Low resolution spectroscopy of hot post-AGB candidates II. LS, LSS, LSE stars and additional IRAS sources
aa r X i v : . [ a s t r o - ph . S R ] D ec Low resolution spectroscopy of hot post-AGB candidatesII. LS, LSS, LSE stars and additional IRAS sources ∗ Mudumba
Parthasarathy,
John S.
Drilling, J. Vijapurkar, and Yoichi Takeda National Astronomical Observatory of Japan (NAOJ)2-21-1 Osawa, Mitaka, Tokyo 181-8588, [email protected] McDonnell Center for the Space Sciences, Department of Physics,Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA Aryabhatta Research Institute of Observational Sciences,Nainital, 263129, India Department of Physics and Astronomy, Louisiana State University,Baton Rouge, Louisiana 70803, USA Homi Bhabha Center for Science Education, Tata Institute for Fundamental Research,V.N. Purav Marg, Mankhurd, Mumbai 400088, India (Received ; accepted )
Abstract
Hot (OB) post-AGB stars are immediate progenitors of planetary nebulae (PNe).Very few hot post-AGB stars are known. Detecting new hot post-AGB candidates andfollow-up multiwavelength studies will enable us to further understand the processesduring the post-AGB evolution that lead to the formation of PNe. Case-HamburgOB star surveys and their extension (LS, LSS, and LSE catalogues) and IRAS (pointsource) catalogues are good sources for detecting new hot post-AGB candidates fromlow resolution spectroscopy. Spectral types are determined from low resolution opticalspectra of 44 stars selected from the LS, LSS, and LSE catalogues. Unlike the starsin the first paper, the stars in this paper were selected using criteria other thanpositional coincidence with an IRAS source with far IR (IRAS) colours similar topost-AGB supergiants and planetary nebulae. These included high galactic latitude,spectral types of O, B, A supergiants, emission lines in the spectrum and knownspectral peculiarity. From the present study we find that LSS 1179, LSS 1222, LSS1256, LSS 1276, LSS 1341, LSS 1394, LSS 2241, LSS 2429, LSS 4560, LSE 16, LSE31, LSE 42, and LSE 67 to be new hot post-AGB candidates. Further study of thesecandidates is needed.
Key words: stars: AGB and post-AGB — stars: evolution — stars : early-type1 stars: emission-line — stars: classification — stars: atmospheres
1. Introduction
From an analysis of the IRAS point source catalogue data, several post-asymptotic giantbranch (post-AGB) candidates were detected (Parthasarathy & Pottasch 1986; Parthasarathy& Pottasch 1989; Pottasch & Parthasarathy 1988; Parthasarathy 1993; Kwok 1993; Van Winckel2003). These stars have far-infrared (IRAS) colours similar to proto-planetary nebulae (PPNe)and planetary nebulae (PNe) (van der Veen & Habing 1988; Pottasch et al. 1988). Several ofthem are at high galactic latitude, like HD 161796 (F3Ib) (Parthasarathy & Pottasch 1986)and LS II + 34 26 (B1Iae) (Parthasarathy 1993). Multiwavelength studies of these objectsby several investigators have confirmed that these are indeed in post-AGB stage of evolution(Kwok 1993; Van Winckel 2003). The post-AGB stars seem to form an evolutionary sequence( K, G, F, A to OB supergiant types) in the transition region from the tip of the AGB to theearly stages of planetary nebulae (Parthasarathy 1993; Parthasarathy et al. 1993). In orderto understand the evolution from cooler to hotter post-AGB types and then into the youngPN stage, it is important to detect and study several hot post-AGB candidates. Vijapurkaret al. (1997); Vijapurkar et al. (1998) and Parthasarathy et al. (2000) presented the results oflow resolution blue spectra of 40 IRAS sources with far-IR colours similar to PNe and PPNe(van der Veen & Habing 1988; Pottasch et al. 1988). A few of these objects have shown rapidchanges in MK spectral type (Parthasarathy et al. 1993; Parthasarathy et al. 1995).The importance of the detection of new hot post-AGB candidates relies in objects such asSAO 244567 (Hen 3-1357) and LS II +34 26. SAO 244567 evolved rapidly from a B-type post-AGB star into a young PN within a period of 20 years (Parthasarathy et al. 1993; Parthasarathyet al. 1995; Bobrowsky et al. 1998). The variations in the spectrum of SAO 244567 weredramatic. Another star LS II +34 26 initially classified as a massive B supergiant (Turner& Drilling 1984) turned out to be a rapidly evolving B-type post-AGB star (Parthasarathyet al. 1993; Smith & Lambert 1994; Garcia-Lario et al. 1997). Other such object are LSE 162(SAO 85766) (Volk & Kwok 1989; Arkhipova et al. 2007) (references therein) and LSIV -12111 (Conlon et al. 1993). Another reason to detect more hot post-AGB candidates is theirabundance peculiarities (McCausland et al. 1992) which are different when compared with thechemical composition of cooler post-AGB stars (Van Winckel 2003). Very few hot post-AGBstars are known and most of them are at high galactic latitudes and several of them are notIRAS sources (McCausland et al. 1992; Moehler & Heber 1998). We started a program toidentify new hot post-AGB candidates by obtaining spectra of selected stars from LS, LSS andLSE catalogues (Hardrop et al. 1959; Stephenson & Sanduleak 1971; Drilling 1994; Drilling ∗ Based on observations obtained at the Cerro Tololo Inter-American Observatory (CTIO), Chile.
2. Selection criteria
The observing list consisted of stars selected from the LS, LSS, and LSE catalogues(Table 1). The stars selected are either IRAS point sources and or O, B and A supergiants athigh galactic latitudes, according to MK types given in the literature. The sample is biasedtowards stars which show some post-AGB characteristic, i.e. positional coincidence with anIRAS point source with far-IR colours similar to post-AGB stars and PNe (van der Veen& Habing 1988; Pottasch et al. 1988), high galactic latitude or known spectral peculiarity,luminous O, B, and A spectral types, and emission lines in the spectrum. Our sample includes29 stars, which are not IRAS sources so that we may be able to detect, post-AGB stars withoutdust shells, similar to BD+ 39 4926 (Kodaira 1973). We selected 38 stars from the LSScatalogue, 4 stars from LSE catalogue and two stars from LS catalogue. In the total selectedsample of 44 stars only 15 stars are IRAS sources (Table 2, Figure 1).
3. Observations
Digital spectra of 44 selected (Table 1) southern candidates were obtained during 19thto 27th April 1994 using the spectrograph and 2d-Frutti two-dimensional photon countingdetector on the 1m telescope of the Cerro Tololo Inter-American Observatory in Chile. Theexposure times ranged from 10 minutes to 30 minutes. Because of limited observing time at ourdisposal we have observed the selected stars only once. The wavelength coverage is 3800˚A to5000˚A, and judging from the comparison spectra, the resolution is about 3.5 ˚A. The data wereextracted, wavelength calibrated, and normalized to the continuum with the standard IRAFsoftware. Spectra of all the 44 stars are shown in Figure 2. Results of analysis of spectra of40 IRAS sources obtained during the above mentioned observing dates were given in paper - I(Parthasarathy et al. 2000). 3 . Analysis
We have compared the spectra of our program stars with the spectra of standard OBstars (Walborn & Fitzpatrick 1990). Walborn and Fitzpatrick made a digital atlas of the spectraof OB stars which they observed with the same instrument, but at higher resolution than thatdescribed above. We found that smoothing the Walborn and Fitzpatrick spectra by 3.5 ˚Aproduced nearly identical looking spectra for the O9.5V star HD 37468, which we observed,and the Walborn and Fitzpatrick O9.5V standard HD 93027. The errors in the spectral atlas ofWalborn and Fitzpatrick are of the order of 0.2 to 0.3 subtypes. Among the O-stars they couldclassify O9.5 and O9.7 and among B stars they could classify B0.5 and B0.7. Spectral types fornon OB stars were estimated by comparison with the photographic atlas of Yamashita et al.(1978). The spectral types determined from the present investigation are given in Table 1. Fornon-OB stars we have also used the digital spectral atlases of Silva & Cornell (1992), Jacobyet al. (1984), and Pickles (1998), however these atlases are of much lower resolution than ourspectra. The errors in our spectal types that we gave in Table 1 are of the order of 0.3 to 0.5subtypes. For example we were able to distinguish spectral differences among B1, B2, and B3stars. The letters e, f, p, and n by the side of spectral types in Table 1 are of standrard MKKnotation. Letter ”e” indicates emission line(s) in the spectrum, ”f” indicates O-type star withemission lines, (f) indicates N III emission is present and the notation ((f)) signifies that inaddition to strong He II 4686˚A weak N III λλ In our sample of stars there are only 15 IRAS sources and they are listed in Table 2.Their IRAS fluxes from SIMBAD are also listed in Table 2. The letter ”L” on the side of someof the fluxes indicates that the error in the flux value is large. We have not listed the 100 micronflux in Table 2 for some of the objects as their 100 micron flux is not relaiable (see SIMBADdata base). In Figure 1 we show the location of the objects (fillled circles) listed in Table 2 inthe IRAS colour-colour diagram (Pottasch et al. 1988). The location of the star LSS 207 is notshown in Figure 1 as its 60 micron flux quality is very low and therefore the 25 to 60 micron fluxratio is beyond the X-axis scale. The Figure 1 is adopted from the paper by (Pottasch et al.1988). The 14 objects (Table 2) are in the region defined by PNe. PNe, PPNe and post-AGBsupergiants have similar IRAS colours (Pottasch et al. 1988) and occupy the same region inthe colour-colour diagram defined by the PNe (Figure 1). Most of the stars listed in Table 24 able 1.
Spectral types of LS, LSS and LSE stars based on our spectra
No. Star b Sp. New Sp.Type m V comment1 LSS 3169 -4.24 W(C) pec.em. 13.2 PN [WC9]2 LSS 3299 +3.99 WRh pec.em. 11.9 PN [WC11]3 LSS 207 -4.40 OB+ O6V((f)) 10.9 Post-AGB ?4 LSS 3888 -5.14 OB+ O6V((f))e 12.6 PN5 LSS 827 +0.36 OB O6Vn 9.2 O6:nne., in nebulosity6 LSS 3119 +0.04 OB O8Iaf 9.2 HD117797(Oe); O8.57 LSS 3418 -9.66 OB:(ce),lep,h O9Iae 11.0 HD 141969; PN8 LSE 67 -13.58 OB+ O9IIe 12.2 -29 15495, PN, post-AGB9 LSS 1448 -0.03 OB+r O9.5III 11.0 CD -55 319610 LSS 1947 -0.51 OB O9.5V 10.1 HD 30559911 LSS 4349 +3.89 OB B0III 9.6 -22 4400, Herbig Ae-Be12 LSS 2354 -1.55 OB- B0V 9.6 HD 99898; B0.5V:, HII13 LSS 1394 -8.23 OB+ce,h B2:nep 10.5 CPD -64 1154, Post-AGB?14 LSS 2241 -10.58 OB+ B1Ib 10.1 CD -71 730, Post-AGB15 LSE 42 +14.59 OB+ B1Ib 12.7 post-AGB16 LSS 1245 -6.57 OB B1III-V 11.4 CD -56 260317 LSS 1021 -1.18 OBce,h B1II-Vne 9.1 HD 69425; B1Vpe18 LSS 968 +8.21 OB- B1V 10.7 -17 235719 LSS 3434 -0.46 OBh B1Vn 11.1 -53 6867, Herbig Ae-Be20 LSS 1256 -6.28 OB+ce,le,h B2ne 12.2 Post-AGB?21 LSS 1341 -10.68 OB(ce) B2ne 9.6 HDE 309784; Post-AGB22 LSS 2429 +7.51 OB+h B2:ne 12.7 Flat continuum; Post-AGB?23 LSS 1276 +5.37 OB+h B2:nep 9.8 HD80834; B5nne, post-AGB?24 LSS 866 -5.94 OB- B2III 8.5 -39 3775 = HD 6505425 LSS 1263 +6.72 OB- B2IIIn 10.1 -38 541026 LSVI+5 5 -2.09 OB B2IIIn 7.8 +5 127927 LSS 1060 -6.20 OB+ B2III-V 12.728 LSS 1367 +4.37 OB B2III-Ve 12.0 CD -48 510329 LSS 327 -1.76 OB B2V 12.230 LSS 2832 +0.79 OB+ B2:V: 13.031 LSS 1339 -7.34 OB-h B2Ve 10.7 CPD -62 129032 LSS 1392 -5.96 OB+ce,le,h B2Ve 10.7 HD 30746733 LSS 1996 -9.91 OB B2Ve 11.6 CPD -69 141734 LSS 1213 +1.90 OB- B2Vp 9.7 CD -42 481935 LSS 1179 -1.59 A1Ia:h B3Ibp 11.4 CD -46 4657, post-AGB36 LSE 3 +12.26 OB+ B3IIIe 11.5 BD-18 4436, post-AGB37 LSS 4560 -7.37 OB B3IIIep 11.3 Hen 3 - 1557, Post-AGB38 LSS 1222 -7.10 B7I-II B9Iap 11.6 Post-AGB39 LSS 1340 -3.19 A1II A1II 11.4 PN, Binary Central Star40 LSS 3309 +8.65 A5Iab A3I 7.6 HD 133656, post-AGB41 LSE 16 +8.88 OB+ A3I 12.0 LSS 4079, post-AGB42 LSVI+10 15 9.99 F5I F5Ia 8.1 +10 1470, post-AGB43 LSS 1033 +7.49 OB: F7V 13.544 LSS 1120 +7.22 OB+ F7V 12.9 able 2. Stars which are IRAS sources
Star IRAS 12 µ m (Jy) 25 µ m (Jy) 60 µ m (Jy) 100 µ m (Jy)LSS 207 IRAS 07077-1825 0.80 6.66 0.40L —–LSS 827 IRAS 07502-2618 8.49 69.32 183.30L —–LSS 1179 IRAS 08487-4623 0.25L 0.91 2.12 ——LSS 1340 IRAS 09418-5703 0.42 4.88 6.40 —–LSS 2354 IRAS 11265-6239 1.37L 11.38 28.98L —–LSS 3169 IRAS 13487-6608 1.19 9.16 11.75 —–LSS 3299 IRAS 14562-5406 92.41 310.50 176.60 71.30LSS 3309 IRAS 15039-4806 0.25L 4.29 3.61 —–LSS 3418 IRAS 15513-6600 2.16 48.18 43.81 19.58LSS 3434 IRAS 15543-5342 3.65 11.86 26.55L —-LSS 3888 IRAS 16577-5018 0.25L 1.05 1.99 —LSS 4349 IRAS 17408-2204 1.61 13.28 33.60 44.18LSS 4560 IRAS 17591-3731 0.35L 1.22 0.91 —LSE 3 IRAS 17074-1845 0.50 12.20 5.66 3.47LSVI +10 15 IRAS 07134+1005 24.51 116.70 50.13 18.72 (Figure 1) are classified as post-AGB objects (Table 1). The evolutionary status of LSS 827,LSS 2354, and LSS 4349 is not clear. An occasional H II region, or a nebula or a T Tau orHerbig Ae-Be star have IRAS colours similar to PNe and post-AGB objects (Pottasch et al.1988). Notes on individual IRAS sources is given in the next section. β . We also find emission lines at 4934˚A and 5009˚A which may be of circumstellar ornebular origin. In the Michigan spectral classification catalogue the spectral type of LSS 827 isgiven as A1(Ia)p. Spectral type variations appear to be present. It may be an early type shellstar. It is in the region of the open cluster NGC 2467 and is involved in nebulosity. Clustermembership is not certain. ig. 1. IRAS colour-colour diagram adopted from the paper by Pottasch et al. (1988). The filled circlesare the objects listed in Table 2
000 4500 5000Wavelength (Å)
LSS 3169 (pec. em).LSS 3299 (pec. em.)LSS 207 (O6Vf)LSS 3888 (O6Vfe)LSS 827 (O6Vn)LSS 3119 (O8Iaf)LSS 3418 (O9Iae)LSE 67 (O9IIe)LSS 1448 (O9.5III)LSS 1947 (O9.5V)LSS 4349 (B0III)LSS 2354 (B0V)LSS 1394 (B1Ib)LSS 2241 (B1Ib)LSE 42 (B1Ib)LSS 1245 (B1III-V)LSS 1021 (B1II-Vne)LSS 968 (B1V)LSS 3434 (B1Vn)LSS 1256 (B2ne)LSS 1341 (B2ne)LSS 2429 (B2;ne)(+20Å) H b H g H d H e H8 He HeHe Si ++ He + Si +++ HeHe + N + H9 [O ++ ]1.0 LSS 1276 (B2:nep)LSS 866 (B2III)LSS 1263 (B2IIIn)LSVI+5 5 (B2IIIn)LSS 1060 (B2III-V)LSS 1367 (B2III-Ve)LSS 327 (B2V)LSS 2832 (B2V)LSS 1339 (B2Ve)LSS 1392 (B2Ve)LSS 1996 (B2Ve)LSS 1213 (B2Vp)LSS 1179 (B3IIIe)LSE 3 (B3IIIe)LSS 4560 (B3IIIep)LSS 1222 (B9Iap)LSS 1340 (A1II)LSS 3309 (A3I)LSE 16 (A3I)LSVI+10 15 (F5Ia)LSS 1033 (F7V)LSS 1120 (F7V)Si + HeCa + Ca + H e H d H g H b Ca [O ++ ]H8H9 1.0 Fig. 2.
Specra of 44 stars (Table 1) (normalized with respect to the continuum level), based on whichwe conducted spectral classifications. Each of the spectra are arranged according the spectral type, andshifted by 1.0 relative to the adjacent ones. Positions of lines important for spectral classifications (e.g.,Balmer lines; He i ii ii iii ] 4959/5007; Si ii iii iv i II iii ] emission in LSS 1340. β and H γ are in emission. The higher members of the Balmer series also appear to beaffected by emission. β and H γ are in emission. Other higher members of the Balmer series appear to befilled in by emission. It is classified as a variable star QQ Vel. δ are in emission. β and H γ are in emission. HeI lines also appear to be affected by emission. Many of theH and HeI lines seems to have emission in the cores. High galatic latitude and the appearenceof the spectrum suggests that it may not be a massive pop. I OB star. It may be a post-AGBstar. β and H γ lines appear to be affected by emission. β and H γ lines may be affected by emission. β and H γ are in emission. Other members of the Balmer series also appear to beaffected by emission. The spectrum is similar to that of LSS 1394. β is in emission. H andHeI lines are partially filled in. The far-IR colours, high galactic latitude and B3IIIep spectraltype indicate that it is most likely a post-AGB star. UBV photometric observations were madeby Drilling (1991). β is filled in. The B1Ib spectral type and high galactic latitude indicate that it maybe a post-AGB star. It may not be a massive pop. I B star. β in emission. Based on these emission lines, we conclude that itmay be a new low excitation planetary nebula. The spectral type of the central star is foundto be O9IIe. . Discussion and Conclusions The presence of circumstellar dust with far-IR colours similar to PNe, high galacticlatitude, OB supergiant type spectrum, and emission in the Balmer lines are some of thecharacteristics of hot post-AGB stars. In the sample of observed stars we found a few casesof B type dwarfs with H β emission. The presence of circumstellar material around Be dwarfsindicates that they may be related to other Be stars, shell stars or Herbig Ae/Be stars. In Table1 we have a few other objects which are not known to be post-AGB stars. They were includedin the observing program as some of them have IRAS colours overlapping with the IRAS coloursof some known post-AGB stars. The comments given in Table 1 for some of the stars are fromthe OB star catalogues and SIMBAD data base. For more information on previous spectralclassification, notes and comments etc. refer to OB star catalogues and SIMBAD data base.The m V given in Table 1 is from the SIMBAD data base.New spectral types for 44 O, B and A stars in the LS, LSS, and LSE catalogs haverevealed several new hot (OBA supergiant) post-AGB candidates on the basis of either coin-cidence with IRAS point sources or high galactic latitude. None of these objects shows anyevidence of a changing spectral type, but we plan to continue our monitoring program. Someof the post-AGB candidates we detected in our sample are not associated with an IRAS source,indicating they do not have dust shells. These are low mass objects and their evolution in theHR diagram from the tip of the AGB is rather very slow and by the time they evolve to A,B, O post-AGB spectral type, the dust shells seems to have disappeared and they may neverappear as planetary nebulae, hence they can be called naked post-AGB stars (e.g. BD +394926). Mutliwavelength study of post-AGB candidates discussed in this paper is needed tofurther understand their chemical composition and evolutionary stage. Some of the post-AGBcandidates given in Table 1 may show light variations similar to the high galactic latitude hotpost-AGB stars LS II +34 26 and SAO 85766 (Arkhipova et al. 2007).All the spectral data used in this study are given in the digital form in electronic tableE, which is available at the PASJ web site.
6. Acknowledgements
This research was supported in part by a grant from the National Science Foundation(NSF) (AST-9819835) to JSD. MP is very thankful to Prof. Shoken Miyama and Prof.Ramanath Cowsik for their kind encouragement and support, JV participated in the workduring her stay at IUCAA. We are thankful to Dr. Shashikiran Ganesh for his help in convert-ing the data into fits files. We are thankful to the referee for helpful comments.12 eferences
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