THE EXTINCTION OF HD BY DUST IN NGC 7023*

Transcription

1 Publications of the Astronomical Society of the Pacific 92: , August 1980 THE EXTINCTION OF HD BY DUST IN NGC 7023* G. A. H. WALKER,f S. YANG, AND G. G. FAHLMANf Department of Geophysics and Astronomy, University of British Columbia, Vancouver, B.C., Canada AND A. N. WITT Department of Physics and Astronomy, University of Toledo Received 1980 March 17 HD is obscured by the dust of the bright reflection nebula NGC In ground-based Reticon spectra between ÀÀ5500 and 6800 the diffuse interstellar features are extremely weak and curvature, or very broad-band structure, in the extinction curve is significantly less than for other stars. The UV extinction curve has been generated from IUE spectra between ÀA1250 and The À2175 absorption is weak and the À1540 feature found by Nandy et al. (1976) is absent, which resembles the case of 6 1 Ori, but the marked rise in extinction for À -1 > 6jU, _1 is typical of the average extinction curve. Differences between the extinction curve for HD and the average curve and that for 0 1 Ori give profiles of the À À1540 features, and the rapid increase in UV extinction, respectively. It is suggested that a three-component dust model is necessary to explain these observations: fine grains for the UV extinction, large grains for the optical region, and a third component, possibly a volatile mantle which is responsible for the diffuse absorption features and the very broad-band structure. Key words: interstellar extinction Be stars reflection nebulae interstellar grains diffuse interstellar features I. Introduction The Be shell-star HD illuminates the brightest known reflection nebula, NGC 7023 (Racine 1968; Strom et al. 1972). The star is probably imbedded in the nebula. Witt and Cottrell (1980a) suggest that the unusual brightness is caused by the strong forward scattering of the nebular dust grains. Duke (1951) was the first to find that the strength of the À4430 diffuse interstellar band in HD was unusually weak despite a substantial color excess (E(B V) = +0.56). In this it resembles 0 1 Orionis (Morgan 1944) and other stars with circumstellar shells (Snow 1973). It seems to be generally agreed (see for example review by Creenberg (1978)) that the full wavelength dependence of interstellar extinction can be best explained by the presence of large (diameter ~ 0.25 fi) possibly silicateous grains and a much larger number of fine particles (~ 0.01 jul) with the latter possibly causing the strong diffuse absorption band at A2175 due to the presence of graphite or carbonaceous compounds. A similar, weaker, diffuse feature is seen at À1540 in the average UV extinction curve of Nandy et al. (1976) but is not seen in the curves published earlier by Bless and Savage (1972). The NGC 7023/HD system offers an excellent opportunity to examine both the scattering and absorp- Based in part on observations by the International Ultraviolet Explorer collected at the Villafranca Satellite Tracking Station of the European Space Agency, and at NASA, Goddard Space Flight Center. fvisiting Astronomer Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. 411 tion properties of the grains. In this paper we discuss their extinction properties from direct observations of HD II. The Diffuse Interstellar Features and Very-Broad-Band Structure, ÀÀ5500 to 6800 HD was observed as part of a larger program on interstellar reddening using a refrigerated RL 1024-C Reticon linear array of silicon diodes as detector mounted at the focus of a Schmidt Camera on the KPNO White spectrograph. The system has been described in detail by Walker (1978). Observations were made with the number m and the 2.1-m telescopes. An effective resolution of 2 Â was achieved. Reddening curves were formed in the standard way from the ratio of spectra of the reddened star to that of an unreddened, or less reddened, standard of similar spectral type. Each reddening curve was normalized to give a 45 slope between À6800 and À5500 when plotted against A -1. Six reddening curves covering the full range of diffuse interstellar feature strengths relative to color excess are shown in Figure 1. The pairs of stars and their relevant characteristics are listed in Table I. The diffuse feature identifications in Figure 1 are taken from Herbig (1975). The spectral match was not complete in the region of the stronger stellar features, notably: Ha, He i, and Si n. These regions have consequently been omitted. In order to emphasize departures from a simple curve a third-order polynomial was fitted to the reddening curve for HD This polynomial was subtracted from each reddening curve to give the differences shown

2 412 WALKER, YANG, FAHLMAN AND WITT WRVELENGTH FIG. 1 Reddening curves normalized to a 45 slope for the pairs of reddened and standard stars in Table I. The curves are arranged in decreasing diffuse interstellar feature strengths. The stronger diffuse features identified by Herbig (1975) are indicated for curve A. Curve sponds to HD Tj O

3 EXTINCTION OF HD BY DUST IN NGC in Figure 2. Apart from a weak feature at À6284 and possibly one at À5780 the diffuse interstellar features are absent from the reddening curve for HD Further, the curve WAVELENGTH FJG- 2 A third-order polynomial was fitted to curve E (HD ) in Figure 1 and subtracted from each reddening curve. This serves to emphasize any departures from a simple curve. The apparent emission wing associated with the À6180 feature has been discussed by Walker (1979).

4 414 WALKER, YANG, FAHLMAN AND WITT Star TABLE I Pairs of Stars Used to Derive Reddening Curves in Figure 1 S P B7Ia A2Ia A2Ia A2Ia BO.51a BOIb 09.51a BOIb B2V B2V B3Ve B3V AE(B-V) 1.20 Curve A for HD in Figure 2 shows the reflex of the polynomial indicating that there is much less curvature or very-broad-band structure (Whiteoak 1966; Hayes et al. 1973; Whittet, van Breda, and Glass 1976) than for the other stars. The short wavelength emission wing associated with the À6180 feature (Walker 1979) is also absent from the HD reddening curve. III. UV and UBV Reddening Curve IUE low-dispersion short, and long, wavelength camera spectra of HD were obtained on 1979 August 3. Spectra of the unreddened standard HD were obtained on 1979 August 31. In both cases only spectra obtained with the large-entrance diaphragm were used. Details of the observations and basic photometric data are given in Table II, and the IUE instrumentation is described by Boggess (1978). The problem of matching spectral types is difficult in the case of HD which has been variously classified as B2 Ve (Guetter 1968), B5e (Racine 1968), and B3e III-IV (Mendoza 1958). HD was chosen as a conveniently placed rapidly rotating B3 V star. The spectral match appears, qualitatively, to be good in Figure 3. The reddening curve was formed by calculating E(V X) from the ratio of the spectral intensities corrected for the different exposure times and using a value of AV = A value of E(B V) = was used and the data were filtered to 35% of the Nyquist frequency. The resulting curve is shown in Figure 3. The smooth fit of the two portions of the curve is remarkable as no adjustment was made. In Figure 4 a mean curve for HD is shown to- star HD HD TABLE II IUE Observations Exposure Times Short Wavelength 100 s 1.6 s (Large Aperture) Long Wavelength 60 s 1 s gether with the average extinction curve of Savage and Mathis (1979) and Nandy et al. (1976), and the curves for 6 1 Ori and a Scorpius from Bless and Savage (1972). The differences between the curves and those for 6 1 Ori and the average for HD are also shown. IV. Discussion Some caution must be exercised when trying to interpret apparently anomalous interstellar reddening curves. Two normalizations have been applied to the data shown in Figures 3 and 4. The color excess E(B V) is assumed to be due entirely to interstellar reddening and the curves therefore correspond to E(B V) = 1. The total visual extinction, A v, is assumed to be the same in each case and, for convenience, is set to zero. Altamore et al. (1979) argue that the strength of the À2200 diffuse feature is the best indicator of color excess from which they derive a value of E(B V) = 0.25 for HD The remaining 0.31 magnitude they ascribe to an intrinsic reddening caused by emission from an extensive shell. Witt and Cottrell (1980b) suggested that the À2200 feature is anomalously weak and that the intrinsic color excess probably does not exceed magnitude. To our knowledge the size of the intrinsic color excess suggested by Altamore et al. (1979) is unprecedented for an early-type shell star. Three arguments led us to favor a larger value of E(B V); A. The point for U at A -1 = 2.78 ju -1 lies on the average extinction curve. If the Balmer emission found by Garrison (1978) seriously affected E(V-U) the point at U would be significantly below the average curve. B. If the À2200 diffuse feature strength is set equal to that of the average extinction curve, then for A -1 > 6 /x _1 the curve for HD lies one or two magnitudes above the average curve. C. The gradients of the ÀÀ6750 to 5500 reddening curves for the pairs of stars in Table I (i.e., prior to normalization to an overall 45 slope) correlate well with the formally derived values of AE(B V). The correlation is shown in Figure 5. None of these arguments is conclusive but, in our view, together they favor a larger value for E(B V) than that adopted by Altamore et al. (1979). The question of the true value of A v cannot be re-

5 EXTINCTION OF HD BY DUST IN NGC E(x : V) E(B-V) l/xc fjl 1 ] Fig. 3 The ultraviolet extinction curve for HD from IUE observations compared with HD as standard. The dashed line connects the points for U,B,V. (The IUE data have been filtered to 35% of the Nvquist frequency.) Fig. 4 A comparison of the UV extinction curves for HD (solid line) with the average extinction and the curves for o Sco and UOri (Savage and Mathis 1979). The differences between the curves are emphasized in the plots of (average extinction HD ) and (HD Ori,)

6 416 WALKER, YANG, FAHLMAN AND WITT this region cannot also be responsible for the diffuse features. The curve for o Sco like that for 0 1 Ori shows no large rise for A -1 > 6 /x -1 (recently confirmed by Snow and York (1976)). In this case fine particles appear to be absent but A2175 is of almost average strength with the A1540 feature also possibly present. Fig. 5 Color excesses E(B V) for the pairs of stars in Table I used to generate the reddening curves A to F of Figure 1 plotted against the the observed gradient (R/Y) between À6750 and À5550 for each curve before normalization. solved from existing data. It is normally possible to obtain a value of A v independently by extrapolating photometry to the infrared where extinction approaches zero. HD , like 0 1 Ori, is at the center of an infrared source probably caused at least in part by heating of the dust (Sneden et al. 1978). Such thermal emission vitiates any independent attempt to estimate the stellar infrared brightness. There are theoretical grounds for supposing that if the grain size is larger than average in NGC 7023 as in M42, a value of R( = A v /E(B V)) > 3.1 should be adopted. It is consequently unlikely that for the four curves in Figure 4 we are dealing with equal optical depths at V or equal column densities of particles. A larger value of R implies that the scale of the curves for both HD and 6 1 Ori should be reduced before a direct comparison is made with the average. With the reservations of the previous paragraphs in mind the differences between the extinction curves for HD and 6 1 Ori and the average can only be discussed qualitatively. The two difference curves in Figure 4 indicate (a) the form of the diffuse absorption bands at AA2175 and 1540, and (b) the extinction due to fine particles. The weakness of both the red/yellow diffuse interstellar features and that at A2125 in both HD and Q 1 Ori strongly suggests that these features have a common origin. However, the considerable difference between the extinction curves for Q 1 Ori and HD for A -1 > 6 ju -1 implies that the fine grains normally considered responsible for the rapid rise in extinction in V. VBS and the AA2175, 1540, and Red/Yellow Diffuse Interstellar Features The similar weakness of all of these features in Figures 2 and 4 for HD strongly suggests that they have a common origin in some component of the interstellar dust. The fact that they are very weak or absent for both 6 1 Ori and HD which have heated the grains responsible for the observed extinction suggests that grain temperature is an important factor. NGC 7023, unlike M42, appears to contain both large and fine particles. The much lower UV luminosity of HD compared to 6 1 Ori and the greater optical depth in NGC 7023 compared to M42 may have limited the segregation by radiation pressure of the fine particles. The presence of fine particles in NGC 7023 implies that they cannot also be responsible for the diffuse absorption features. This suggests that a third component of the dust must be responsible for the diffuse features. We suggest that this is a volatile mantle on the grains which has evaporated in the cases of NGC 7023 and M42. Both nebulae are known sources of CO (Elmegreen and Elmegreen 1978) and OH emission (Lépine and Nguyen-Quang-Rieu 1974; Pankonin and Walmsley 1978). These molecules could conceivably be derived from the evaporated mantles. The extinction of a Sco and HD is caused by the so-called p Ophiuchi dust cloud (Carrasco, Strom, and Strom 1973). As pointed out in the previous section there is an absence of fine grains obscuring o Sco. On the other hand, the AA2175, 1540, red/yellow diffuse features, and VBS for both a Sco and HD are of more normal strength. There is no infrared source or molecular cloud associated with either star despite their being of a higher temperature than HD This suggests that the dust in the p Oph cloud is far enough removed from them that no appreciable heating has occurred. We thank the Stellar Data Center, Strasbourg, for extensive bibliographic information. REFERENCES Altamore, A., Baratta, G. B., Cassatella, A., Grasdalen, G. L., Persi, P.. and Viotti, R (preprint). Bless, R. C., and Savage, B. D. 1972, Ap. J. 171, 293. Boggess, A. 1978, Nature 275, 371. Carrasco, L., Strom, S. E., and Strom, K. M. 1973, Ap. J. 182, 95. Duke, D. 1950, Ap. J. 113, 100.

7 EXTINCTION OF HD BY DUST IN NGC Elmegreen, O. M., and Elmegreen, B. G. 1978, Ap. J. 220, 510. Garrison, L. M. 1978, Ap. J. 224, 535. Greenberg, J. M. 1978, in Cosmic Dust, J. A. M. McDonnell, ed. (New York: Wiley & Sons), p Guetter, H. H. 1968, Pub. A.S.P. 80, 197. Hayes, D. S., Greenberg, J. M., Mavko, G. E., Radick, R. R., and Rex, K. H. 1973, in Interstellar Dust and Related Topics, I.A.U. Symposium No. 52, J. M. Greenberg and H. C. van de Hulst, eds. (Dordrecht: Reidel), p. 83. Herbig, G. H. 1975, Ap. J. 196, 129. Lépine, J. R. D., and Nguyen-Quang-Rieu 1974, Astr. and Ap. 36, 469. Mendoza, E. E. 1958, Ap. J. 128, 207. Morgan, W. W. 1944, A.J. 51, 21. Nandy, K., Thompson, G. I., Jamar, C., Monfils, A., and Wilson, R. 1976, Astr. and Ap. 51, 63. Pankonin, V., and Walmsley, C. M. 1978, Astr. and Ap. 67, 129. Racine, R. 1968, A.J. 73, 233. Savage, B. D., and Mathis, J. S. 1979, Ann. Rev. Astr. and Ap. 17, 73. Sneden, C., Gehrz, R. D., Hackwell, J. A., York, D. G., and Snow, T. P. 1978, Ap. J. 233, 168. Snow, T. P. 1973, Pub. A.S.P. 85, 590. Snow, T. P., and York, D. G. 1976, in Solid State Astrophysics, N. C. Wickramasinghe and D. J. Morgan, eds. (Dordrecht: Reidel), p. 19. Strom, S. E., Strom, K. M., Yost, J., Carrasco, L., and Grasdalen, G. 1972, Ap. J. 173, 353. Walker, G. A. H. 1978, J.R.A.S. Canada 71, , J.R.A.S. Canada 73, 304. Whiteoak, J. B. 1966, Ap. J. 144, 305. Whittet, D. C. B., van Breda, I. G., and Glass, I. S. 1976, M.N.R.A.S 111, 625. Witt, A. N., and Cottrell, M. J. 1980a, A.J. 85, b, A.J. (in press).