A NEW CATALOG OF AGB STARS BASED ON INFRARED TWO-COLOR DIAGRAMS

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1 Journal of the Korean Astronomical Society 5: 8, 7 August pissn: 5-6 eissn: 88-89X c 7. The Korean Astronomical Society. All rights reserved. A NEW CATALOG OF AGB STARS BASED ON INFRARED TWO-COLOR DIAGRAMS Kyung-Won Suh and Jinju Hong Department of Astronomy and Space Science, Chungbuk National University, Chungdae-ro, Seowon-Gu, Cheongju 86, Korea; kwsuh@chungbuk.ac.kr Received June 8, 7; accepted July, 7 Abstract: We present a new catalog of AGB stars based on infrared two-color diagrams (CDs) and known properties of the pulsations and spectra. We exclude some misclassified objects from previous catalogs. We identify color areas in two IR CDs where most O-rich and C-rich objects listed in previous catalogs of AGB stars are found. By collecting new objects in these color selection areas in the two IR CDs, we find candidate objects for AGB stars. By using the color selection method, we identify 996 new objects in the O-rich areas, 87 new objects in the C-rich areas, and 95 new objects in the overlap areas of the two CDs simultaneously. We have found that 7 O-rich and 9 C-rich objects are Mira variables with positive spectral identification and they are newly identified AGB stars. We present a new catalog of 88 O-rich AGB stars and 68 C-rich AGB stars excluding misclassified objects and adding newly identified objects. Key words: stars: AGB and post-agb dust, extinction circumstellar matter infrared: stars. INTRODUCTION Asymptotic giant branch (AGB) stars are generally believed to be in the last evolutionary phases of a low mass star (M M ). Most AGB stars are longperiod variables (LPVs) which show large amplitude pulsations. The cool envelopes around AGB stars are believed to be the main site of dust formation (e.g., Suh ). Based on chemical properties of the photosphere or the outer dust envelope, AGB stars are classified as oxygen-rich or carbon-rich. The AGB stars with approximately equal quantities of C and O are classified as S type stars. O-rich AGB stars show µm and 8 µm features in emission or absorption due to amorphous silicate dust grains in the outer envelopes (e.g., Jones & Merrill 976; Suh 999). Dust envelopes around C- rich AGB stars are believed to have amorphous carbon dust grains which do not produce special features and SiC grains which produce the. µm emission feature (e.g., Suh ). The Infrared Astronomical Satellite (IRAS) and AKARI space telescope (Murakami et al. 7), which carried out all sky surveys, provided infrared (IR) observational data for a large sample of AGB stars. The IRAS point source catalog (PSC) provides photometric data in four bands (, 5, 6, and µm) and AKARI PSC contains photometric data in two bands (9 and 8 µm). The Two-Micron All-Sky Survey (MASS; Cutri et al. ) PSC provides fluxes in J (.5 µm), H (.65 µm) and K (.7 µm) bands. The angular resolutions of the IRAS, AKARI, and MASS images are , 9. (in the 9 µm band), and, respectively. Corresponding author: K.-W. Suh IR two-color diagrams (CDs) have been used in studying the structure and evolution of many types of celestial objects (e.g., van der Veen & Habing 988; Suh & Kwon ). The IR CDs were useful in characterizing changes of the properties of the central stars and circumstellar envelopes during the evolution from AGB stars to planetary nebulae (PNe) also (e.g., Suh 5). In this paper, we present a new catalog of AGB stars based on IR CDs. We find the color areas in two IR CDs where most O-rich and C-rich objects listed in previous AGB star catalogs (e.g., Suh & Kwon ) are located. By collecting new objects in the color selection areas in the two IR CDs, we may find new candidate objects for AGB stars. We identify new AGB stars by verifying the properties of the pulsations and spectra for the candidate objects.. SAMPLE OF AGB STARS AGB stars show characteristic spectral features from the central stars (identified by the optical spectra) or dust and gas envelopes (identified by the IR spectra or radio maser emission) and most AGB stars can be identified as LPVs (e.g., Kwon & Suh ; Suh )... Search for Information on AGB Stars In identifying important dust features for AGB stars, IRAS Low Resolution Spectrograph (LRS; λ = 8 µm) data are useful (Kwok et al. 997). Most O- rich AGB stars are class E (the µm silicate feature in emission) or class A (the µm silicate feature in absorption) objects and typical C-rich AGB stars are class C (the. µm SiC feature in emission) objects (see Section ). Kwok et al. (997) also provide useful information on the optical spectra for the IRAS LRS sources.

2 Suh & Hong y = (.687 ±.5) x + (-.67 ±.) [5] - [6] x = (-. ±.7) y + (.59 ±.) y = (.89 ±.) x + (.957 ±.) - O-rich AGB stars (58) C-rich AGB stars (Group A; ) C-rich AGB stars (Group B; 679) C-rich AGB stars (Group ; 7) 5 5 y = (.66 ±.9) x + (.6 ±.5) y = (. ±.58) x + (.79 ±.8) : for all C-rich AGB stars O-rich AGB stars (58) C-rich AGB stars (Group A; ) C-rich AGB stars (Group B; 679) C-rich AGB stars (Group ; 7) K - [] Figure. IR CDs for the new sample of AGB stars. The least-square fit lines are shown for each class of AGB stars. The general catalogue of variable stars (GCVS; Samus et al. ) contains the list of LPVs for different variable types and provides information on their pulsation properties and optical spectroscopic types. LPVs in AGB phase are classified into various types (Miras, semi-regulars, irregular, and RV Tauri) according to the amplitude and regularity of the period. The American association of variable star observers (AAVSO) international Variable star index (VSX; Watson et al. 7) also provides updated information on the pulsation properties of LPVs. Using the IRAS PSC name, we find the IRAS LRS class and optical spectral type from Kwok et al. (997). Using the position of the object, we find the correspond-

3 A New Catalog of AGB Stars Based on Infrared Two-Color Diagrams Table objects to be excluded from the previous catalogs of AGB stars IRAS PSC AKARI PSC Other Name Remark Reference O-rich AGB 5+ + GI Tau T Tauri star Carr & Nafita () IC interstellar matter Reipurth & Aspin (997) AFGL 96 YSO Robinson et al. () PN Vo PN Kohoutek () MR 7 Wolf-Rayet star Roberts (96) NGC 65 PN Kohoutek () NGC 6 PN Kohoutek () PN Th - PN Kohoutek () 7 88 Quintuplet cluster open cluster Shin & Kim (6) PN RPZM 9 PN Kohoutek () GAL HII region Koo et al. (996) 86 GAL.-. HII region Woodward et al. (98) PN Vy - PN Kohoutek () OY CMa S type star Chen & Kwok (99) R Cyg S type star Wallerstein et al. () C-rich AGB PN M -9 PN Kohoutek () PNG PN Sahai et al. () DY Gem S type star Ramstedt et al. (9) AC Cas M type star GCVS (SRA) M type star Kwok et al. (997) M type star Kwok et al. (997) V7 Cyg M type star GCVS (SRB:) V5 Lac M type star GCVS (PPN) RU Aqr M type star GCVS (SRB) ing AAVSO and GCVS sources (see Section.)... Cross-Identification Using the position given in the IRAS PSC (version.), we cross-identify the AKARI PSC counterpart by finding the nearest source within 6 for each object. If we use a smaller angular distance limit (e.g., ), we may miss the AKARI source for the same object of both AKARI and IRAS PSC sources with large angular separations (larger than ). This is because the IRAS beam size is very large (.75.5 at µm). Using the position of the AKARI PSC counterpart, we cross-identify MASS counterpart and find corresponding AAVSO and GCVS sources for each object (see Section.). This method decreases the possibility of finding wrong counterparts (MASS, AAVSO, and GCVS). When the AKARI PSC counterpart is not available, we use the position in the IRAS PSC for the object... Previous Catalogs of AGB Stars and Verification Suh & Kwon () presented a catalog of AGB stars which contains O-rich, 68 C-rich, 6 S type, and 5 silicate carbon stars in our Galaxy. Kwon & Suh () presented a revised list of 7 O-rich AGB stars. Kwon & Suh () presented a revised catalog of 9 silicate carbon stars, from which two of them are post-agb stars. We re-examine O-rich and C-rich objects in the previous catalogs of AGB stars by verifying the spectroscopic and pulsational properties (see Section.) and looking into more references. We find that 5 objects from the O-rich AGB star list and 9 objects from the C-rich AGB star list were obviously misclassified. Most of them are young stellar objects (YSOs), planetary nebulae (PN), or S type stars. We also exclude the six objects, which were classified to be C-rich AGB stars, because they show the M type optical spectra and it is not certain whether they are AGB stars. We list the misclassified objects in Table. We exclude them from the previous catalogs of AGB stars. Even though each class of heavenly bodies occupies roughly a specific area in a CD (e.g., van der Veen & Habing 988), it is difficult to identify them just by using the CD. Significant portions of evolved stars (Orich and C-rich AGB stars, S type stars, and PNe) share similar colors in IR CDs (see Suh & Kwon ; Suh 5). It is difficult to distinguish YSOs from evolved stars on an IR CD because they share similar properties of the dust envelopes and central stars... The New Sample of AGB Stars Upon excluding the misclassified objects (see Section.) from the previous catalogs of AGB stars (7 O- rich and 68 C-rich objects), the new sample of AGB stars have 58 O-rich and 59 C-rich objects. Table lists the new sample of AGB stars. The

4 Suh & Hong Table The new sample of AGB stars Class No. AKARI Mira E class A class C class M type 5 C type 5 M type 6 C type 6 O-AGB 58 7 (97) 5 () 85 (8) 88 (68) (9) C-AGB 59 (75) 7 () 98 () () Number of IRAS PSC sources; number of cross-identified AKARI PSC sources; Mira variables from AAVSO and GCVS; IRAS LRS spectral class; 5 optical spectral type from GCVS for the object without IRAS LRS identification (E, A, or C); 6 optical spectral type from Kwok et al. (997) for object without IRAS LRS or GCVS spectral identification; the number of Mira variables is parenthesized. table also lists the numbers of the objects in each class (or type) for which the spectral or pulsational information is collected from IRAS LRS, Kwok et al. (997), GCVS, and AAVSO (see Sections. and. for identification). 97 Mira variables are identified from the 58 O-rich AGB stars mainly based on optical observations (GCVS and AAVSO). Many Mira variables with thick dust envelopes could be missed from the GCVS and AAVSO because they cannot be observed in optical wavelength bands..5. IR Two-Color Diagrams Figure shows the IR CDs for the new sample of AGB stars (58 O-rich and 59 C-rich objects; see Section.). For these stars, we obtain photometric data in three bands (, 5, and 6 µm) of the IRAS PSC and MASS data (see Section. for cross-identification) in K (.7 µm) band. Generally, the stars in the upperright region of the CDs have thick dust shells with large optical depths. The color index is defined by M λ M λ =.5 log F λ /ZMC λ F λ /ZMC λ () where ZMC λi is the zero magnitude calibration at given wavelength (λi) (see Suh & Kwon ). In plotting the IR CDs, we use only those objects with good quality observational data in all wavelength bands. The upper panel of Figure plots AGB stars in an IRAS CD using [5] [6] versus [] [5]. van der Veen & Habing (988) divided this CD into eight regions so that objects in each region share similar characteristics. The lower panel of Figure plots AGB stars in an IRAS-MASS CD using [] [5] versus K []. O-rich and C-rich AGB stars are distinguished more clearly on this CD because silicate dust produces larger [] [5] for given K [] than carbon dust (e.g., Suh & Kwon )..6. Fitting Lines in IR CDs To find a general trend in observed data in an IR CD for each class of AGB stars, we use a simple fitted line. For the least-squares line fitting, we use the Marquardt- Levenberg algorithm to find the coefficients of the individual variables (Press et al. 986) of the following equation: y = ax + b, () where x is the horizontal axis and y is the vertical axis. We find the best fit parameters (a and b) for each class of AGB stars on a CD. We perform the line fitting using the same weighted value for all the observational data. In Figure, the fit parameters as well as the fitted lines are shown in the two CDs. For O-rich stars in both panels and C-rich stars in the lower panel, we use a single fitted line with some vertical dispersion. But in the upper panel of Figure (the IRAS CD using [5] [6] versus [] [5]), C-rich AGB stars are distributed roughly along the shape of a L. Therefore, we use two separate fitted lines for different groups of C-rich AGB stars for this CD only. Stars in Group A are located in the left region ([] [5] <.) and are fitted by an almost vertical line, while stars in Group B are located in the right region ([] [5].) and are fitted by an almost horizontal line. Stars in Group do not have good quality data to be plotted on the CD. Note that this split of C-rich AGB stars into groups is used only for finding the fitted lines in the IRAS CD using [5] [6] versus [] [5]. Group A stars consist of optical carbon stars, among which are the objects with large [5] [6] colors that show excessive fluxes at 6 µm due to the remnant of an earlier phase when the stars were O-rich AGB stars (e.g., Chan & Kwok 99). Group B stars in the lower region, which extend to the right side, are infrared carbon stars. The infrared carbon stars on the right side have thick carbon dust envelopes with large optical depths (see Suh & Kwon ). Minor members of Group B stars in the upper region could be in the transition phase from O to C because the thicker remnants of O-rich dust would produce excessive fluxes at 6 µm as well as larger [] [5] colors (see Kwon & Suh ).. NEW AGB STAR CANDIDATES For many studies since the IRAS mission, the IR CDs have been very useful to make color selection and rough classification for many types of heavenly bodies: AGB stars, post-agb stars, PNe, YSOs, and galaxies (van der Veen & Habing 988). IR CDs can be useful to distinguish statistically between O-rich and C-rich AGB stars (e.g., Suh & Kwon ). We may use IR CDs to find new candidate objects for AGB stars... Color Selection Using two IR CDs For the color selection of new candidate objects, we use two different IR CDs (the IRAS CD using [5] [6]

5 A New Catalog of AGB Stars Based on Infrared Two-Color Diagrams 5 y = [.687 x -.67] ±. [5] - [6] x = [-. y +.59] ±.88 y = [.89 x +.957] ±. - O-rich AGB star (58) C-rich AGB star (59) O-rich candidates (996) C-rich candidates (87) overlap candidates (95) 5 5 y = [.66 x +.6] ±.7 y = [. x +.79] ±.89 O-rich AGB star (58) C-rich AGB star (59) O-rich candidates (996) C-rich candidates (87) overlap candidates (95) K -[] Figure. Known AGB stars and new candidate objects in the color selection areas. versus [] [5] and IRAS-MASS CD using [] [5] versus K []) simultaneously to make the process more reliable and efficient. After the color selection of the new candidate objects, we may use further verification process using other known information on the spectra and pulsations. Two panels of Figure show the color selection areas, which use the fitted lines obtained for the new sample of AGB stars (see Section.6). The CDs also show the new candidate objects in the areas as we will explain in Section.. For O-rich stars in both panels of Figure and C- rich stars in the lower panel, we use the selection area for which the fitted line (see Section.6) extends by

6 6 Suh & Hong Table New AGB star candidates CDs No. AKARI Mira E class A class C class M type 5 C type 5 M type 6 C type 6 O-rich (6) 7 () () () 8 () 5 (7) C-rich 87 7 (58) 87 (8) 8 () (5) 9 () 9 (5) 5 (5) overlap 95 9 (7) 7 () (8) 7 (8) Nnumber of new IRAS PSC sources in the color selection; number of cross-identified AKARI PSC sources; Mira variables from AAVSO and GCVS; IRAS LRS spectral class; 5 optical spectral type from GCVS for the object without IRAS LRS identification (E, A, or C); 6 optical spectral type from Kwok et al. (997) for object without IRAS LRS or GCVS spectral identification; the number of Mira variables is parenthesized; the number of newly identified AGB stars is bold-faced. ±σ (the standard deviation) in the vertical direction (see Figure ). For C-rich AGB stars in the upper panel (the IRAS CD using [5] [6] versus [] [5]), we use two color selection areas based on the two obtained fitted lines (see Section.6): the area in the left region for which the almost vertical fitted line extends by ±σ in the horizontal direction and the area in the right region for which the almost horizontal fitted line extends by ±σ in the vertical direction. Note that we use the larger extension (±σ) in the horizontal direction because Group A stars show very small dispersion in the horizontal direction (see Figure ). We find that major portions of known AGB stars are located in the color selection areas and expect that new objects in these areas would be possible candidate objects for AGB stars... New Candidate Objects We look up all the 5,889 IRAS PSC sources (and 59,87 cross-identified AKARI sources; see Section.) to find candidate objects observed in the color selection areas of AGB stars of the two IR CDs simultaneously (see Section.). We find that 579 objects are in the O-rich areas, 7 objects are in the C-rich areas, and 77 objects in the overlap areas of the two IR CDs simultaneously (see Figure ). Excluding the AGB stars known from previous catalogs (58 O-rich, 59 C-rich, 6 S-type stars, and 9 silicate carbon stars; see Sections. and.) located in these color selection areas, there are 996 new objects in the O-rich areas, 87 new objects in the C- rich areas, and 95 new objects in the overlap areas of the two CDs simultaneously. These are new candidate objects for AGB stars. Figure shows these new candidate objects (996 O-rich; 87 C-rich; 95 overlap) in the color selection areas... New Identifications of AGB stars To make firm identification of AGB stars for the new candidate objects, we use further verification processes using known information on the pulsations and spectra. Among many types of LPVs with large amplitude pulsation, Mira type variables are known to be most close to the typical nature of AGB stars (e.g., Suh ). Table The new catalog of AGB stars Class Number O-AGB previous 7 () excluded 5 () the new sample 58 (7) new candidates 996 (856) newly identified 7 (67) total 88 (67) C-AGB previous 68 (5) excluded 9 (9) the new sample 59 () new candidates 87 (7) newly identified 9 (9) total 68 (5) Number of IRAS PSC sources (numbers of cross-identified AKRAI PSC sources are parenthesized); this work. The objects of E or A classes from the IRAS LRS spectra have O-rich dust envelopes and C class objects have C-rich dust envelopes (see Section.). The objects of optical spectral type M (or C) have O-rich (or C-rich) stellar atmospheres. Table lists the new candidate objects for AGB stars. It lists the numbers of the objects in each class (or type) for which the spectral or pulsational information is collected from IRAS LRS, Kwok et al. (997), GCVS, and AAVSO (see Sections. and. for identification). We find that 9 objects from the 996 new candidate objects with O-rich colors (see Section.) are Mira variables with O-rich spectra (IRAS LRS class E or A; M type from the optical spectroscopy). 9 objects from the 87 new candidate objects with C-rich colors are Mira variables with C-rich spectra (IRAS LRS class C; C type from the optical spectroscopy). And 79 objects from the 95 new candidate objects with overlap colors are Mira variables with O-rich spectra. These 79 Mira variables (7 objects with O-rich spectra and 9 objects with C-rich spectra) are the best candidate objects for AGB stars or newly identified AGB stars. Table summarizes the final results. The Mira variables in the color selection without spectroscopic identification ( candidate objects with O-rich colors, 9 objects with C-rich colors, and 8 objects with overlap colors) are also likely to be AGB stars. Figure shows the newly identified AGB stars (7 O-rich and 9 C-rich objects) on the IR CDs as well as

7 A New Catalog of AGB Stars Based on Infrared Two-Color Diagrams 7 [5] - [6] - O-rich AGB stars (58) C-rich AGB stars (59) O-rich newly identified (7) C-rich newly identified (9) 5 5 O-rich AGB stars (58) C-rich AGB stars (59) O-rich newly identified (7) C-rich newly identified (9) K - [] Figure. IR CDs for known AGB stars and newly identified AGB stars. the previously known AGB stars (58 O-rich and 59 C-rich objects; see Section ). Finally, we present a new catalog of 88 O-rich AGB stars and 68 C-rich AGB stars. Among the newly identified C-rich AGB stars (the 9 objects with C-rich colors), there is one object in the overlap area for both of the two IR CDs in Figure. Note that these two observed points are for different objects. This is because we find the new candidate objects of which the observed points are inside each area (O-rich, C-rich, or overlap) of the two IR CDs simultaneously for the color selection (see Sections. and.).

8 8 Suh & Hong. SUMMARY In this work, we present a new catalog of AGB stars based on IR CDs. Using various IR CDs and known properties of the pulsations and spectra, we find new candidate objects for AGB stars. We have found that 5 objects from the list of O- rich AGB stars and 9 objects from the list of C-rich AGB stars were misclassified in the previous catalogs of AGB stars. When the misclassified objects are excluded, the new sample of AGB stars have 58 O-rich and 59 C-rich objects. To find new candidate objects for AGB stars, we have used the color selection method in two IR CDs simultaneously: the IRAS CD using [5] [6] versus [] [5] and IRAS-MASS CD using [] [5] versus K []. We have found the color areas in the two CDs where most O-rich and C-rich objects listed in previous catalogs of AGB stars are located. We have found new candidate objects for AGB stars by collecting new objects in the color selection areas in the two IR CDs. 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