mapped the H I of IC 1613 to a radius of 12@.5. Hodge et al. (1991) have investigated its overall optical structure. The observations are valuable to compare with the extent of the C stars spatial distribution, as we shall describe in 5.1. Recent distance determinations of IC 1613 were done from B, V, R, I photometry of its Cepheids by Freedman (1988); from the apparent magnitudes of the tip of the red giant branch (RGB) of Population II by Lee, Freedman, & Madore (1993); and by using RR Lyrae stars by Saha et al. (1992). These determinations are within 0.2 mag from each other. We adopt for our dis-

2 SURVEY OF THE LOCAL GROUP DWARF GALAXIES. I TABLE 1 OBSERVING LOG FOR IC 1613 Western Field (1998 Dec 9) Eastern Field (1998 Dec 10) Filter (s) (s) R... 4] ] 240, 3 ] 300 I... 4] ] 180, 1 ] 150, 3 ] 120 TiO... 4] ] 600, 1 ] 420 CN... 4] ] 600, 1 ] 420 cussion the Freedman (1988) result, namely, (m [ M) \ 24.3 with a color excess of E(B[V ) \ 0.03, leading to A 0 \ Her magnitudes are used to calibrate our photometry. I 2. OBSERVATIONS Observations of IC 1613 were conducted on two consecutive nights, on 1998 December 9 and 10, at the 2.5 m Du Pont Telescope on Las Campanas, Chile. The wide-ðeld (WF) CCD with an unvignetted Ðeld of 24@ in diameter was used. With a Tektronix 2048 ] 2048 CCD, the pixel scale is 0A.78 pixel~1. To photometrically identify C stars, we employed the same technique as described by Brewer et al. (1995) for their M31 survey. However, an R Ðlter was deemed preferable to a V Ðlter because AGB stars are 1 mag brighter in R than in V. The e ect of the moonlight is the same in both Ðlters. Our CN and TiO interference Ðlters are centered at 8150 and 7700 A, respectively, with a bandwidth of 300 A. Standard Cousins R and I Ðlters were used along with these two narrow Ðlters. In order to discover C stars as far as possible in the periphery of IC 1613, we imaged the entire galaxy in two Ðelds, east and west, thus producing a 40@ ] 24@ mosaic. The J coordinates of the center of the eastern and western Ðelds are, respectively, a \ 1h5m35s.5, d \]2 8@42A.4 ; a \ 1h4m15s.3, d \]2 6@44A.5. Table 1 lists the observing log. The seeing varied from 1A.0 to 1A.5 on 1998 December 9 but was very steady at 0A.9 on 1998 December 10. Images were acquired at air masses ranging from 1.2 to DATA ANALYSIS 3.1. IRAF Reduction Bias subtraction and trimming to a 2048 ] 2048 format were applied on our images using IRAF. All images were Ñat-Ðelded using dome Ñats. We Ðnally multiplied all 35 images by a circular step function to e ectively remove the vignetted contours produced by the WF CCD, the CCD being square while the WF CCD Ðeld is circular. The resulting Ñat-Ðelded images revealed some undesirable artifacts. First, because of the moonlight, sky lines polluted the images taken through the two narrow interference Ðlters, producing a radially varying sky background. Through the CN Ðlter, the sky brightness gradually increases by 25% from the center to the periphery of the frames, while through the TiO Ðlter it decreases by 10%. This e ect resulted from the position of the Ðlters in the converging beam of the telescope. Fortunately, this did not hamper the ability of ALLFRAME to measure accurately star brightnesses, providing that the Ðtting radius parameter was set correctly. Thus, no action was taken to correct this minor problem. A second gremlin, Moire fringing, afflicted all I-band images and to a lesser extent all CN and TiO images. After trying to Ðx the problem in the I band with our best but yet noisy sky Ñat, we decided not to apply any correction in order not to introduce more noise to the images than the ¹1% fringe amplitude. Later, looking at the subtracted images produced by ALLFRAME, we were convinced that fringes did not alter our photometry; they are large-scale background variations that ALLFRAME can deal with. The last artifact and the only one that required some image correction was caused by a poor Ñat Ðelding in the western Ðeld I-Ðlter images. We attribute this to the presence of a 7 mag star in the vignetted corner of the Ðeld. We leveled the background using a second-order Legendre polynomial DAOPHOT -AL L FRAME Reduction The photometric reduction was achieved using the DAOPHOT and ALLFRAME packages (Stetson 1994). Images were not combined; the analysis was made on the 35 di erent images. Stars were found in two passes and point-spread functions (PSFs) determined with DAOPHOT on each image. Then, ALLFRAME was performed in two runs, one for the 19 eastern Ðeld images, the other one for the 16 western Ðeld images. A great deal of care was spent in characterizing the PSF in every single image because the WF CCD yields undersampled PSFs with FWHM \ 1.2 pixels (east) and FWHM \ 1.5È1.8 pixels (west), comparable to what is achieved with the Wide Field Planetary Camera 2 on the Hubble Space T elescope (Guharthakurta et al. 1996). We used a constant PSF over the entire Ðeld of view and saw no need to model a radially varying PSF. We typically selected 50 to 70 stars to build a PSF for each image and iteratively excluded their neighbors to obtain the Ðnal solution. A Mo at analytical function with b \ 2.5 best Ðtted our stars. Before launching ALLFRAME, we built for each Ðeld a deep reference image by combining all the R frames. The DAOPHOT analysis found 32,732 and 30,813 stars, respectively, in the western and eastern Ðelds. Finally, we picked only the best possible stars by keeping those that ALL- FRAME had Ðtted in all of the 16 (west) and 19 (east) frames. This gave two Ðnal lists of 19,686 (west) and 19,481 (east) stars. We combined the two lists into a Ðnal list of 33,762 stars. In the overlapping region, we adopt the eastern Ðeld photometry, which is of better quality than the western Ðeld one Photometric Calibration Our photometry is fully calibrated only in the R and I Ðlters for which we used the IC 1613 observations of Freedman (1988) as our reference. No attempt has been made to put the CN and TiO photometry on an absolute magnitude scale since the color indices of C stars have the same zero point as the indices of M stars. We calibrate every R and I image individually before combining the results. We use FreedmanÏs (1988) Ðeld 1 (100 bright stars) to calibrate our eastern Ðeld and her Ðeld 2 (80

3 2782 ALBERT, DEMERS, & KUNKEL Vol. 119 bright stars) for our western Ðeld. The zero-point shift is calculated from the median di erence between the reference (Freedman 1988) and instrumental magnitudes. The di erences show typically a dispersion of ^0.14 mag in the R band and ^0.08 mag in the I band. The dispersion is not due to a color term. For the CN and TiO Ðlters, the instrumental magnitudes were shifted to a common 10 minute exposure time. The comparison of the calibrated photometry, for a given Ðlter, revealed that small systematic di erences remained (¹0.02 mag) between di erent frames of the same Ðeld; this was corrected. We Ðnally combined the results of the 4 or 5 images to obtain for each star in the four Ðlters the mean magnitude and the standard deviation of the mean. We did not use the errors given by ALLFRAME. To check the constancy of the magnitude zero points for the western and eastern Ðelds, we computed the mean magnitude di erence of stars in the overlapping region between x \ 1600 and x \ 2000 (see Fig. 2). Magnitude di erences in the four bands show a similar dispersion of B0.1 mag. However, unexplained nonzero systematic di erences are seen. They are rather similar in the four Ðlters and do not show a radial dependency. They cannot be explained by di erent air masses nor by variable atmospheric transparency. They are CN east [CN west \[0.14 mag, TiO east [TiO west \[0.10 mag, R east [R west \[0.10 mag, I east [I west \[0.11 mag. We accordingly corrected all magnitudes by half these amounts, increasing magnitudes in the eastern Ðelds, decreasing them in the western Ðelds. An assessment of the quality of the Ðnal photometry is shown in Figure 1. For p \ 0.4, the R photometry reaches almost 1.5 mag fainter than the I photometry, as can be judged by the cuto magnitudes at I \ 22 and R \ This is due to the sky level, which was brighter in the I band near full moon. The photometry in the two interference Ðlters is of similar quality, with a cuto at about instrumental magnitude 18. Also, the eastern-ðeld photometry is on average better than its western counterpart. It is attributed to the two di erent seeing conditions. FIG. 1.ÈVariation of the uncertainties for the four bands in the two Ðelds

4 No. 6, 2000 SURVEY OF THE LOCAL GROUP DWARF GALAXIES. I FIG. 2.ÈMap of the two contiguous Ðelds. North is to the top, and east is to the left T he InÑuence of V ignetting Users of focal reducers should always be wary of the possible e ect of vignetting on the apparent magnitudes at the periphery of the Ðeld. This is particularly important in our case because IC 1613 is o center, on the edge of each Ðeld, as shown in Figure 2. We checked the uniformity of sensitivity in two ways. For the Ðrst test, four short exposures of the LMC were obtained. The telescope was displaced in declination by B4@ between exposures, thus recording hundreds of stars on four positions in the Ðeld. The frames were analyzed in the same fashion as the program frames. Mean instrumental magnitudes were computed for 15 declination strips ranging in pixel coordinates from 300 to We Ðnd for this region a standard deviation of the mean magnitudes of ^0.06, con- Ðrming the uniformity of the Ðeld. The e ect of vignetting in the outer D400 pixels could not, however, be checked with this observation. For the second test, we use the blank ÏÏ Ðeld of the IC 1613 western and eastern Ðelds, that is, the extremities of Figure 2 without IC 1613 stars. In that region, the stellar population is expected to be fairly uniform. Here we Ðnd that the average magnitudes in radial zones of 100 pixel width vary to ^0.08 in the outer 500 pixels. We thus conclude that the e ect of vignetting is less than 0.1 mag and is undetectable with our data FIG. 3.ÈCMD of the central section of Fig. 2, as deðned in the text. Error bars are evaluated from our photometry. Ðlters than theirs. Nevertheless, as we shall see, our index does discriminate between C and M stars. To conðrm this statement, we have obtained, concurrently with our observations, short exposures of an LMC Ðeld surveyed by Blanco, McCarthy, & Blanco (1980). They spectroscopically identiðed M and C stars and published the R[I colors for most of them. These colors were used to roughly calibrate our instrumental magnitudes. Figure 4 presents the colorcolor diagram of the LMC Ðeld stars. Large dots correspond to stars with known spectral types, while small dots are all the stars seen in our CCD Ðeld and with p \ There are about 3000 of them. R~I We use Figure 4 to deðne the limits of the C star and the M 4. RESULTS Figure 2 displays a map of individual stars, 33,762 in total, detected in the two combined Ðelds. As one can see, about half of the area surveyed is outside of IC The small scale of this map fails, however, to reveal the true morphology of IC This map allows us to roughly divide the Ðeld into two regions: the galaxy. (800 \ x \ 2600) and the two extremities populated by foreground galactic stars. The CMD of the IC 1613 region is shown in Figure 3. It convincingly reveals the brightest 2 mag of the RGB above which lies the AGB IdentiÐcation of C Stars Carbon stars are identiðed from their CN[TiO index on a color-color diagram. Brewer et al. (1995) and Brewer, Richer, & Crabtree (1996) determined by photometry and spectroscopy that C stars have V [I [ 1.8, corresponding to R[I [ 0.9. For the CN[TiO axis, we cannot use the Brewer et al. (1995, 1996) criteria because we have wider FIG. 4.ÈColor-color diagram of stars in the LMC Ðeld. Large dots are stars with known spectral types.

5 2784 ALBERT, DEMERS, & KUNKEL star regions. The blue limit of the R[I index is set at 0.9 to agree with Brewer et al. (1995, 1996); it Ðts well the C stars of Blanco et al. (1980). This color corresponds to spectral type M0. We adopt CN[TiO º 0.1 for the C star region and CN[TiO \ 0.0 for the M star region. The large number of RGB stars seen in IC 1613 complicates things. This leads to the truncated corner of the C region, which is described below. One may notice two interesting facts in Figure 4: (1) Blanco et al. (1980) have identiðed mostly late-m stars and very few early-m stars. Indeed, R[I \ 1.7 corresponds to about M6 (The, Steenman, & Alcaino 1984). The adoption of our criterion for M stars will lead to low C/M ratios. (2) A number of stars in the C star region were not identiðed by Blanco et al. (1980). All but two of them are in fact outside of the Blanco et al. surveyed area, which is smaller than our Ðeld. The two missed by Blanco et al. (1980) are faint and near a bright star. Having produce a color-color diagram for stars of known spectral types, we are now in position to look at IC Figure 5 presents a color-color diagram for the whole Ðeld. Only bright stars (18 \ I \ 21; R \ 22.5) with low photometric errors (p \ 0.09) are plotted. The C stars are R~I evident in the top right part of the diagram. Unfortunately, the bluest C stars overlap with the bulk of the RGB stars. We thus deðne arbitrarily, by eye, a line to separate C stars from M stars. This line can be represented by the following equation: y \ [ 0.267x. Finally, to well separate M and C stars, we adopt CN[TiO \ 0.0 as the upper boundary of the M stars region. The 195 C stars identiðed in Figure 5 are listed in Table 2, along with their J equatorial coordinates, I and R magnitudes, and CN[TiO indices. Following Green, Schmidt, & Liebert (1986), one can determine the probability of detection of C stars for one observation from the multiple detections in the overlapping section. In the central overlapping area of Figure 2, 29 C stars were found twice and 10 were found only once. This yields a probability p of 85% of detecting a C star, from one FIG. 5.ÈColor-color diagram of the bright stars in the whole IC 1613 Ðeld with p \ C stars are in the top right outlined area. R~I TABLE 2 CSTARS IN IC 1613 ID R.A. (J2000.0) Decl. (J2000.0) R I CN[TiO

6 TABLE 2ÈContinued ID R.A. (J2000.0) Decl. (J2000.0) R I CN[TiO TABLE 2ÈContinued ID R.A. (J2000.0) Decl. (J2000.0) R I CN[TiO NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.

7 2786 ALBERT, DEMERS, & KUNKEL Vol. 119 observation. We cannot, from our data, pursue this exercise and determine the completeness of our survey because the distribution of the C stars is far from uniform over the Ðeld, most of them being concentrated in the center. 5. DISCUSSION 5.1. Spatial Distribution of C Stars in IC 1613 For our discussion, we adopt the dynamic center of IC 1613 determined by Lake & Skillman (1989), at a \ 1h4m46s.4, d \]2 8@46A (J2000.0), as the center of the galaxy. Figure 6 shows the distribution in the plane of the sky of the 195 C stars discovered in the two CCD Ðelds. On this Ðgure, the above center corresponds to x \ 1644 and y \ 990. One notices that the stars are distributed over an area larger than the WF CCD Ðeld, more than 24@. Indeed, 24 C stars are found at more than 10@ from the center of IC This contrasts with the Hodge et al. (1991) star counts; Hodge et al. adopted, for background,ïï counts at distances greater than 7@.2. C stars are found as far as the H I limit detected by Lake & Skillman (1989). The radial distribution of these C stars agrees, however, fairly well with Hodge et al. (1991) Ðndings for the whole stellar population. Following Hodge et al. (1991) and for the sake of simplicity, we assume that the projected surface distribution of the stars of IC 1613 is circular. We then present in Figure 7 the radial surface density of the C stars. The densities in annuli larger than 6@ have been corrected for the incomplete annuli. The dashed line has the slope estimated by Hodge et al. (1991) and corresponds to a scale length of 760 pc. The Ðt is rather good considering the small number statistics. The surface distribution of M stars presents, however, a quite di erent picture, as can be seen on Figure 8. The 671 M stars, seen on our color-color diagram, include stars in IC 1613 as well as foreground stars in our galaxy. In order to establish reliably the C/M ratio of IC 1613, one must take into account the M stars seen along the line of sight TheC/M Ratio of IC 1613 When dealing with the ratio of the number of C and M stars that have been identiðed from a photometric survey, it is quite important, as stressed by Pritchet et al. (1987), to specify what we mean exactly by M ÏÏ and C ÏÏ stars. In our case, the outlined areas of Figures 4 and 5 deðne the two types. This deðnition is not based on spectroscopic observations. For M stars, stars of spectral type M0 and later are included. This is in agreement with Brewer et al.ïs (1995) deðnition based on V [I FIG. 7.ÈRadial surface density proðle of the C stars. The dashed line has a slope determined by Hodge et al. (1991). Our survey has identiðed 195 C stars and 671 M stars in the Ðeld of IC Among the M stars, there are numerous M dwarfs near the Sun. Because we observed a large area around IC 1613, we are able to make a good evaluation of the foreground contamination. For this purpose, we adopt, as foreground sky,ïï the area of Figure 8 farther than 15@ from the center of IC This sky ÏÏ corresponds to two crescents at each extremity of the Ðeld. They amount to 37% of the total area surveyed and contain 137 M stars. Thus, there are 305 M stars in IC The M stars of IC 1613 have a limited range of colors and magnitudes, as can be seen in Figure 9. This Ðgure shows the magnitude and color distributions of the M stars. The Ðlled areas correspond to the foreground distributions multiplied by the area ratio. Thus, the Ðlled areas must be subtracted from the histogram to give the IC 1613 contribution. The top panel of Figure 9 shows that there are a few supergiant M stars in IC 1613 at I B From the color distribution, we note that there is no M star redder than R[I \ 1.6 in IC 1613, corresponding to spectral type M5 (The et al. 1984). From our deðnition of M stars, we Ðnd a global C/M ratio of 0.64 for IC As expected from the FIG. 6.ÈDistribution of the 195 C stars over the Ðeld. They outline the extent of IC FIG. 8.ÈDistribution of the M stars. One can see that the foreground contamination is important.

8 No. 6, 2000 SURVEY OF THE LOCAL GROUP DWARF GALAXIES. I FIG. 9.ÈMagnitude and color distribution of M stars. The Ðlled areas represent the foreground contribution to the total number in each bin. morphology of an irregular galaxy, where the most recent star formation has taken place close to its center, we do observe a slight C/M gradient from the center outward, as displayed in Figure 10. The above C/M ratio contrasts with the value of C/ M3]\2.8 determined for IC 1613 by Cook et al. (1986) from 16 C stars. Can we reconcile these numbers? First of all, if we adopt a blue limit for M stars at R[I B 1.25 (for M3] stars), inspection of Figure 9 shows that we would have only 35 M stars, thus giving C/M [ 5. However, their survey is complete to M \[4.4, at least 1 mag I brighter than ours. This is particularly important for M stars because they are generally fainter than C stars, as can be seen from their luminosity function (plotted in Fig. 12). Omitting stars fainter than I \ 20.0 would double our C/M ratio. The variation of the C/M, as a function of the limiting magnitude of the survey, has been discussed in detail by Brewer et al. (1995). Inspection of our color-color diagram is, in this respect, misleading. Indeed, inclusion of bluer M and C stars does not necessarily mean a huge increase of M stars because the bluer ones are fainter and can very well be fainter than Cook et al. (1986) limit. This underlines the fact that when comparing the C/M ratios of di erent galaxies, the limiting magnitude of the survey is an important factor Photometric Properties of C Stars in IC 1613 The metallicity of IC 1613 was estimated by Freedman (1988), from the mean color of the tip of the its RGB, to be [Fe/H] \[1.3. We adopt this value. IC 1613 represents a metal-poor population. The 195 C stars are plotted on the CMD of IC 1613 and are reproduced in Figure 11. The median R[I of the C star population of IC 1613 is 1.03, or (R[I) \ 1.01 with the adopted color excess. This median color Ðts 0 rather well the relationship between the median color of C stars and [Fe/H] of the parent galaxy, when compared with the Magellanic Cloud carbon star populations. Richer (1981) gives for the Large and Small Magellanic Clouds, respectively, (R[I) \ 1.28 and 1.20, which J,0 correspond in the Cousins system to (R[I) \ 1.20 and The M distribution of the C stars in IC 1613 is displayed I in Figure 12. For comparison, we also include, not at the same scale, the distribution of magnitudes of M stars; the foreground stars have been subtracted. This comparison shows that the drop in the number of C stars at M B [4.4 I is not due to our magnitude limit. From this distribution, we determine that the median absolute magnitude of C stars in IC 1613 is M \[4.69 ^ This result reaffirms I the universality of the median absolute magnitude of C stars in di erent galaxies. This value is within 0.1 mag of the values quoted by Brewer et al. (1995), for example. One of the goals of our LG survey is to establish whether the numerous dwarfs have a similar star formation history. Carbon stars are then used as probes to evaluate the extent FIG. 10.ÈC/M ratio varies as expected toward the periphery of IC FIG. 11.ÈC stars plotted on the CMD of IC 1613

9 2788 ALBERT, DEMERS, & KUNKEL N FIG. 12.ÈLuminosity function of C stars in IC The dashed histogram represents the magnitude distribution of M stars in IC 1613 (not at the same scale to better see the comparison). of the intermediate-age population relative to the global stellar population of the galaxy. One relationship, the total number of C stars (N ) versus the M of the galaxy, and one C V parameter, log (N ) ] 0.4M, provide information in this C V respect. The total number of C stars is known for very few galaxies: Phoenix, four or Ðve; Fornax, D100; Leo I, a dozen or so. Along with IC 1613, these galaxies do form a linear relationship, indicative of the increase of the C star population with the mass. For IC 1613, log (N C ) ] 0.4M V equals [3.59, a value quite typical according to Groenewegen (1999). 6. CONCLUSIONS The identiðcation of numerous C stars in IC 1613 has not only provided an additional datum for the LG dwarf irregulars, it has also revealed the extent of the stellar population of IC This is not unique to IC 1613; Cook (1987) found C stars in the Sagittarius dwarf irregular galaxy well outside of its Holmberg radius. Like what was done for the Magellanic Clouds, carbon stars may then serve as kinematic probes to better understand the dynamics of the periphery of dwarf galaxies. IC 1613 represents only the Ðrst step of our LG survey; it is thus too early to fully compare its carbon star population with those of other galaxies. We already have on hand four-color photometry for seven other dwarf irregulars. The use of a uniform criterion for C identiðcation will provide observations that will greatly facilitate the comparison between galaxies. Therefore, we defer to a future paper a detailed comparative study of the carbon star population in LG dwarfs. This project is supported Ðnancially in part (S. D.) by the Natural Sciences and Engineering Research Council of Canada. Aaronson, M., Blanco, V. M., Cook, K. H., & Schechter, P. L. 1989, ApJS, 70, 637 Baade, W. 1935, Annu. Rep. Mt. Wilson Obs., 1934È1935, 185 Blanco, V. M., McCarthy, M. F., & Blanco, B. M. 1980, ApJ, 242, 938 Brewer, J. P., Richer, H. B., & Crabtree, D. R. 1995, AJ, 109, 2480 ÈÈÈ. 1996, AJ, 112, 491 Cook, K. H. 1987, Ph.D. thesis, Univ. Arizona Cook, K. H., Aaronson, M., & Norris, J. 1986, ApJ, 305, 634 Frappier, B. 1998, MasterÏs thesis, Univ. Montre al Freedman, W. L. 1988, ApJ, 326, 691 Green, R. F., Schmidt, M., & Liebert, J. 1986, ApJS, 61, 305 Groenewegen, M. A. T. 1999, in IAU Symp. 191 Asymptotic Giant Branch Stars, ed. T. Le Bertre, A. Lèbre, & C. Waelkens (San Francisco: ASP), 535 Guharthakurta, P., Yanny, B., Schneider, D. P., & Bahcall, J. N. 1996, AJ, 111, 267 REFERENCES Hodge, P. W., Smith, T. R., Eskridge, P. B., MacGillivray, H. T., & Beard, S. M. 1991, ApJ, 369, 372 Hubble, E. 1936, The Realm of the Nebulae (New Haven: Yale Univ. Press) Lake, G. R., & Skillman, E. D. 1989, AJ, 98, 1274 Lee, M. G., Freedman, W. L., & Madore, B. F. 1993, ApJ, 417, 553 Mateo, M. 1998, ARA&A, 36, 435 Minniti, D., Zijlstra, A., & Alonso, M. V. 1999, AJ, 117, 881 Pritchet, C. J., Richer, H. B., Schade, D., Crabtree, D., & Yee, H. K. 1987, ApJ, 323, 79 Richer, H. B. 1981, ApJ, 243, 744 Saha, A., Freedman, W. L., Hoessel, J. G., & Mossman, A. E. 1992, AJ, 104, 1072 Stetson, P. B. 1994, PASP, 106, 250 The, P. S., Steenman, H. C., & Alcaino, G. 1984, A&A, 132, 385 Wolf, M. 1906, MNRAS, 67, 91