THE INITIAL MASS FUNCTION IN THE TAURUS STAR-FORMING REGION 1. K. L. Luhman and Lee Hartmann. John R. Stauffer. and J.

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1 The Astrophysical Journal, 580: , 2002 November 20 # The American Astronomical Society. All rights reserved. Printed in U.S.A. THE INITIAL MASS FUNCTION IN THE TAURUS STAR-FORMING REGION 1 César Briceño 2 Centro de Investigaciones de Astronomía (CIDA), Apartado Postal 264, Mérida 5101-A, Venezuela; briceno@cida.ve K. L. Luhman and Lee Hartmann Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; kluhman@cfa.harvard.edu, lhartmann@cfa.harvard.edu John R. Stauffer SIRTF Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125; stauffer@ipac.caltech.edu and J. Davy Kirkpatrick Infrared Processing and Analysis Center, California Institute of Technology, MS , 770 South Wilson Avenue, Pasadena, CA 91125; davy@ipac.caltech.edu Received 2002 March 14; accepted 2002 July 25 ABSTRACT By combining a deep optical imaging (I; z 0 ) survey of 8 deg 2 in the Taurus star-forming region with data from the Two-Micron All-Sky Survey (2MASS) and follow-up spectroscopy, we have performed a search for low-mass Taurus members that is complete to 0.02 M for reddenings of A V d4. We report the discovery of nine new members with spectral types of M5.75 M9.5, corresponding to masses of M by recent evolutionary models. The new M9.5 member is the least massive brown dwarf found to date in the Taurus star-forming region. We derive an initial mass function (IMF) for the fields surveyed in this work and in our previous studies, which encompass 54% of the known Taurus membership. We compare the Taurus IMF with a similarly derived one for the Trapezium Cluster and to mass functions for the M35 and Pleiades open clusters. While the IMFs in all of these regions flatten near 0.8 M, the mass function in Taurus is more narrow and sharply peaked at this mass. Our survey indicates that Taurus has 2 fewer brown dwarfs at M than the Trapezium. We discuss the implications of these results for theories of the IMF, and suggest that the lower frequency of brown dwarfs in Taurus relative to the Trapezium may result from the low-density star-forming environment, leading to larger minimum Jeans masses. Subject headings: infrared: stars stars: evolution stars: formation stars: low-mass, brown dwarfs stars: luminosity function, mass function stars: pre main-sequence 1. INTRODUCTION Understanding the formation of the least massive stars and brown dwarfs and determining the properties they exhibit as an ensemble during their youth are fundamental problems in studies of star formation. Key issues are the form of the initial mass function (IMF) across the hydrogen-burning mass limit and any variation with environment from one region to another. Some theories of star formation have proposed that the IMF should be a function of the physical conditions in molecular clouds (Larson 1985), while other studies propose a more universal IMF (Elmegreen 1997, 1999; Gilmore 2001). To test these ideas, accurate determinations of the IMF across widely differing starforming conditions are needed. One approach to the determination of the IMF comes from studies of volume-limited samples of stars in the solar 1 Based on observations obtained at the Kitt Peak National Observatory, Keck Observatory, Steward Observatory, and the MMT Observatory. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. 2 Visiting Astronomer, Kitt Peak National Observatory. KPNO is operated by AURA, Inc. under contract to the National Science Foundation. 317 neighborhood. However, stellar evolution and dynamical considerations complicate the analysis of such samples. In contrast, the young stellar populations in star-forming regions offer several distinct advantages for deriving the IMF under a variety of environmental conditions. First, because these young populations are mostly coeval (Hartmann 2001), biases due to evolutionary effects, both stellar and dynamical, are minimized. Second, young low-mass stars and brown dwarfs are hotter and brighter than their older counterparts; thus, they are easier to detect and sensitive searches can reach down to lower masses than for old field brown dwarfs at the same distance. A major difficulty in studying the youngest populations in many regions is the large dust extinction within the natal molecular clouds, which requires observations near 1 lm and longward. This limitation is being largely overcome with improvements in the sensitivity of infrared (IR) and optical cameras and spectrographs that make it possible to conduct sensitive studies of very low mass populations in these heavily obscured regions. Most surveys for young low-mass stars and brown dwarfs have concentrated on star-forming clusters like the Orion Nebula cluster (Hillenbrand 1997; Luhman et al. 2000; Hillenbrand & Carpenter 2000; Lucas et al. 2001; Muench et al. 2002), Orionis (Barrado y Navascués et al. 2001; Béjar et al. 2001), IC 348 (Herbig 1998; Luhman 1999), and Chamaeleon I (Neuhäuser & Comerón 1999; Comerón, Neu-

2 318 BRICEN O ET AL. Vol. 580 häuser, & Kaas 2000). Studies of extended star-forming regions with low stellar density have lagged behind, mainly because of the difficulty of conducting deep surveys over large areas of the sky. However, this approach has become feasible given the completion of the Two-Micron All-Sky Survey (2MASS) and the availability of wide-field optical cameras. The Taurus dark cloud complex, with a pre mainsequence population (1 2 Myr) and a relatively low density of stars (n 1 10 pc 3 ), offers ideal conditions in many respects for a wide-field search for young low-mass stars and brown dwarfs. First, it can be considered an example of the nonclustered mode of star formation, at the other end of the spectrum when compared to the Orion Nebula cluster. The Taurus molecular cloud complex is a region where stars are forming in relative isolation, free from the disturbing effects of nearby massive stars. Second, at its low distance of 140 pc (Kenyon, Dobrzycka, & Hartmann 1994a; Wichmann et al. 1998), objects can be detected at lower masses than in regions like Orion (d 400 pc; de Zeeuw et al. 1999). Taurus members at masses of 0.08 M have I 14 (Briceño et al. 1998; Luhman et al. 1998a), which are easily observable with imaging from a 1 m class telescope. Finally, because a majority of the members of Taurus have relatively low extinction (A V d4), the region is suitable for an optical survey. Recent surveys in Taurus have identified a few lowmass objects (Strom & Strom 1994; Briceño et al. 1998; Luhman & Rieke 1998; Luhman 2000; Martín et al. 2001), but have been restricted in their spatial coverage (d2 deg 2 )and hence included only a fraction of the young stellar population in Taurus. In our initial studies (Briceño et al. 1998; Luhman 2000), we noted an apparent deficit of brown dwarfs in Taurus when compared to the Trapezium cluster in Orion. To improve the number statistics of the measurements in Taurus and to better test for variations in the low-mass end of the IMF with the environment, we have conducted a deep optical CCD survey of 8 deg 2 in the Taurus dark cloud complex, which encompasses 54% of the known pre mainsequence population in the region. This survey represents a large expansion of the area covered compared to previous searches for low-mass stars and brown dwarfs in Taurus. We combine our optical photometry with near-ir data from 2MASS and identify candidate low-mass members of Taurus. From follow-up spectroscopy of these sources, we find nine new members of Taurus, one of which has a spectral type of M9.5, which is equivalent to M, making it the least massive brown dwarf found in this region. For the new survey fields and those from Briceño et al. (1998) and Luhman (2000), we measure an IMF that is representative to 0.02 M and compare it to mass functions in other star-forming regions and open clusters. We conclude by discussing the implications of these measurements for theories of the formation of stars and brown dwarfs. 2. OBSERVATIONS AND DATA ANALYSIS 2.1. Optical Imaging Even with large-area detectors, the extent of Taurus on the sky (100 deg 2 ) is so large that it is difficult to survey the entire region with reasonable amounts of telescope time. We therefore selected fields to match the clustering of the known pre main-sequence population. Approximately 60% of the known pre main-sequence stars are concentrated in six groups with an average radius of 1 pc (25 0 ; Gómez et al. 1993), which are well matched to the field of view of the new generation of large field cameras on 1 2 m class telescopes. Thus, a judicious selection of target fields can encompass a large fraction of the pre main-sequence population. While our survey is biased somewhat to regions of higher density, we also observed fields like L1544 (Briceño et al. 1993) and Taurus V (Gómez et al. 1993), toward which the column density of gas (Ungerechts & Thaddeus 1987) is relatively low. Most importantly, our observations represent a substantial expansion of the area covered in previous studies and provide better constraints on any possible highly dispersed population. During the nights of 1999 December 26 30, we obtained deep images in I and z 0 filters of eight fields centered on Taurus stellar aggregates from Gómez et al. (1993) and the group in the L1544 cloud reported by Briceño et al. (1993). The observed areas are defined in Table 1 and illustrated in Figure 1. We used the KPNO CCD mosaic camera on the 0.9 m telescope; this configuration yields a plate scale of 0>43 pixel 1 and a field of view of 1 1, corresponding to a total survey area of 8 deg 2. Exposure times ranged from 300 to 1800 s (Table 1). We attempted to obtain up to three individual images in a dither pattern to fill in the gaps between the CCDs of the mosaic array; the individual exposures were dithered by offsets of about 100 pixels (43 00 ). However, because of weather we could only obtain full dithers under clear skies for Tau I, Tau V, and L1544. We also obtained short exposures (30 60 s) of Landolt (Landolt 1992) standard fields at similar air mass to those at which the program fields were observed. The individual CCD images were bias-subtracted and flat-fielded using the standard data-reduction techniques and tools for mosaic images in the MSCRED package within IRAF. 3 Pixel-topixel sensitivity variations were corrected with dome flats; the large-scale illumination gradients were corrected with sky frames constructed by median combining the deep Taurus images previous to correcting for the spatial offsets. We then registered and combined the individual dither frames for each field Optical Photometry and Astrometry Point sources in the combined images were identified with DAOFIND with a 4 threshold. Images were then visually inspected to check that no faint objects were missed, and to remove obvious false detections (e.g., in overly saturated stars, etc.). As these fields were not crowded (at b 20, the Taurus star-forming region is well off the Galactic plane), we measured instrumental magnitudes for the point sources with the APPHOT package and an aperture radius of 6 pixels (2>4), or roughly twice the typical FWHM. The photometry was calibrated in the Cousins I C system through observations of Landolt (1992) standard fields SA 95 and SA 101. Since the standard stars and the target fields were observed at similar air masses, no extinction correction was applied. The z 0 data were calibrated in the Sloan (SDSS) system using the I C and z 0 magnitudes for Landolt (Landolt 3 IRAF is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.

3 No. 1, 2002 IMF IN TAURUS 319 TABLE 1 Survey Fields Field a (J2000) (J2000) l (deg) b (deg) Associated Dark Cloud Area Imaged (deg 2 ) Exposure Time per Filter (s) Completeness I C (mag) Tau I B209, L Tau II L Tau IIIw L Tau IIIe L Tau IVe L , Tau V L Tau VIe L L L Note. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Aggregate designations from Gómez et al ) standard stars in fields SA 95 and SA 110 and the relationship between I C z 0 and (R I ) C in Krisciunas, Margon, & Szkody (1998). The color term was found to be very small, so we corrected instrumental magnitudes only for the nightly zero point. We found similar results if we instead assumed I C z 0 ¼ 0 for standard stars with neutral ([R IŠ C 0) colors as in Luhman (2000) and Barrado y Navascués et al. (2001). An approximate calibration of the I C z 0 color is adequate because only the relative colors are important for separating field stars and candidate low-mass objects. The internal errors in our photometry can be estimated from the typical dispersion in instrumental magnitudes for the same stars in each of the individual dithers, which was 0.02 mag for I C d18 and 0.1 for I C 20:5. External and systematic errors are more difficult to assess, in part because little I C photometry exists for the lower mass, fainter T Tauri stars in the dynamic range of our sample, but also because the majority of young low-mass stars are intrinsically variable at the mag level (e.g., Herbst et al. 1994; Briceño et al. 2001). With these caveats in mind, our I C photometry for V410 Anon 13 (Luhman et al. 1998a) and the two new young brown dwarfs identified by Martín et al. (2001) agree at the 0.2 mag level. To examine the overall effects of photometric completeness in our survey, we summed all sources in each of the fields in both 0.5 and 1 mag wide bins, covering the range I C ¼ 13 22, and constructed histograms of the number of stars per magnitude bin. We define our completeness limit as the magnitude at which the number count of stars per magnitude interval stops rising. The values obtained are shown in Table 1. In tests with other magnitude bin widths, these results turned out to be rather insensitive to the width one chose, within values of mag. Because we could not obtain similarly deep exposures for all fields, the optical completeness is not homogeneous across the entire region. However, the optical photometry is used only for selecting candidate members, whereas the completeness of the IMF measurement will rely on the 2MASS data. Coordinates were measured with the astrometric routines in the MSCRED package and the WCSTools stand-alone package (Mink 1995, 1996, 1998, 2001). The global astrometric solution was derived using reference stars from the Hubble Space Telescope (HST) ACT catalog (Lasker, Russel, & Jenkner 1996). The average residuals between these positions and those of the 2MASS Spring 1999 Release Point Source Catalog are d0> Infrared Photometry Measurements at J, H, and K s for our survey fields are taken from the 2MASS Spring 1999 Release Point Source Catalog. 2MASS data are not available for portions of two of our optical fields, namely, west of ¼ 5 h 04 m 21 s in L1544 and between ¼ 4 h 35 m 43 s and 4 h 36 m 12 s in Tau IVe. These regions correspond to 20% and 10%, respectively, of the areas imaged in those locations. Both our optical data and 2MASS have very accurate coordinates, so we used the optical coordinates as input to search the 2MASS catalog for an IR counterpart, using a 2 00 radii for this search, much larger than the errors in the coordinates. In this way we obtained the following number of objects that were present in both the optical and IR data: 3354 sources in Tau I, 1534 in Tau II, 4361 Tau IIIw, 4172 in Tau IIIe, 2315 in Tau IVe, 3916 in Tau V, 1400 in Tau VIe, and 5581 in L Spectroscopy In x 3.1, the optical and IR photometry for the Taurus fields is used to identify candidate low-mass members of the star-forming region, which are listed in Tables 2 and 3. We now describe the optical spectroscopic observations of these candidates one of which, KPNO Tau 2, was also a candidate from Briceño et al. (1998) and Luhman (2000) and a sample of previously known late-type members of Taurus. We observed KPNO Tau 2 on 2000 November 26 and KPNO Tau 3, KPNO Tau 4, KPNO Cand 1, KPNO Cand 3, and KPNO Cand 4 on 2001 November 13 with the Keck I low-resolution imaging spectrometer (LRIS; Oke et al. 1995). The long-slit mode of LRIS was used with the 150 and 400 l mm 1 gratings ( blaze ¼ 7500 and 8500 Å) and GG570 and GG495 blocking filters during the two nights. The slit width was 1>0 for each night, producing spectral resolutions of FWHM ¼ 13 and 6 Å. On 2001 December 25 26, we used the Boller and Chivens Spectrometer on the Steward Observatory 2.3 m Bok Reflector to obtain spectra of the KPNO Tau 5, KPNO Tau 8, and KPNO Cand 5, and the previously known members CFHT-BD Tau J (CFHT-BD Tau 4), MHO 4 through MHO 9, V927 Tau A+B, V410 X-ray 3, and V410 Anon 13. The spectrometer was operated with the 400 l mm 1 grating ( blaze ¼ 7506 Å), Y48 blocking filter, and 1>5 slit, providing a resolution of FWHM ¼ 7Å. On 2002 January 12, we observed the candidates KPNO Tau 1, KPNO Tau 6, KPNO Tau 7, KPNO Tau 9, and KPNO Cand 2, and the

4 Fig. 1. Distribution of young sources in the Taurus star-forming region, shown with contours of 12 CO emission (Ungerechts & Thaddeus 1987). The groups identified by Gómez et al. (1993) are labeled. In this work, images at I and z 0 were obtained for the eight 1 1 fields that are indicated. The fields within the smaller rectangles were surveyed by Briceño et al. (1998) and Luhman (2000). For clarity, in the upper panels we show an expanded view of the dense groupings Taurus I and II, projected against images from the Digitized Sky Survey.

5 IMF IN TAURUS 321 TABLE 2 New Low-Mass Members in Survey Fields ID 2MASS ID (J2000) (J2000) I C I C z 0 J H H K s K s Spectral Type W (H) (Å) KPNO Tau 1... J M þ10 5 KPNO Tau 2... J a M KPNO Tau 3... J b M KPNO Tau 4... J M9.5 0:25 þ0:5 150 KPNO Tau 5... J M KPNO Tau 6... J M KPNO Tau 7... J M KPNO Tau 8... J M KPNO Tau 9... J M d20 Note. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Iz 0 data are from this work. Coordinates are from Luhman 2000 for KPNO Tau 2 and from 2MASS for the remaining sources. JHK s data are from 2MASS. a Identified as a candidate both in this work and previously by Briceño et al and Luhman b W (He 6678Þ ¼6:5 1Å. known members IRAS , CFHT-BD Tau J (CFHT-BD Tau 2), and CFHT-BD Tau J (CFHT-BD Tau 3) with the Blue Channel spectrometer at the Multiple Mirror Telescope (MMT) Observatory. We used the 600 l mm 1 grating ( blaze ¼ 9630 Å), LP495 blocking filter, and 1>0 slit. The spectral resolution of these data was 2.8 Å. All spectra were obtained with the slit rotated to the parallactic angle. The exposure times ranged from 300 to 1800 s. After bias subtraction and flat-fielding, the spectra were extracted and calibrated in wavelength with arc lamp data. The spectra were then corrected for the sensitivity functions of the detectors, which were measured from observations of spectrophotometric standard stars. 3. NEW LOW-MASS MEMBERS OF TAURUS 3.1. Selection of Candidate Members Candidate low-mass members of Taurus can be identified with color-magnitude and color-color diagrams generated from our new optical photometry and the 2MASS data. These methods have been used successfully in extracting refined samples of candidate young low-mass stars and brown dwarfs in star-forming regions and young clusters (e.g., Luhman 1999, 2000; Barrado-Navascués et al. 2001; Béjar et al. 2001). We first describe the regions in Figures 2 4 where lowmass members of Taurus are expected (and observed) to reside. For the models of Baraffe et al. (1998), masses of 0.02 and 0.08 M at 1 Myr correspond to spectral types of M9 and M6.5, respectively (Luhman 1999). The I C z 0 colors for these spectral types are taken from Luhman (2000). By the data in Luhman (1999) and x 4.2, the typical intrinsic J H color of a young M6.5 object appears to be similar to that of a dwarf, so we adopt a dwarf value of J H ¼ 0:575 (Leggett 1992). However, based on the results in x 4.2, we adopt an intrinsic color of J H ¼ 0:85 for young M9 objects, which is redder than the value of 0.73 for M9 V. For the I C K s colors, we adopt the dwarf values from Leggett (1992). We compute apparent I C and H magnitudes for 0.02 and 0.08 M at 1 Myr by combining theoretical luminosities (Baraffe et al. 1998), bolometric corrections for dwarfs (x 4.3), and a distance modulus of 5.76 (Wichmann et al. 1998). The unreddened positions of 0.02 and 0.08 M are indicated in Figure 2. We have plotted the reddening vectors from A V ¼ 0 4 for these masses in Figures 3 and 4. Taurus members with masses greater than 0.02 M should fall above the dashed reddening vector in Figure 2. After plotting all objects for which both optical and IR data are available, we find 250 sources above this boundary. These candidates can be either late-type objects (field dwarfs or low-mass Taurus members) or reddened, earlier type stars (background field stars or high-mass Taurus members). The mass expected for a candidate if it is a member is indicated by its position in J H versus H in Figure 4. We first consider the candidate higher mass members. There are a few dozen candidates at greater than 0.2 M (Hd11), most of which have high reddenings by their J H colors. Because most of these objects are beyond the reddening limit for our IMF measurement (A V ¼ 4, x 4.4.1), they were not observed spectroscopically. For the candidates that TABLE 3 Candidate Objects Rejected as Field Stars ID 2MASS ID (J2000) (J2000) I C I C z 0 J H H K s K s Spectral Type W (H) (Å) KPNO Cand 1... J M4.75 V KPNO Cand 2... J M3.75 V KPNO Cand 3... J early/giant... KPNO Cand 4... J early/giant... KPNO Cand 5... J early/giant... Note. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.

6 322 BRICEN O ET AL. Vol. 580 Fig. 2. I C z 0 vs. I C for the eight 1 1 fields of the Taurus star-forming region in Fig. 1. We have indicated the previously known members at <M6 (larger filled circles) and M6 (outlined circles), spectroscopically identified new members ( filled diamonds) and field stars (outlined triangles) from this work. The unreddened positions of 0.08 (M6.5) and 0.02 M (M9) for an age of 1 Myr (Baraffe et al. 1998) are shown as the upper and lower open starred symbols, respectively. Taurus members with masses of 0.02 M are expected to fall above the dashed reddening vector. Stars below this line are labeled as field stars (small points). Among the sources that are above the reddening vector and lack spectra, five objects have the appropriate colors and brightnesses in Figs. 3 and 4 to be Taurus members with masses below 0.2 M and with A V 4(outlined diamonds), while the remaining objects are likely field stars or members with higher extinctions or masses (small open circles). The two Taurus members below the reddening vector are DG Tau B and IRAS ; the first is known to exhibit an edge-on disk, and the latter is probably detected primarily in scattered light as well. appear near and below the hydrogen-burning mass limit in Figure 4, we can better determine their nature before obtaining spectroscopy by incorporating the diagram of J H versus I C K s in Figure 3. With progressively later M spectral types, I C K s /J H quickly increases and the corresponding reddening vector separates from the band occupied by reddened early-type background stars. By checking whether a source exhibits the appropriate colors in I C z 0 versus I C and J H versus I C K s for the mass implied by J H versus H, we arrive at a final sample of 19 candidate low-mass members of Taurus (d0.1 M ). We have obtained spectra of 14 of these sources; nine are identified as new Taurus members and are shown in Table 2 (see x 3.2), and five are classified as field stars and are listed in Table 3. The other five candidates will be presented in a later work after they have been observed spectroscopically (outlined diamonds in Figs. 2 4). The one promising brown dwarf candidate that lacked spectra in the fields observed by Briceño et al. (1998) and Luhman (2000), KPNO Tau 2, is confirmed here as a late-type pre main-sequence object and appears in this list of new members as well. A test of the reliability of these selection criteria is whether previously known low-mass young stars and brown dwarfs in the survey area could be recovered in our final Fig. 3. J H vs. I C K s for the eight 1 1 fields of the Taurus starforming region in Fig. 1. Symbols are as in Fig. 2. The lower and upper lines represent the reddening vectors from A V ¼ 0 4 for spectral types of M6.5 and M9, which correspond to 0.08 and 0.02 M for an age of 1 Myr (Baraffe et al. 1998). Objects that are field stars by the I C and z 0 data in Fig. 2 have been omitted. Among the sources above the reddening vector in Fig. 2 and lacking spectra, five objects have the appropriate colors and brightnesses in this diagram and Fig. 4 to be Taurus members with masses below 0.2 M and with A V 4(outlined diamonds), while the remaining objects are likely field stars or members with higher extinctions or masses (open circles). candidate list. The Taurus members in our survey fields with spectral types of M6 or later and below our saturation limit are V410 Anon 13, V410 X-ray 3, CFHT-BD Tau 2, and CFHT-BD Tau 3. These objects were all recovered with our selection method, the photometry for which is presented in Table Spectral Classification of Candidate Members To identify the Taurus members within our list of candidates, we used low-resolution optical spectroscopy from 6000 to 9000 Å. At the spectral types of late M that are expected for low-mass Taurus members, the K and Na absorption lines vary significantly between dwarfs and pre main-sequence stars (Martín et al. 1996; Luhman et al. 1998a, 1998b; Luhman 1999). Because these features are easily detected in low-resolution spectra, higher resolution data for Li were unnecessary for distinguishing young members from field dwarfs. Meanwhile, both high sensitivity and accurate spectral types can be achieved with low spectral resolution for faint M-type sources. Our photometrically selected candidates can be background stars, foreground stars, or young members of Taurus. Background stars are predominantly giants and earlytype dwarfs; in low-resolution optical spectra they exhibit only a few absorption lines (H, Caii) and are otherwise featureless. Three of our candidates have these spectral characteristics, and are thus rejected as field stars (Table 3). The spectra of 11 candidates have spectral features such as TiO and VO absorption bands that are characteristic of cool

7 No. 1, 2002 IMF IN TAURUS 323 Fig. 4. J H vs. H from 2MASS for the eight 1 1 fields of the Taurus star-forming region in Fig. 1. Symbols are as in Fig. 2. The upper and lower solid lines represent the reddening vectors from A V ¼ 0 4 for 0.08 (M6.5) and 0.02 M (M9) for an age of 1 Myr (Baraffe et al. 1998). Objects that are field stars by the I C and z 0 data in Fig. 2 are omitted. Among the sources above the reddening vector in Fig. 2 and lacking spectra, five objects have the appropriate colors and brightnesses in this diagram and Fig. 3 to be Taurus members with masses below 0.2 M and with A V 4(outlined diamonds), while the remaining objects are likely field stars or members with higher extinctions or masses (open circles). We also show the 2MASS sources that were not detected in our optical images (small filled triangles). The dashed line represents the completeness limits of J ¼ 15:75 and H ¼ 15:25. atmospheres. The spectra for two of the candidates, KPNO Cand 1 and KPNO Cand 2, show both the strong K and Na absorption lines that are found in M dwarfs and the significant reddening that is indicative of background stars. These spectra are consistent with those of background dwarfs with spectral types of M4.75 V and M3.75 V and reddenings of A V 2. If these objects are placed on the Hertzsprung-Russell (H-R) diagram for the distance of Taurus, they fall near or just below the main sequence, indicating that they are probably near the opposite side of the starforming region. The data for the remaining nine late-type candidates and a sample of previously known low-mass members are presented in Figures 5 and 6. These candidates exhibit the weak K and Na absorption features that are signatures of pre main-sequence objects. The various absorption bands in these data are matched well by averages of standard dwarfs and giants (Luhman 1999), further demonstrating their pre main-sequence nature. The strengths of H emission in KPNO Tau 1, KPNO Tau 2, KPNO Tau 8, and KPNO Tau 9 are consistent with those of both active field dwarfs and young pre main-sequence stars, while the much stronger emission lines from KPNO Tau 3, KPNO Tau 4, KPNO Tau 5, KPNO Tau 6, and KPNO Tau 7 are expected for only the latter (see Table 2). In addition, the spectrum of KPNO Tau 3 exhibits He emission and the signal-to-noise ratio is sufficient for the detection of strong Li absorption, both of which are indicative of a T Tauri star. To derive spectral types for these new Taurus members and the previously known members observed here, we used the methods of spectral classification for young late-type stars described by Luhman (1999). In that study, the average of optical spectra of a dwarf and a giant at a given spectral type, when normalized at 7500 Å, was found to produce the best match to data at Å for young objects at M6 M9. The stars used as standard dwarf and giants and the references for their spectra are provided in Luhman (1999). Here we add for M9.5 V, 2MASSs J and 2MASSs J (Reid et al. 1999a; Gizis et al. 2000); and for M10+ III, IO Vir (Kirkpatrick, Henry, & Irwin 1997). M9.5 III is the average of M9 III and M10+ III when normalized at 8250 Å. The spectral types for the new Taurus members are given in Table 2. For the known members, we measure spectral types of IRAS ¼ M01, CFHT-BD Tau 2 ¼ M7.50:25, CFHT-BD Tau 3 ¼ M7.750:25, CFHT-BD Tau 4 ¼,MHO5¼M60:25, MHO 6 ¼ M4.750:25, MHO 7 ¼ M5.250:25, MHO 8 ¼ M6 0:25, MHO 9 ¼ M4.250:25, V927 Tau A+B ¼ M4.75 0:25, V410 X-ray 3 ¼ M60:25, and V410 Anon 13 ¼ M5.750:25. The data for the three M7.5 members match the spectrum of GG Tau Bb (White & Ghez 1999), which was also classified as M7.5 with the same methods used here (Luhman 1999). The spectra of the three new M8.5 sources agree closely with the spectrum of the M8.5 object GY141 in Ophiuchus (Luhman, Liebert, & Rieke 1997; Luhman 1999) Meanwhile, CFHT-BD Tau 3 is clearly earlier than GY141 and these three sources, and matches the average of dwarfs and giants at M7.75 under a small amount of reddening (A V 1), whereas Martín et al. (2001) reported a spectral type of M9 from data for a smaller wavelength range. The best match to the spectrum of the coolest new member of Taurus, KPNO Tau 4, is provided by an average of M9.5 V and M9.5 III in which their spectra are normalized at 8250 Å M7 0:5 þ0:25,mho4¼m7þ0:25 0:5 TABLE 4 Previously Known Low-Mass Members in Survey Fields ID (J2000) (J2000) I C I C z 0 J H H K s K s V410 X-ray V410 Anon CFHT BD Tau CFHT BD Tau Note. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Iz 0 and JHK s data are from this work and the 2MASS survey. Coordinates are from Luhman 2000 for V410 X-ray 3 and V410 Anon 13 and from 2MASS for CFHT-BD Tau 2 and 3.

8 324 BRICEN O ET AL. Vol. 580 Fig. 5. Low-resolution spectra of M-type members of the Taurus starforming region. KPNO Tau 2 and KPNO Tau 3 were identified as candidate members of Taurus by Figs. 2 4 and are confirmed as pre mainsequence objects by the weak absorption in K and Na. The other sources are previously known late-type members. These objects are near and above the hydrogen-burning mass limit by the H-R diagram in Fig. 10. The spectra of IRAS , V410 Anon 13, and KPNO Tau 3 exhibit noticeable reddening (A V e1). All data are smoothed to a resolution of 18 Å and normalized at 7500 Å. Fig. 7. Spectrum of KPNO Tau 4 compared to data for M9.5 III and M9.5 V and the combination of the two that produces the best match. All data are smoothed to a resolution of 18 Å and normalized at 7500 Å. and scaled by 0.4 and 0.6, respectively, as shown in Figure 7. The VO absorption in KPNO Tau 4 is stronger than in the average of the dwarf and giant, which is a behavior that has been observed in other young objects at M8 M8.5 (Luhman 1999). In summary, we have discovered nine new members of Taurus with spectral types from M5.75 to M9.5 ( M ), including the least massive brown dwarf detected to date in this star-forming region. After taking into account these new objects, we arrive at the distribution of known members as a function of spectral type in Figure 8 for all of the Taurus star-forming region. Fig. 6. Low-resolution spectra of M-type members of the Taurus starforming region. The CFHT sources and MHO 4 are previously known latetype members. The remaining objects were identified as candidate members of Taurus by Figs. 2 4 and are confirmed as pre main-sequence objects by the weak absorption in K and Na and the shape of the spectra from Å. These objects are below the hydrogen-burning mass limit by the H- R diagram in Fig. 10. The spectra of CFHT-BD Tau 4, CFHT-BD Tau 2, and CFHT-BD Tau 3 exhibit noticeable reddening (A V e1). All data are smoothed to a resolution of 18 Å and normalized at 7500 Å Completeness The completeness in mass and reddening of our survey is readily evaluated with near-ir data. A diagram of J H versus H from 2MASS is shown in Figure 4 for the fields observed in this study. The completeness limits of the 2MASS data are taken to be the magnitudes at which the logarithm of the number of sources as a function of magnitude departs from a linear slope and begins to turn over (J 15:75, H 15:25) for locations adjacent to the Taurus dark clouds. Sources that have been spectroscopically confirmed as members or field stars are indicated in Figure 4, while the objects that were identified as probable field stars by the diagram of I C z 0 versus I C have been omitted. Out of the remaining sources, there are only five objects that lack spectra and are candidate low-mass (d0.1 M ) members of Taurus (x 3.1). Four objects have H > 15 and one object

9 No. 1, 2002 IMF IN TAURUS 325 In this section, we begin by tabulating photometry and spectral types for all known Taurus members within the fields surveyed in this work and by Briceño et al. (1998) and Luhman (2000) (x 4.1). We estimate extinctions, effective temperatures, and bolometric luminosities for these sources when possible and place them on the H-R diagram (xx 4.2 and 4.3). Because young stars in Taurus exhibit reddenings that range from A V ¼ 0 to 10, while our surveys for lowmass members are complete to 0.02 M only for lower reddenings of A V d4, we restrict the IMF sample to include only sources within the latter extinction limit. After applying a few other criteria, we arrive at an IMF sample that should be representative of the surveyed Taurus fields. We then use theoretical evolutionary models to infer masses for the objects in the IMF sample and construct an IMF (x 4.4.1). Finally, we examine the spatial distribution of Taurus members as a function of mass (x 4.4.2), compare the Taurus IMF to mass functions from other studies (x 4.4.3), and explore the implications of these data for theories of the formation of stars and brown dwarfs (x 4.4.4). Fig. 8. Distribution of spectral types for the entire Taurus star-forming region. Binary star systems with separations less than 2 00 are treated as single objects. The vertical dashed line is M6.5, which approximates the hydrogen-burning mass limit for 1 Myr (Baraffe et al. 1998). has H ¼ 11, which should correspond to masses of 0.01 and 0.1 M, respectively, if they are members. In addition, there are a few objects at H < 11 and low J H in Figure 4 that lack spectroscopy and that could be stellar members of Taurus. Based on these results and the 2MASS completeness limits, our census should be complete for Taurus members with A V 4, M 0:02 M, and 1 Myr, except for a few remaining candidates at M. Because the reddening vector for 0.02 M at 10 Myr is only 0.5 mag fainter than the one shown for 1 Myr, the survey should be only slightly incomplete for ages of 10 Myr. Comparable completeness limits apply to the observations of Luhman (2000) and the fields therein. If the distribution of reddenings is the same for stellar and substellar members, an IMF that includes members in the fields surveyed in this work and by Luhman (2000) with A V 4 should be representative and unbiased in mass. The census completeness as a function of mass is unaffected by the lack of 2MASS photometry for two small sections of our fields (x 2.3) because those areas do not contain any known members of Taurus. Just as this survey is incomplete for low-mass members with high extinction (A V > 4), it is unlikely to detect low-mass objects that have edge-on disks, as discussed in x THE TAURUS STELLAR POPULATION 4.1. Members and Adopted Data We have compiled a list of all known members of the Taurus star-forming region from Herbig & Bell (1988), Strom & Strom (1994), Kenyon & Hartmann (1995), Briceño et al. (1998, 1999), Luhman & Rieke (1998), Martín (2000), Martín et al. (2001), and this work. We include young objects across a range of evolutionary stages, as indicated by their spectral energy distributions (SEDs), from class I through class III (Lada & Wilking 1984). The resulting compilation contains 269 sources. To facilitate comparisons of the Taurus population to other young clusters, a binary star system in Taurus with a separation less than 2 00 is treated as one object in the remainder of this study (Luhman 2000). After combining this definition of a single source with measurements of separations from Ghez, Neugebauer, & Matthews (1993), Simon et al. (1995), Duchene (1999), White & Ghez (2001), and Woitas, Leinert, & Koehler (2001), we arrive at a list of 228 sources in Taurus. The 183 objects for which spectral types are available are plotted in the map of Taurus in Figure 1. Note that the brown dwarf companion GG Tau Bb is not shown in this map because it is at a distance of less than 2 00 from its primary. For the remainder of x 4, we consider the 123 Taurus members that fall within the fields surveyed by Briceño et al. (1998) and Luhman (2000) and in this work, which are listed in Table 5; thus, our compilation encompasses 54% of the total of 228 systems in Taurus. In order of preference, we adopt the R I colors from White & Ghez (2001), Hartigan, Strom, & Strom (1994), Briceño et al. (1998, 1999), Strom & Strom (1994), and Kenyon & Hartmann (1995); the I-band measurements from White & Ghez (2001), Hartigan et al. (1994), Luhman (2000), Briceño et al. (1998, 1999), Strom & Strom (1994), Kenyon & Hartmann (1995), and this work; and the near-ir data from 2MASS and Kenyon & Hartmann (1995). These data and the available spectral types are presented in Table 5. From this sample, we will construct an H-R diagram and measure an IMF. Ten sources lack accurate spectral types, and three objects have spectral types that are unresolved from their primaries (DK Tau B, IT Tau B, and CoKu Tau 3B). These 13 objects cannot be placed on the H-R diagram. HK Tau B, PSC , and CoKu- Tau/1 are omitted as well from the H-R diagram because they are occulted by edge-on disks or circumstellar material, and thus are observed primarily through scattered light (Stapelfeldt et al. 1998; Koresko 1998; Ménard et al. 2001; Padgett et al. 1999), which precludes estimates of the luminosities of the central stars. Therefore, extinctions, effective temperatures, and bolometric luminosities will be estimated for 107 of the 123 Taurus members in the surveyed fields. The IMF will be measured from a reddening-limited sample containing 86 of the 123 sources, as described in x

10 TABLE 5 Young Sources in Selected Fields of Taurus ID (J2000) (J2000) Reference Spectral Type Reference Adopt Teff a AJ Lbol R I I Reference J H H K s K s In IMF? LkCa M4 2 M yes Anon M0 2 M yes IRAS A no IRAS B no V773 Tau A+B K3, K2 2, 4 K yes FM Tau M0 2 M yes FN Tau M5 2 M yes CW Tau K3 2 K yes CIDA C no MHO M2.5 8 M , no MHO M2.5 8 M , no MHO K7 8 K , no LkCa 3 A+B M1 2 M yes FO Tau A+B M2 2, 9 M yes CIDA M4.5, M5.5 2, 8 M yes 2M J M M yes LkCa K7 2, 9 K yes CY Tau M1, M2, K7 M3 2, 9, 10 M yes LkCa M2 2, 9 M yes V410 X-ray M2,M4 2, 9 M , yes V410 X-ray M4.5 M8.5, M6, M6, M6 10, 9, 11, 8 M , yes V410 Anon M5, M6 M6.5, K2 K6, M6, M , 8, 10, 12, 3 M , yes V410 Anon F9 G3 10 G no V410 Anon K7 M3 10 M no V410 Tau A+B+C K3, K7 2, 9 K yes DD Tau A+B M1, M4, M3 2, 9, 6 M yes CZ Tau A+B M1.5, M3 2, 9 M yes PSC M0.5 M M no V410 X-ray K6 M2 10 M no V410 X-ray M3 M5 10 M no V892 Tau A6, B9 2, 9 B no LR K K no V410 X-ray M1, M0.5 9, 8 M , no V410 Anon K1 K5 10 K no Hubble K7 2, 9 K yes 2M J M M , yes CoKu Tau/ M0, M2 13, 9 M , no PSC M3, K7 M3 13, 10 M , no V410 X-ray M5, M5.5, M2.5 M6.5 9, 14, 10 M , yes V410 X-ray 5a M5, M5.5, M5.5, M3 M5 9, 14, 8, 10 M , yes FQ Tau A+B M2, M4 2, 9 M yes V819 Tau K7 2, 9 K yes LkCa7 A+B K7 2, 9 K yes 2M J M M yes FV Tau A+B K5 2 K no FV Tau/c A+B M3.5 2 M yes 326

11 TABLE 5 Continued ID (J2000) (J2000) Reference Spectral Type Reference Adopt Teff a AJ Lbol R I I Reference J H H K s K s In IMF? DG Tau B no DF Tau A+B M3 2 M yes DG Tau K6, K7 M0 15, 2 K yes 2M J M9.5 þ0:5 0:25 3 M yes IRAS M2 13 M no J M4 2, 8 M yes FW Tau C no FW Tau A+B b M4, M5.5 2, 8 M yes DH Tau M1 2 M yes DI Tau A+B M0 2 M yes 2M J M M yes IQ Tau M0.5 2 M yes 2M J M M yes DK Tau A c K7 2 K yes DK Tau B yes 2M J M M yes MHO M4, M , 3 M , yes MHO M6 M6.5, M7 þ0:25 0:5 8, 3 M , yes L1551/IRS G K 2, no LkHa M5.5, M5 M6 2, 13 M no HH M0? no HL Tau K7 2 K yes XZ Tau A+B M3 2 M yes L1551NE no HK Tau A c M0.5 2 M yes HK Tau B M2 18 M no V710 Tau A M1, M0.5 2, 6 M yes V710 Tau B M3, M2 2, 6 M yes J M5, M5.5 2, 8 M yes L K7 2 K yes V827 Tau K7 2 K yes Haro C no V826 Tau A+B K7 2 K yes MHO M6 M6.5, M , 3 M , yes V928 Tau A+B M0.5 2 M yes MHO M5, M , 3 M , yes MHO M5, M , 3 M , yes FY Tau K5, K7 6, 2 K yes FZ Tau M0, C 6, 2 M yes UZ Tau Ba+Bb M2 6 M yes UZ Tau A M1 2 M yes L K7 2 K yes MHO M5.5, M , 3 M , yes GH Tau A+B M2,M , 5 M yes V807 Tau A+B K7 2 K yes V830 Tau K7 2 K yes IRAS M0 1 3 M no GI Tau K7 M0, K6 6, 2 K yes 327