Investigating New Planetary Nebulae in the Southern Hemisphere

Transcription

1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 112:542È554, 2000 April ( The Astronomical Society of the PaciÐc. All rights reserved. Printed in U.S.A. Investigating New Planetary Nebulae in the Southern Hemisphere F. KERBER Space Telescope European Coordinating Facility, ESO, Karl-Schwarzschild-Strasse 2, DÈ85748 Garching, Germany; Ñorian.kerber=eso.org E. FURLAN1 Institut fu r Astronomie der Leopold-Franzens-Universita t Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria M. ROTH AND G. GALAZ Las Campanas Observatory, Carnegie Institution of Washington, Casilla 601, La Serena, Chile J. C. CHANAME Departamento de Astronom a y Astrof sica, PontiÐcia Universidad Cato lica de Chile, Casilla 306, Santiago 22, Santiago, de Chile AND STUDENTS OF THE SECOND ANDES-CARNEGIE ASTRONOMY SUMMER SCHOOL2 Received 1999 September 27; accepted 2000 January 18 ABSTRACT. The major purpose of the investigation presented in this paper was to prove or disprove the nature of planetary nebula (PN) candidates in the southern hemisphere, selected from the third volume of the Atlas of Galactic Nebulae by Neckel & Vehrenberg. We present imaging and spectroscopic observations of seven PNe, Ðve of them identiðed for the Ðrst time. An additional object probably is an H II region. All observed PNe represent evolved stages, their angular diameter ranging from 15A to 170A, and exhibit low surface brightnesses. 1. INTRODUCTION Planetary nebulae (PNe) are usually identiðed by narrowband imaging and/or optical spectroscopy. The combination of images and spectra gives insight into the complex physics of the nebular plasma and its relation to the central star; see Weinberger & Kerber (1997) for a recent review. In some, mostly highly evolved, PNe, interaction with the environmentèthat is, the interstellar medium (ISM)Ècan also be identiðed, opening up new possibilities to study the ISM on a small-scale basis (Tweedy & Kwitter 1994; Xilouris et al. 1996). The decay of the nebulae and the mechanism of returning processed material to the ISM, which drives in part the chemical evolution of galaxies, are also important issues, as is the understanding of the transition from PNe nuclei to white dwarfs (Napiwotzki 1995). Therefore a number of fundamental astrophysical processes can be addressed and be demonstrated by investigating PNe. For these reasons we have selected a project to identify and study new PNe as part of the Second Andes- Carnegie Astronomy Summer School. ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Current address: Cornell University, Space Sciences Building, Ithaca, NY 14853; furlan=astrosun.tn.cornell.edu. 2 The students of the Second Andes-Carnegie Astronomy Summer School were A. Belmar, M. C. Ca ceres, J. Corte s, K. Lastarria, J. F. Martin, A. Navarette, D. A. Quezada, M. Sotomayor, M. A. Tapia, and M. Valenzuela. Members of the Innsbruck Institute for Astrophysics have, for a long time, been successfully engaged in the detection (more than 10% of the registered galactic PNe) and investigation of extended PNe (e.g., Weinberger et al. 1983; Hartl & Tritton 1985; Ishida & Weinberger 1987; Melmer & Weinberger 1990; Tamura & Weinberger 1995; Kerber et al. 1996). The PNe discussed in our paper represent a selection of objects from the third volume of the Atlas Galaktischer Nebel by Neckel & Vehrenberg (1990). With one exception, all of the objects are located very close to the Galactic plane. Some had been suspected to be PNe by the authors of the Atlas, referred to as NeVe 3-n in Table 1, while others were given as objects of unknown nature and are identiðed as PNe by us. The Atlas is an example of a highly valuable data set which has not been exploited in full. Recently Corradi et al. (1997) published a very nice example of a large bipolar PN, taken from the same atlas. Until now only coordinates had been known for the objects under study. With the spectra and images presented in this article we are able to discuss these nebulae on a more solid basis. 2. OBSERVATIONS All observations have been obtained in 1998 February during the course of the Second Andes-Carnegie Astronomy Summer School at Las Campanas Observatory 542

2 TABLE 1 BASIC DATA FOR THE PLANETARY NEBULAE INVESTIGATED NEW PLANETARY NEBULAE 543 R.A. Decl. Diameter Name Designation (2000.0) (2000.0) (arcsec) NeVe Remarks (1) (2) (3) (4) (5) (6) (7) NeVe PN G250.4[ [ ] 165 GN BBWo 100 NeVe PN G251.2] [ GN NeVe G288.0[ [ ] 80 GN H II region? NeVe PN G292.5] [ ] 42 GN NeVe PN G296.5] [ ] 25 GN KFR 2... PN G298.5] [ ] 43 GN Po 1... PN G298.3] [ GN NeVe 3-8 KFR 3... PN G321.2[ [ ] 45 GN NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. 40" FIG. 1.È[N II] image of NeVe 3-3

3 544 KERBER ET AL. FIG. 2.ÈSpectrum of NeVe 3-3 (LCO), Chile. Narrowband images have been obtained for all objects at the LCO 1 m Swope telescope. The SITE 1 chip (2048 ] 2048 pixels) gave us a Ðeld of view (FOV) of almost 24@ at a scale of 0A.69 pixel~1; the actual usable FOV was between 12@ and 18@ due to vignetting by the narrowband Ðlters. For identifying the PNe we used broad (bandwidths about 100 A ) Ha and [O III] Ðlters and complementary o -band Ðlters centered at 7000 and 5200 A (bandwidth 100 A for both). For further study we also employed two Ðlters that are centered on Ha and [N II] only (bandwidth 30 and 35 A ). For spectroscopy we employed the modular spectrograph at the 2.5 m du Pont telescope covering the spectral FIG. 3.ÈHa image of NeVe 3-4

4 NEW PLANETARY NEBULAE 545 range from 4000 A to 7300 A for six objects. Using the Tek 5 CCD detector we obtained a spectral resolution of 2 A pixel~1. The exposure time ranged from 900 to 1800 s. A long slit (length 2@.5) with a width of 1A.2 was used. The raw data were calibrated using the standard stars LTT 3218 and 3864 from the lists of Hamuy et al. (1992, 1994). The raw spectra were reduced within MIDAS using the context long.ïï This performs a two-dimensional Ðt to the calibration lamp spectrum for the wavelength solution. The area of the object and of the night sky were chosen interactively on the two-dimensional spectrum. First the rows containing the objectïs spectrum were summed up. In order to get a one-dimensional spectrum of the sky background, several rows above and beneath the object were averaged and extracted. Finally, the background was subtracted from the one-dimensional spectrum of the object. To correct for atmospheric extinction, a MIDAS task was applied which uses the air-mass value (from either the descriptor of the frame or the observing log). The spectra were calibrated in Ñux by using the standard stars mentioned above. Spectra of these stars were obtained in each night of spectroscopic observations. The lines were measured using the package ALICE, which allows for interactive Ðtting of multiple FIG. 4.ÈSpectrum of NeVe 3-4 Gaussian proðles to the observed lines. The radial velocities were measured relative to the night-sky lines present in the spectra and are given as heliocentric velocity values. The resulting spectra are presented for each object. It may seem odd that new objects can still be found in this survey after so many years of intense searches but the discovery of new PNe on the Palomar Observatory Sky Survey (POSS) or ESO/SERC has been continuing at a TABLE 2 LINE FLUXES FOR THE NEW PLANETARY NEBULAE Parameter NeVe 3-3 NeVe 3-4 NeVe 3-5 NeVe 3-6 NeVe 3-7 KFR 2 Line: Hc j He II j Hb j [O III] j [O III] j [N I] j [N II] j : : 32: He I j [O I] j [O I] j [N II] j Ha j [N II] j He I j [S II] j [S II] j [Ar III] j v (km s~1) ^ ^ ^ ^ 10 [31 ^ 5 48 ^ 10 hel Excitation class c n... \100 \100 \ \ e T ([N II]) (K) : : 7500: e N(He)/N(H) : 0.204: 0.061: : 0.138: N(N)/N(H) ] 10~4: 4.61 ] 10~5: 1.26 ] 10~5: 4.72 ] 10~5: 8.28 ] 10~5: 7.34 ] 10~4 N(O)/N(H) ] 10~4: 3.04 ] 10~4: 1.54 ] 10~5: 4.21 ] 10~ ] 10~4: 7.01 ] 10~4 N(S)/N(H) ] 10~6: 6.77 ] 10~7: 8.37 ] 10~7: 1.07 ] 10~6: 1.08 ] 10~6: 9.77 ] 10~6 N(Ar)/N(H) ] 10~6: 3.67 ] 10~6: 1.77 ] 10~6: 6.14 ] 10~6 log [N(N)/N(O)]... [0.097: [0.821: [0.088: [0.950: [0.295: 0.020

5 546 KERBER ET AL. FIG. 5.ÈSpectrum of NeVe 3-5 more or less constant rate for many years now. There are two main reasons for this. First, many objects discovered in recent years do not look like a textbook ÏÏ PN, but come in a variety of shapes; second, most are nondescript objects of low surface brightness. The Atlas Galaktischer Nebel was published in 1990, but many of the objects presented have never been studied in any detail. From the third (southern) volume we have extracted a sample of PN candidates for study as a project during the Second Andes-Carnegie Astronomy Summer School in The naming of the objects starts with NeVe 3-3 as NeVe 3-1 and 3-2 have already been reported in Kerber et al. (1998). For the object reported as PNe for the Ðrst time we used the conventional FIG. 6.ÈHa image of NeVe 3-5

6 NEW PLANETARY NEBULAE 547 FIG. 7.ÈHa image of NeVe 3-6 common name starting with KFR 2 as KFR 1 has been reported in Kerber et al. (2000). Table 1 summarizes some basic data of the PNe. In column (1) we give the common name and in column (2) the designation following the IAUÏs recommendations outlined in the Strasbourg-ESO Catalogue of Planetary Nebulae (Acker et al. 1992). Columns (3) and (4) give equatorial coordinates, and column (5) lists the diameter in arcseconds measured on the narrowband images. The largest diameter observed (usually in the Ha Ðlter) is given. In column (6) the original designation in the Atlas by Neckel & Vehrenberg is listed. Table 2 lists the line Ñuxes as extracted from the spectra, relative to Hb \ 100. The radial velocities are accurate to about ^10È15 km s~1. We used the ratio [I(4959) ] I(5007)]/10I(Hb) to derive an excitation class as given by Aller (1956). When possible, basic plasma parameters have been derived, by employing the plasma diagnostics program HOPPLA by J. Ko ppen (Acker et al. 1989). The electron temperatures, obtained from the [N II] lines, while giving reasonable values, can only be considered an estimate, given the small signal-to-noise ratio (S/N) of the third [N II] line at 5755 A. The abundances derived with the same program also have to be considered as an indication only, due to the very limited numbers of lines involved. In particular, for objects NeVe 3-4, 3-5, and 3-6 we assumed an electron temperature of 10,000 K as the actual value could not be derived from the spectra. Note also that no attempt has been made to correct for unseen ionization stages, and the abundances stated in Table 2 give only the total ionic abundances for ionization stages for which a line was mea-

7 548 KERBER ET AL. FIG. 8.È[N II] image of NeVe 3-6 sured. Still, we tentatively assign Peimbert type I [deðned by Peimbert & Torres-Peimbert 1983 as N(He)/N(H) º and log (N/O) º [0.3] to at least two of the objects. FIG. 9.ÈSpectrum of NeVe RESULTS In all images north is up and east is to the left. We present only a selection of the image data obtained; in particular, no o -band images are shown. The position of the long slit is indicated by the solid bars. A scale allows for the comparison of sizes. First, all nebulae could be conðrmed to be emission-line objects by comparison with the o -band images. From

8 NEW PLANETARY NEBULAE 549 FIG. 10.ÈHa image of NeVe 3-7 spectroscopy we classify one object (NeVe 3-5) as a possible H II region, while the rest are true PNe. All but one object are very close to the Galactic plane. We now discuss the objects individually. FIG. 11.ÈSpectrum of NeVe Individual Objects NeVe 3-3 was Ðrst listed as number 100 in a catalog of nebulous objects by Brandt et al. (1986). Only the bright inner part of the nebula is evident on the POSS plates, but a deep image reveals faint caps in the polar direction, making this an evolved bipolar PN of considerable size and an axis ratio of about 3. In fact, the students dubbed it manzana ÏÏÈ the apple ÏÏ in SpanishÈfor its appearance;

9 550 KERBER ET AL. FIG. 12.È[N II] image of KFR 2 FIG. 13.ÈSpectrum of KFR 2 see Figure 1. The inner nebula shows the symmetrical double peaked brightness distribution typical for a toruslike structure found in many bipolar PNe. Strong [N II] emission is observed from this torus (Fig. 2), and a pronounced ionization stratiðcation is evident in the spatially resolved long-slit spectrum. The caps are well deðned, but no connection to the central nebula is apparent. Nebulae with this double conus structure are commonly called double hourglass or butterñy nebulae (Balick 1987). The increased brightness of the upper and lower planes of the conusèthe capsèmight be the result of a terminal shock where the expansion has been impeded by the interstellar medium. This is consistent with the fact that the caps are

10 NEW PLANETARY NEBULAE 551 FIG. 14.ÈHa image of Po 1 brightest in the light of the [N II] lines. The distances of the caps from the main body are not the same, which may be the result of di erent outñow velocities and di erent densities in the surrounding ISM. The spectrum of the bright torus shows strong emission of [N II], and the abundances show the PN to be of type I (see Peimbert & Torres Peimbert 1983). The electron density is very low, even in the torus, conðrming that the PN is already in an evolved stage. NeVe 3-4 is a round, ring-shaped nebula (Fig. 3) not mentioned in SIMBAD, but we later accidently found the object in Capellaro et al. (1991). It is interesting to note that marked di erences exist between their spectrum and ours, which shows much stronger [N II] lines (Fig. 4). This might indicate that this PN actually is a bipolar PN seen face-on. A pronounced ionization stratiðcation is evident in the spectrum, and the electron density is low. NeVe 3-5 is an elliptical nebula that probably rather is an H II region than a low-excitation PN judging from its IRAS colors and the spectrum (Fig. 5). To put this classiðcation on a more quantitative basis, we used the diagnostic diagrams given by Baldwin et al. (1981) and Garcia Lario et al. (1991). In both diagrams NeVe 3-5 is located within the area of H II regions clearly separated from all other objects in this paper, which are evidently PNe according to the diagnostic diagrams. Furthermore, the object is close to the g Carina nebula complex. There is a complicated asymmetric brightness distribution within the object (Fig. 6). NeVe 3-6 is an elliptical PN with markedly di erent appearances in the lights of di erent atomic lines. While it is

11 552 KERBER ET AL. FIG. 15.È[N II] image of KFR 3 homogeneous in both Ha and [O III], much more and Ðlamentary structure can be seen in the [N II]; see Figures 7 and 8 for comparison. The brightness distribution is asymmetric, with a pronounced rim resembling a bow shock. The strong [N II] emission may be the result of shock excitation. Together with the morphology and the overall spectrum, this lets one think of a PN interacting with the ISM, a process that has been found to be much more common among evolved PNe than once thought (Xilouris et al. 1996; Kerber et al. 1998, 2000). The spectrum (see Fig. 9) reveals a very low value of 140 cm~3 for the electron density. Excitation is high with pronounced He II emission while [N II] is strong even for this slit position which did not intersect the [N II] bright rim. NeVe 3-7 looks like an elliptical PN at Ðrst sight, but a ringlike structure can easily be discerned in the broad Ha image (Fig. 10). [N II] emission is very pronounced. The abundances make this a borderline case of a type I PN. From the image we deduce that this is a bipolar nebula with the equatorial plane seen from an angle of about 45. The density is low (see Table 2 and also Fig. 11). Figure 12 illustrates that KFR 2 is a beautiful example of a butterñy nebula with a high contrast in brightness between the equatorial ring and the polar lobes. The spectrum (see Fig. 13) reveals extremely strong [N II] lines with the ratio of [N II] (j6584) to Ha of 5, among the largest values on record (Guerrero et al. 1995). The abundances derived from the good S/N spectrum clearly show it to be a type I PN. This is also the only one of our objects for which the abundances can be considered to be reliable. The inner torus is seen from an angle of about 40. It shows a knotty structure and an apparent warp.

12 NEW PLANETARY NEBULAE 553 FIG. 16.È[O III] image of KFR 3 Po 1 is a small round PN of homogeneous brightness distribution, with only a slight enhancement in the northwest sector (see Fig. 14). At 6.5 Galactic latitude it is the object farthest from the Galactic plane in our sample. Although we have not obtained a spectrum for this object, it is certainly a PN since a blue central star is evident. This object has independently been discovered by S. Pottasch (quoted as private communication in the supplement of the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae; Acker et al. 1996), we therefore use this common name. KFR 3 is yet another bipolar PN but has a complex morphology. The bright central part is reminiscent of a double ring forming an hourglass structure. In contrast to other such nebulae such as the famous MyCn 18, the axis of the hourglass is not the long axis, which is perpendicular to it as the [N II] (Fig. 15) and [O III] (Fig. 16) images clearly show. Unfortunately, the seeing was not better than about 1A.5, so high-resolution imaging is certainly warranted in order to fully reveal the rich structural details just indicated in our images. A spectrum of this unusual object will be obtained in the future. 4. CONCLUSION As a project for the Second Andes-Carnegie Summer school we have observed a sample of PN candidates taken from the Atlas of Galactic Nebulae by Neckel & Vehrenberg (1990). The students learned about the excitement of discovery and also got a taste of the work involved in such endeavor. We have been able to conðrm Ðve of the objects as true PNe, while another one probably is an H II region. All of the

13 554 KERBER ET AL. objects found are in an evolved state of the PN phase as indicated by the low electron densities. One object shows indication of interaction with the ISM. For three (including one borderline case) PNe we tentatively assign a Peimbert type I to the object on the basis of the abundances. This classiðcation and the abundance pattern is generally believed to be associated with a high-mass progenitor star. The new objects, which were not much more than mere entries in a list of coordinates, have now gained an identity, but much more remains to be done in understanding their physical properties. This work nicely demonstrate that many more relatively large and therefore close PNe remain to be found. It may also be noted that new detections of (extended) PNe are not only interesting from a statistical point of view, but are also of considerable signiðcance due to the extreme heterogeneity of the class: each PN is, to a certain extent, unique. We thank the referee, G. Jacoby, for his valuable suggestions that resulted in a signiðcant improvement of the paper. Thanks go to J. Ko ppen for providing the analysis software HOPPLA and for helpful discussions. It is a pleasure to thank Fundacion Andes and the Carnegie Institution of Washington for their support of the Andes-Carnegie Summer School and the education of young Chilean astronomers. Special thanks are also due to the members of the sta at Las Campanas Observatory for their assistance and in particular to O. Duhalde, our night assistant. We gratefully acknowledge the Ðnancial support by the Fonds zur Fo rderung der wissenschaftlichen Forschung project P11675-AST and travel grants by the Austrian ministry of Science and the University of Innsbruck. This research has made use of the Simbad database, operated at CDS, Strasbourg, France. Acker, A., Jasniewicz, G., Ko ppen, J., & Stenholm, B. 1989, A&AS, 80, 201 Acker, A., Marcout, J., Ochsenbein, F., Beaulieu, S., Garcia-Lario, P., & Jacoby, G. 1996, First Supplement of the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (Garching: ESO) Acker, A., Ochsenbein, F., Stenholm, B., Tylenda, R., Marcout, J., & Schon, C. 1992, Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (Garching: ESO) Aller, L. H. 1956, Gaseous Nebulae (New York: Wiley), 66 Baldwin, J. A., Phillips, M. M., & Terlevich, R. 1981, PASP, 93, 5 Balick, B. 1987, AJ, 94, 671 Brandt, J., Blitz, L., & Wouterloot, J. G. A. 1986, A&AS, 65, 537 Cappellaro, E., Sabbadin, F., Salvador, L., & Turatto, M. 1991, ESO Messenger, 64, 39 Corradi, R. L. M., Villaver, E., Mampaso, A., & Perinotto, M. 1997, A&A, 324, 276 Garcia Lario, P., Manchado, A., Riera, A., Mampaso, A., & Pottasch, S. R. 1991, A&A, 249, 223 Guerrero, M. A., Manchado, A., & Stanghellini, L. 1995, ApJ, 444, L49 Hamuy, M., et al. 1992, PASP, 104, 533 ÈÈÈ. 1994, PASP, 106, 566 Hartl, H., & Tritton, S. B. 1985, A&A, 145, 41 REFERENCES Ishida, K., & Weinberger, R. 1987, A&A, 178, 227 Kerber, F., Gro bner, H., Manchado, A., & Roth, M. 1998, A&AS, 130, 501 Kerber, F., Lercher, G., & Weinberger, R. 1996, A&AS, 119, 423 Kerber, F., Rauch, T., Furlan, E., & Roth, M. 2000, in PASP Conf. Ser. 199, Asymmetrical Planetary Nebulae II: From Origins to Microstructures, ed. J. H. Kastner, N. Soker, & S. A. Rappaport (San Francisco: ASP), in press Melmer, D., & Weinberger, R. 1990, MNRAS, 243, 236 Napiwotzki, R. 1995, European Workshop on White Dwarfs, ed. D. Koester & K. Werner (Lecture Notes in Physics 433; Berlin: Springer), 176 Neckel, Th., & Vehrenberg, H. 1990, Atlas of Galactic Nebulae III (Du sseldorf: Treugesell) Peimbert, M., & Torres-Peimbert, S. 1983, in IAU Symp. 103, Planetary Nebulae, ed. D. R. Flower (Dordrecht: Reidel), 233 Tamura, S., & Weinberger, R. 1995, A&A, 298, 204 Tweedy, R. W., & Kwitter, K. B. 1994, AJ, 108, 188 Weinberger, R., Dengel, J., Hartl, H., & Sabbadin, F. 1983, ApJ, 265, 249 Weinberger, R., & Kerber, F., 1997, Science, 276, 1382 Xilouris, K. M., Papamastorakis, J., Paleologou, E., & Terzian, Y. 1996, A&A, 310, 603