A NEW DISTANCE INDICATOR TO GALACTIC PLANETARY NEBULAE BASED UPON IRAS FLUXES

Transcription

1 THE ASTRONOMICAL JOURNAL, 115:1989È2008, 1998 May ( The American Astronomical Society. All rights reserved. Printed in U.S.A. A NEW DISTANCE INDICATOR TO GALACTIC PLANETARY NEBULAE BASED UPON IRAS FLUXES AKITO TAJITSU AND SHINÏICHI TAMURA Astronomical Institute, Tohoku University, Sendai , Japan Received 1997 July 1; revised 1998 January 13 ABSTRACT It is well known that the IRAS colors of planetary nebulae (PNs) are very similar to blackbody colors. Taking account of this characteristic, we deðne a new method to obtain some information about the distance to PNs by blackbody Ðtting of IRAS four-band Ñuxes, assuming these Ñuxes are due to thermal emission from the nebular dust envelope. The Ðt should have two free parametersèthe dust temperature T and the distance-dependent scaling factor ÏÏ AÈunder the assumption of uniform dust mass. We Ðnd that D the A-values have a good correlation with published distances. The scaling factor A could be a more e ective distance scale than others, because many PNs have been detected as IRAS sources and the extinction of IRAS Ñuxes is not so severe, and is known. We also Ðnd that T is concentrated between 100 and 200 K, with a typical value of about 150 K. D Key words: infrared radiation È planetary nebulae: general 1. INTRODUCTION Almost all investigators of planetary nebulae (PNs) believe that PNs are remnants expelled from the extended envelopes of postèasymptotic giant branch stars. It is considered that these remnants consist of ionized gas, neutral gas, and dust. Nebular scientists have made clear the massloss mechanism related to such materials, and the subsequent nebular evolution. tances for 62 PNs on the basis of their istical parallaxes. Acker (1978) compiled all of these istical and individual distance determinations for 330 PNs. In the 1980s, the Shklovskii methods were modiðed by empirical mass-radius relations (Maciel & Pottasch 1980; Daub 1982). A variant of the Shklovskii method to avoid the use of angular diameter was proposed by Barlow (1987). Maciel (1984, hereafter Ma84) recalculated the distances for The distances to planetary nebulae are the most important PNs in the Cahn & Kaler (1971) catalog according to parameters in the study of their evolution and of the Maciel & Pottasch (1980). Cahn, Kaler, & Stanghellini distribution within our Galaxy, but it has been very difficult (1992, hereafter CKS92) calculated the distances for the to determine them precisely. Many methods have been thought of to date. In principle, the most reliable method is trigonometric parallaxèbut the PNs for which distances largest sample of Galactic PNsÈ778Èaccording to the Daub (1982) scheme. Recently, Zhang (1995) and Van de Steene & Zijlstra (1995) proposed new istical distances can be determined by this method are very few (Pier et al. based upon the 5 GHz radio continuum. Schneider & 1993). Expansion distances can be obtained by comparing Buckley (1996) developed an improved method of distance nebular expansion velocities with angular expansions determination for PNs based upon the general Shklovskii derived from studying nebular features in old and new images over a rather long time span. The limitations of this method are the assumption of spherically symmetric expansion and the difficulty of measuring accurate angular expansions. Interstellar extinction distances can be obtained, because the extinction in the nebular spectra should be mostly caused by interstellar dust. Gathier, Pottasch, & Pel (1986) measured extinction distances of 12 nebulae. But this method has limitations caused by the inhomogeneity of interstellar matter along the line of sight and by the lack of stars whose distances are well determined in the surrounding regions of the PNs. These methods can determine the distances to individual nebulae independently. On the other hand, in 1956 Shklovskii established a istical method for determining the distances that is based upon the assumption that all nebulae have the same ionized method and upon their radii and radio surface brightness. But these istical distances are largely in conñict with each other, and there has not been any other comparable distance scale that has been determined with the largest reliable sample presently available. In this paper, we propose a new istical method to determine the distances to PNs and compare it with other distance scales. This method is based upon the assumption that IRAS four-band Ñuxes from PNs are due to blackbody radiation from dust particles in the nebula. Compared with the Shklovskii method itself, this method, which is quite independent of the Shklovskii method, does not require accurate angular radius information but only IRAS fourband Ñuxes. In the Strasbourg-ESO Catalogue (Acker et al. 1992), more than 800 PNs have been identiðed with IRAS point sources. So, this method will be able to use a large mass (Shklovskii 1956a, 1956b). Basically, this method sample of PNs, like CKS92. requires information about nebular angular radii and the extinction-corrected Hb Ñux. Up to the present time, many istical distance scales for PNs have been investigated, In what follows we will attempt the blackbody Ðtting of IRAS four-band Ñuxes for all available data (more than 600 PNs) in the Strasbourg-ESO Catalogue with two free but almost all of them are modiðed Shklovskii methods. parametersèthe dust temperature T and the scaling They often suppose that some physical conditions are same factor ÏÏ A, which can be deðned as a function D of the distance in all PNs. Cahn & Kaler (1971) measured Shklovskii ÏÏ to the PN. We explain the basic concept of our method in distances for more than 600 PNs. Milne & Aller (1975) 2. In 3, we describe the details of the blackbody Ðtting calculated distances of southern PNs according to 5 GHz radio observations. Cudworth (1974) determined the disand present the sample of PNs examined. The dust temperature obtained by our Ðtting is also discussed in this 1989

2 1990 TAJITSU & TAMURA Vol. 115 section. In 4, we check the correlations between the scaling factor A and the other istical and individual distance estimations. The conclusions and discussion follow in BASIC CONCEPT It is well known that the IRAS colors of PNs are very similar to blackbody colors, compared with galaxies or H II regions (Pottasch et al. 1988). Taking this characteristic into account, we deðne a method to obtain some information about the distances to the PNs by blackbody Ðtting of IRAS four-band Ñuxes. Here we assume a spherical dust cloud with uniform size, mass, temperature, and composition of the grains. The blackbody radiation from this dust cloud at distance D (cm) can be expressed as follows: p F \ N l D D2 Q(l)B(l, T D ) (1) (Hildebrand 1983), where N denotes the total number of grains in the cloud, p (cm2) is D the geometric cross section of a single dust grain, Q(l) is its emissivity efficiency, and B(l, T ) (ergs s~1 cm~2 Hz~1) is the Planck function with D T the dust temperature. If we deðne a (cm) as the radius of D a dust grain, then the mass of all dust, M (g), can be D expressed as 4n M \ N D D 3 a3o \ F l D2 4n a3o, (2) pq(l)b(l, T ) 3 D where o (g cm~3) is the speciðc mass density of one grain of material. Using p \ na2, equation (2) can be rewritten as M \ F l D2 4a D B(l, T ) 3Q(l) o, (3) D and the observed Ñux of this dust cloud, F, is l F \ 3 M Q(l) 1 D B(l, T ) l 4 ao D D2. (4) Assuming that the emissivity efficiency of the dust grain is a simple function of frequency l, Q(l) P ln (5) (Barlow 1983), where n is constant, and assuming a reference frequency of 25 km, the Ñux is given by F l \ 3 4 M D Q 25 km ao A25 kmbn 1 B(l, TD ) j D2 A25 kmbn \ A B(l, TD ), (6) j where j is in microns. Here A is given by A \ 3 M Q 1 D 25 km 4 ao D2 ; (7) we call A the scaling factor.ïï The calculated blackbody radiation represented by equation (6) tends to have the best Ðt to the four-band Ñuxes when the value of n in equation (5) is zero. So, hereafter we use 0 as the value of n in our Ðtting. But n is often mentioned as 1 or 2 (Barlow 1987). The reason for this discrepancy seems to be the excess of 12 km Ñux caused by emission lines. We discuss this subject in detail in 5. The value of A obviously depends on the distance to the nebula, and it is proportional to D~2. Then, if one assumes the other quantities in the scaling factorèm, a, o, Q Èare almost uniform in all PNs, A should be D a distance 25 km indicator to the Galactic PNs. Here we propose a distance scale by supposing that all PNs have the same dust masses, that the IR Ñuxes of PNs originate in the blackbody radiation of spherical and uniform dust clouds, and that the dust cloudsï sizes are almost the same as the nebular size. The Ðrst assumption is not easy to admit, but in the next section we will show that the scaling factor correlates well with other distance scales. And if the nebula has a reliable enough alternate distance scale, the dust mass in the nebula can be estimated by this Ðtting of IRAS Ñuxes. 3. BLACKBODY FITTING OF IRAS FLUXES AND SAMPLES The procedure for the blackbody Ðtting almost follows that used by Kwok, Hrivnak, & Milone (1986). We need at least two useful (not upper limit) data out of the four-band Ñuxes for this Ðtting, for it has two free parametersèthe dust temperature, T, and the scaling factor, A. In the D Strasbourg-ESO Catalogue, we Ðnd 659 PNs that satisfy this requirement. First, the e ect of the IRAS bandwidth should be removed from the Ñuxes, following Kwok et al. (1986). For this purpose, two-point Ðtting is performed, using each pair of Ñuxes from adjacent IRAS bands. Then, following the calculated temperature from this Ðtting, the colorcorrection factor K is determined from volume 1 of the IRAS Point Source Catalog (1988). For the 25 and 60 km bands, if there are two adjacent nonèupper limit ÏÏ bands, we use the average of the two correction factors. Then the raw Ñuxes are color-corrected, the correction applied to the Ðtting of equation (6). Because of the Ðnite IRAS aperture, for PNs that have diameters larger than 25A, the observed IRAS Ñuxes (and thus the scaling factor) must be revised according to the aperture of IRAS. From equation (1), the Ñux from a nebula is proportional to the number of dust grains in the nebula. In other words, when the nebula is partly observed, under our assumption of a dust cloud with uniform density, the observed Ñux is proportional to the observed volume of the nebula. If the nebular diameter, h (rad), is larger than the diameter of the entrance aperture neb of IRAS, h (rad), the observed Ñux F should be corrected according tel,j to obs,j h3 F \ F neb corr,j obs,j h3 [(h2 [h2 )3@2, (8) neb neb tel,j assuming that the dust cloud has almost the same size as the gaseous nebula. Equation (8) is purely led by geometric considerations, comparing projected cylindrical volume, with diameter h, in the spherical nebula with the whole volume of the nebula tel,j itself. Practically speaking, however, it is not always the case that the size of the dust cloud is the same as the optically detected nebular size. So this aperture correction might be an error source. We discuss this error in 4 with the illustrations of the Ðts Samples The Strasbourg-ESO Catalogue (Acker et al. 1992) contains 1143 PNs classiðed as true and probable planetary

3 No. 5, 1998 DISTANCES TO GALACTIC PNS 1991 TABLE 1 SAMPLE NUMBERS CLASSIFIED WITH AVAILABLE IRAS FLUX BANDS Available Bands Number of PNs nebulae. Out of these objects, we Ðnd 785 PNs identiðed as IRAS point sources by the ESO Catalogue. We have classi- Ðed these 785 IRAS planetary nebulae according to the number of nonèupper limit ÏÏ Ñuxes in the four IRAS bands in Table 1. The objects that have at least two available bands were distinguished from them and should be included in our Ðtting sample. These amount to 670 objects. But, in 10 cases, they have IRAS Ñuxes obviously di erent from blackbodies. The 10 such objects are listed in Table 2. Rejecting these objects, we attempt the blackbody Ðtting to 660 objects. The results of the blackbody Ðttings for these 660 nebulae are presented in Table 3. The Ðrst column is the PN designation in the Strasbourg-ESO Catalogue, which gives the FIG. 1.ÈDistribution of dust temperature. The dust temperature distributions of the objects Ðtted by only two IRAS bands (long-dashed line), three bands (short-dashed line), and four bands (dotted line) are shown, with the total represented by the histogram. The temperature distribution of the objects Ðtted by only two points shift to lower temperatures by about 30 K. This seems to be caused by the nonexistence of the 12 km band Ñux. Galactic longitude and latitude. Objects in Table 3 are listed in order of this PN G-number. The second column gives the common name of the nebula. The next four columns are the IRAS point-source Ñuxes at 12, 25, 60, and 100 km, each given in janskys, and the next four are their percentage errors. If the corresponding band Ñux is an upper limit, the character L ÏÏ is listed. The following column (col. [11]) is the optical diameter of the nebula in arcseconds. The next four columns list the color- and diameter-corrected IRAS Ñuxes corresponding to the third through sixth columns, in janskys, as described above. The next two columns present our calculated dust temperatures (in kelvins) and the logarithm of the scaling factors A from equation (6). The next column (col. [18]) is our distance estimation D in parsecs, calculated from A using equation (9) below. IRAS Columns (19) and (20) present two sta- tistical distances, Ma84 and CKS92, in kiloparsecs. In the last column, if the nebula has never had a distance estimation, we list an N.ÏÏ Also in this column, the objects marked with asterisks exhibit the best Ðts in the range of dust temperature 130 K \ T \ 180 K. D 3.2. Dust Temperatures of Planetary Nebulae As a by-product of this Ðtting, we can obtain information about the temperature of the dust in the nebulae. The distribution of the dust temperature for all of our sample is shown in Figure 1. This temperature is distributed between 80 and 200 K for most of the PNs, and one can see that the typical value is about 140 K. Some PNs whose temperatures are higher than 200 K are likely proto-pns (Kwok 1993). Under such higher dust temperatures, the error of the blackbody Ðtting will become larger, because the wavelength at the peak of the Ñux from the dust is moved toward shorter wavelengths. Conversely, for a blackbody whose temperature is 100È200 K, the peak of infrared Ñux is positioned between 25 and 60 km. So, for most of the nebulae, the dust temperature is convenient for our four-band Ðtting, unlike galaxies or H II regions. 4. COMPARISON OF THE SCALING FACTOR WITH OTHER STATISTICAL DISTANCES To verify the relationship between the scaling factor and distance, we have compared our own scaling factor with other published distances. We have chosen two istical distance scales, CKS92 and Ma84, because these catalogs have very large samples. CKS92 have compiled the distances of 778 PNs, and Ma84 contains 468 objects. TABLE 2 NEBULAE REJECTED FROM OUR SAMPLE IRAS FLUX (Jy) FLUX ERROR (%) COLOR-CORRECTED FLUXES (Jy) DIAMETER PN NAME 12 km 25 km 60 km 100 km 12 km 25 km 60 km 100 km (arcsec) 12 km 25 km 60 km 100 km G002.0[ H L L G002.2[ Cn G003.6] M G005.1[ H L G258.0[ Wray L L G293.6] BlDz L... L G305.7[ Wray L... L G353.3] M L G353.5[ H L L Stellar G356.9[ M

4 TABLE 3 RESULTS OF THE BLACKBODY FITTING OF IRAS FLUXES G000.0[ H L St [ N, B, * G000.1] PC L L [ B, * G000.1] H L [ N, B, * G000.1] Al2-J L L [ N, B G000.1[ Bl L L [ N, B G000.2[ M L L [ B G000.3] IC [ * G000.3[ M L L [ B G000.4[ M L L [ N, B G000.4[ M L L [ G000.7] H L [ N, B, * G000.7] He L L [ B G000.7[ M L L [ B G000.7[ M L L [ B, * G000.8[ BlO L L St [ N, B, * G000.9[ M L [ B, * G001.0] K L L [ B G001.0[ Sa L [ N, B G001.2] He L L [ B G001.2[ H L L [ N, B, * G001.3[ BlM L L [ B G001.4] H L L [ B, * G001.5[ SwSt [ B G001.6[ BlQ L [ N G001.7] H L L [ B, * G001.7[ H L L [ N, B G001.7[ H L L [ B, * G002.0[ M L L [ B, * G002.0[ IC [ B, * G002.1] PBOZ L L St [ N, B G002.1[ H L L [ B G002.2[ M L [ B G002.2[ H L [ B G002.4] NGC [ * G002.4[ M L [ N, B, * G002.6] H L L [ B G002.6] Th L L [ N, B G002.6[ M L L St [ N, B, * G002.7[ M L [ G002.7[ IC5148/ L L [ G002.8[ Pe L L [ G002.9] PM L L [ N, B G003.1] H L L [ B G003.1] Hb L [ B, * G003.2[ M L L [ B G003.3[ Ap L L [ B, * G003.4[ H L L [ B G003.5[ IC L [

5 G003.5[ NGC L [ G003.6] M L [ N, B, * G003.7[ M L L [ B G003.8[ H L L [ G003.9[ M L [ B, * G003.9[ Hb L [ B, * G004.0[ M L [ B G004.3[ H L L St [ N, B, * G004.5] H L [ B, * G004.6] H L L [ B G004.8[ M L [ B G004.8[ He L L [ G004.9] M L [ B, * G004.9[ M L L [ G005.0] H L L [ B, * G005.1[ Hf L L [ B G005.2] M L L [ G005.2] M L [ N, B G005.5] M L L [ B G005.5] H L L [ N G005.5[ H L L [ B G005.7[ M L L [ B G005.8] H L L [ B G005.8[ NGC L [ B G005.9[ MaC L [ N, B G006.0[ M L L [ N, B G006.1] M L [ B G006.3] H L [ B G006.3] H L L [ B G006.4] M L [ B, * G006.4[ Pe L [ B G006.5[ H L [ N, B, * G006.8] M L [ B, * G007.0] M [ G007.0[ H L L [ B G007.0[ VY L [ B G007.2] Hb L [ * G007.6] M L L [ B G007.8[ H L L [ B, * G008.0] NGC [ * G008.1[ M L L [ B, * G008.2] He L [ B, * G008.2[ M L L [ B G008.3[ NGC L [ B G008.3[ M L [ * G008.4[ H L L [ B G009.0] Th L [ G009.3] Th L L [ N, B G009.4[ NGC [ * 1993

6 G009.4[ M L [ B G009.6] NGC [ * G009.6[ M L L [ B G009.8[ H L L [ B G009.8[ GJJC [ N G010.1] NGC [ * G010.4] M L L [ G010.6] Th L L St [ N G010.7] Sa L L [ N G010.7[ IC L [ G010.7[ Pe L L [ * G010.8] M [ G010.8[ NGC L L [ G011.0] M L L [ G011.0] NGC L [ G011.0[ M L L [ * G011.1] M L L [ G011.3] Th L [ N G011.3[ H L [ * G011.7[ NGC L L [ G011.7[ M St [ N, * G011.9] M L [ G012.2] PM L [ N G012.6[ M L St [ N G013.1] M L [ G013.4[ M L [ G013.7[ Y-C L L [ N G014.0[ V-V L St [ N G014.2] Sa L [ N, * G014.2[ M L L [ G014.6[ M L L [ G015.4[ M L L [ G015.9] M L [ G016.0] A L [ G016.0[ M L L [ G016.1[ M L [ * G016.4[ M L [ G017.3[ A L L [ G017.6[ A L [ G017.7[ M L L [ G017.9[ M L L [ G018.0] Na L L [ G018.9] M L L St [ N, * G019.4[ M St [ * G019.7] M L [ * G019.7[ M L [ * G020.7[ Sa L L [ G020.9[ M L [ * G021.1[ M L L [

7 G021.2[ We L L [ N, * G021.8[ M L L [ G022.1[ M L [ * G022.5] MaC L L [ N G023.8[ K L St [ * G023.9[ M [ * G024.1] M L [ G024.2] M L [ G024.2[ M L L [ N G024.3[ Pe L L [ G024.8[ M L [ G025.3[ K L St [ N G025.4[ IC L L [ G025.8[ NGC [ * G025.9[ Pe L L [ G026.5[ Pe L L [ G027.3[ Pe L [ * G027.4[ Vy L L [ * G027.6] M L [ G027.6[ IC L L [ * G027.7] M L L [ G028.2[ Pe L L [ G028.5] K L [ * G028.5] M L [ * G028.7] K L L [ G028.7[ Pe L L [ G029.2[ NGC [ * G030.8] A L [ G031.0] K L St [ * G031.0[ M L [ N, * G031.2] K [ * G031.7] PC L L [ N G032.0[ K L [ G032.5[ K L L St [ N G032.7] K L L [ G032.7[ M L [ * G032.9[ K L St [ * G033.1[ NGC L [ G033.8[ NGC L L [ * G034.0] K L L St [ * G034.3] K L L [ * G034.5[ NGC [ G034.6] NGC [ * G035.7[ K L L [ N G035.9[ Sh L [ G036.0] A L [ * G036.1[ NGC L L [ G037.5[ A [ * G037.7[ NGC [ * 1995

8 G038.2] Cn [ * G038.4[ K L L St [ N G038.7[ M L L St [ N G039.5[ M L L [ * G039.8] K L [ * G040.4[ K L St [ G041.8] K L St [ N G041.8[ NGC L [ G042.0] K L L St [ G042.5[ NGC L [ * G042.9[ NGC [ * G043.1] NGC [ * G043.1] M L L [ * G043.3] M L L [ G043.3] PM L L [ N G044.3[ K L [ N G045.4[ Vy [ G045.6] K L L [ * G045.7[ NGC [ G045.9[ K L St [ G046.3[ PB L L [ G046.4[ NGC [ * G047.0] A L L [ G048.0[ PB L [ * G048.1] K L St [ * G048.7] K L L [ G048.7] He L [ * G049.4] He L L [ G050.1] M [ * G050.4] A L L [ * G051.0] He L [ G051.0] WhMe L St [ N G051.0[ PC L L [ N, * G051.3] PM L L [ N G051.4] Hu L [ * G051.5] K L L [ G051.9[ M [ G052.2] K L L [ N, * G052.2[ M L [ * G052.5[ Me L [ G052.9] K L St [ * G052.9[ K L L St [ G053.2[ K L L [ G053.3] Vy L [ * G054.1[ NGC [ * G054.4[ M L [ * G055.1[ K L L [ G055.2] He L [ * G055.3] He L L [

9 G055.5[ M L [ * G055.6] He L [ N G056.0] K St [ * G056.8[ K L L [ N, * G057.2[ NGC L L [ G057.9[ He L [ * G058.3[ IC [ * G058.9] K L L St [ G059.0[ He L [ N, * G059.4] K L L St [ G059.7[ A [ G059.9] K L St [ * G060.1[ NGC [ * G060.4] PM L St [ N G060.5] He L [ * G060.8[ NGC L [ G061.0] K L L [ * G061.3] He [ G061.4[ NGC [ * G062.4] NGC L L [ G062.4[ M L L [ G063.1] NGC [ * G063.8[ K L L St [ G064.6] NGC L [ G064.7] BD] [ G064.9[ K L [ * G065.9] NGC L L [ G066.7[ NGC L [ G066.9[ PC L L [ G067.9[ K St [ * G068.3[ He [ * G068.7] PC L L [ * G068.7] K L St [ G068.8] M L L [ G069.2] K L [ N, * G069.4[ NGC L [ G071.6[ M L [ G072.1] K L [ * G074.5] NGC L [ * G077.5] KjPn [ N, * G077.7] KjPn [ N G078.3[ K L L [ N, * G079.6] M L L [ G082.1] NGC L [ * G083.5] NGC [ * G086.5[ Hu L [ * G088.7] K L [ G088.7[ NGC L L [ G089.0] NGC [ * 1997

10 G089.3[ M [ * G089.8[ IC [ G093.3[ K L L [ G093.3[ M L L [ G093.4] NGC [ * G093.5] M [ * G094.5[ K L L [ G095.1[ M L L [ G095.2] K L [ * G096.3] K L L [ G097.5] A [ * G097.6[ M L L [ G098.1] K L L [ * G098.2] K L [ G100.0[ Me L [ G100.6[ IC L [ * G101.8] A L L [ G103.2] M L [ G104.1] Bl L L St [ G104.4[ M L [ G104.8[ M [ * G106.5[ NGC [ * G107.4[ K L L St [ N G107.4[ K L L [ G107.6[ Vy L L [ G107.7[ M L L [ G107.8] NGC [ * G110.1] PM L [ N, * G111.2] KjPn L L [ G111.8[ Hb [ G112.9[ A L L [ G116.2] M L [ G118.0[ Vy L L [ G119.6[ Hu L [ * G120.0] NGC [ G121.6] BV L L [ N G123.6] IC [ * G125.9[ PHL L L [ G126.3] K L L [ * G130.2] IC L [ * G130.3[ M L L [ * G130.9[ NGC L [ G133.1[ M L [ N G136.1] A L L [ G138.8] IC [ * G142.1] K L L [ G144.5] NGC [ N, * G146.7] M [ G147.4[ M L [ * 1998