Notes on diffuse galactic nebulae

Otto Struve and W.C. Straka

Mount Wilson and Palomar Observatories
Carnegie Institution of Washington
California Institute of Technology

This article represent a resumption of the work on diffuse galactic nebulosities (mainly of the reflection type) that was conducted by members of the Yerkes and McDonald Observatories about a quarter of a century ago. Since that time important advances have been made by a number of other astronomers (see, for example, Bok¹). The magnificent Palomar photographs of the sky obtained with the 48-inch Schmidt telescope now make it possible to extend the earlier investigations. Since each field has been photographed separately on blue- and red-sensitive emulsions, it is possible to distinguish between blue and red nebulae. But the red photographs do not, per se, allow the viewer to distinguish between reddish reflection nebulae and Hα-emitting gaseous nebulae.

Of special interest are the nebulosities in and near Taurus and those forming the great complex in Scorpius-Ophiuchus. Both regions are rich in dark, obscuring clouds, and both are relatively close to the sun. Both are associated with Gould's belt, rather than with the average galactic equator. The Scorpius-Ophiuchus complex is located approximately in the direction of the galactic center and at an average distance of about 200 parsecs. The Taurus group of nebulae lies in the opposite part of the sky, at an average distance of 100 to 140 parsecs. However, A. Adolfson (quoted by Bok) "suspects on the basis of spectral types and color for the brighter stars, that there is considerable range in distance for these same dark nebulae, with the nearest dense dark nebulae no further than 50 parsecs away."

Similar distance estimates by W. Gotz and W. Wenzel, at Sonneberg, seem to indicate that the nearest boundaries of the Taurus dark nebulae may, in fact, be near the sun.

There is no such indication with regard to the Scorpius-Ophiuchus complex. These very prominent dark clouds are apparently bounded, at a minimum distance of perhaps 100 parsecs.

Nevertheless, the suspicion exists that these two great complexes are parts of the same spiral arm of the Milky Way, with the one in Scorpius and Ophiuchus forming the inner part of it. If so, the great density of the obscuring clouds in that region may represent a tendency of the diffuse material in the Milky Way to be more prominent in the inner parts of the arms than in their outer regions.

1. Red Nebula near Antares (α Scorpii). This nebula was recorded in blue light, by E. E. Barnard, as a fairly large but faint objects located mostly north of Antares. The strong red glow of the same nebulosity was found by Struve, Elvey, and Roach on a red-sensitive emulsion.² Its spectrum was obtained by Struve, Van Biesbroeck, and Elvey with the nebulae spectrograph of the McDonald Observatory.³ The great intensity of the visual continuous spectrum is well shown on Plate XI facing p. 562 of volume 87 of the Astrophysical Journal, and the authors tentatively concluded "that the nebula is essentially of the reflection type." This large nebula is not to be identified with the small emission nebula that surrounds the B-type companion of Antares. As far as we know, there have been no new spectrographic observations of the large nebula, and there is still the possibility that Hα may be found to be in emission; the McDonald spectrogram does not eliminate this possibility.

The 48-inch Schmidt Palomar Sky Survey plates show the large nebula as a spectacular red object. Since Antares lies near the southern boundary of the dense obscuring clouds in Scorpius and Ophiuchus, the most intense red glow is observed north of the star. But a fainter red glow extends towards the southeast, over a distance of several degrees, into a relatively less obscured field of stars.

The reddish glow also appears to extend into the opaque lane that runs from the star 22 Scorpii toward the east. In fact, the southern edge of the lane is distinctly luminous on the red Palomar photograph, up to a distance of approximately four degrees from Antares. That this not unreasonable may be shown by the method of Hertzsprung,⁴ as applied by him to the reflection nebulae in the Pleiades, or by the method of Struve and Story,⁵ which led, for the Scorpius-Ophiuchus group of nebulae, to a Hubble relation of the form m_* + 5 log a = 12.0, where m_* is the

apparent magnitude of the star, and a is the maximum distance of the nebular glow from the star, expressed in minutes of arc. For the Pleiades the same authors found m_* + 5 log a = 8.5. This would imply that the Scorpius-Ophiuchus cloud is effectively opaque, that the albedo of the particles is fairly large, and that the light of Antares is not greatly absorbed by the intervening clouds before it reaches the dark lane. Under these conditions, the red glow could extend to distances of four, or even more, degrees from Antares.

2. Reflection Nebulae in Scorpius and Ophiuchus. The Palomar photographs confirm and extend the conclusions of Henyey⁶ and Struve.⁷ The reflection nebula surrounding the star CD -24° 12684 is distinctly redder than the nebula that surrounds ρ Ophiuchi, despite the face that the spectral type of the former star, B3, is earlier than that of the latter, B5. The star CD -24° 12684, apparent magnitude 8.3, is deeply immersed in the obscuring cloud. It experiences a total absorption in the cloud of about 3 magnitudes; the nebula surrounding it is also produced deep in the cloud: both the star and the nebula appear reddened by selective scattering within the cloud. Rho Ophiuchi is located near the surface of the cloud; its nebula is intensely blue.

An interesting blue nebulous filament may be seen northwest of the star CD -24° 12698 (object 58 in Table I, apparent magnitude 9.3). Plate 13 of the Barnard Atlas shows it faintly. The nebulosity is also faintly shown on the red Palomar photograph. Dr. George Herbig has informed us that he has one low-dispersion slit spectrogram of CD -24° 12698 taken in 1949: "The spectral type is A0 or A1, and there is no sign of emission or any obvious peculiarities in the photographic region" of the star's spectrum. The type agrees well with the blue color of the nebulous filament.

A re-examination of the Mount Wilson Observatory spectrogram confirms the spectral types assigned to ρ Ophiuchi, 22 Scorpii, and HD 147889 = CD -24° 12684. The first has very broad absorption lines and the usual very sharp interstellar lines discovered by W.S. Adams. The star 22 Scorpii also has broad lines, but CD -24° 12684 has fairly narrow lines on a background of greatly reddened continuous spectrum.

A small reddish nebulosity appears east of CD -24° 12684 and may be associated with one or more faint red stars in its immediate vicinity, rather than with the brighter B-type star. Still another, bluish, nebulosity of very low surface brightness surrounds the A0 star HD 147702 (object 56 in Table I, apparent magnitude 9.3). Since ρ Ophiuchi has an apparent magnitude of 5.2, the difference between it and HD 147702 is 4 magnitudes. IF both are main sequence stars - as is probable - we should expect, from the H-R diagram, a difference of only 1.5 magnitudes. Apparently HD 147702 is immersed in the cloud to a much greater extent than ρ Ophiuchi. In this respect HD 147702 resembles CD -24° 12684.

The structural details of these nebulae appear to be the same on the red and blue photographs, which supports the view that they are produced by reflection only.

The foregoing results show that the Scorpius-Ophiuchus complex of dark clouds contains not only the previously known B-type stars but also several A-type stars. No illuminating star has yet been found whose brightness is reduced by more than 3 magnitudes. This result is compatible with Bok's determination of total photographic absorptions in the Scorpius-Ophiuchus region.¹ In the vicinity of CD -24° 12684 he found Δmag. = 6.3. Thus this star, and also HD 147702, are probably located deep inside the cloud.

3. σ Scorpii. The foregoing statement does not apply to the nebula that surrounds σ Scorpii: its blue and red photographs differ in a most spectacular manner (Plate I). The presence within this nebula of reflected starlight and Hα emission was first demonstrated in 1937,⁸ and was confirmed a year later with the McDonald Observatory nebular spectrograph by Struve, Van Biesbroeck, and Elvey.³ The Palomar photographs confirm this and show that Hα in emission is especially strong northwest of the star, where there is an almost straight, snake-like filament resembling a shock front. However, the red Hα glow is not limited to that filament: it overlaps in many regions with the reflection nebulosity seen on the blue photograph.

This filamentary emission nebula was photographed by Hase and Shajn,^9,10 who designated it as Simeiz No. 176-177. The

structure shown in Figure 1 of their paper¹⁰ is similar to that shown on the Palomar photographs.

Sigma Scorpii, of spectral class B1, is the hottest star immersed in the Scorpius-Ophiuchus cloud. It is a well-known member of the β CMa variables, and its two interfering periods of about 5^h56^m and either 6^h7^m or 5^h44^m place it, together with β Canis Majoris, near the top of the sequence in the H-R diagram. It has had time to evolve away from the main sequence, which should make it possible to date the entire group. Sigma Scorpii has an additional period of more than one month (this is not the beat period), and its corresponding velocity curve suggests that it is a single-lined spectroscopic binary. Nothing, however, is known about the companion.

Radial velocities of σ Scorpii obtained in 1954 indicated differential motions: the mean departures H–O II = +9.3 km/sec imply an inflow of H relative to O II (or an outflow of O II relative to H). In 1960, no such differential motions were present.¹¹ Beta Canis Majoris itself has never shown differential motions,¹² nor has this phenomenon been found in any other member of the sequence. The occurrence of Hα emission in the nebulosity need not be regarded as contradicting the sharp transition at B0 from emission to reflection. One may infer from the work of Stableford and Abhyankar¹³ that σ Scorpii is probably as luminous and as hot at β Canis Majoris. The only remarkable feature of this nebula is the fact that the emission and reflection come from different regions in the cloud.

4. Listing of Interesting Nebulae. Initially, forty-four selected red and blue pairs of prints (see Fig. 1) from the Palomar Sky Survey were examined by twelve members of an astronomy class at the California Institute of Technology, who volunteered their time for the project. The basis for selection of objects was the nebulous appearance of a star or small number of stars, with a somewhat arbitrary upper limit on brightness. Several of the prints were examined by two or three persons as a cross-check. This preliminary list contained some 250 objects.

The next stage was the re-examination of the objects by the writers, during which many objects were eliminated for various

reasons and some were added. However, not all objects retained fall into the original classification, in particular, object 13. Also, in some cases two or three objects were listed as one, e.g., 19, 41, and 51.

The amount of dark nebulosity on the prints was determined by laying a glass plate with a 10′ grid over the red print and counting squares. The total area for a print was 1521 squares, of about 1.5 x 10⁵ square minutes of arc. (Whether the obscuration was predominantly light (l), moderate (m), or heavy (h) was also noted.) From these measures and the number of objects in dark regions, the density of objects per square degree of dark matter was computed. The average density, determined by the total number of objects in dark regions divided by the number of square degrees of dark matter, is 0.22 objects per square degree of dark area.

The objects are listed in Table I in order of right ascension. The fourth column is the plate designation of the Palomar Sky Survey print. Sometimes the object appears on two prints. In such cases the one from which the position was first determined is listed. Columns 5 and 6 give the position in centimeters from the right (R) or left (L), and from the top (T) or bottom (B) of the print. Column 7 gives the color of the print on which the

measurements were made – red (R) for E-plate prints and blue (B) for O-plate prints. This is usually the print on which the object is more prominent. If it is about equal on both, the red print was used. Column 8 gives the object's name, if known. Comments are in the last column. D means that the object is in a dark lane or a cloud. A question mark means that the region is only slightly darkened. The magnitudes are those in the Durchmusterung, with ranges of variables from the Kukarkin catalog. The spectral types are from the Henry Draper catalog, the Herbig article, and the Kukarkin catalog. T indicates that the star is of T Tauri type. The dark objects named have the Barnard designations. If more than one object is included, the positions of the secondary objects are noted.

Plates II through V contain several enlargements of small portions of the prints used for the construction of Table I. Most of the nebulae in the table have not been investigated spectroscopically; only a few (like T Tauri and its associated nebulosities) are already well known.

Table I suggests that there is a tendency for the nebulae associated with obscuring clouds to be more frequent in certain regions of the sky than in others. Table II gives a summary of the results. Successive columns list: (1) the survey plate center, (2) the number of nebulae associated with dark clouds on each plate, (3) the percentages of the area of each plate occupied by dark clouds, and (4) the number of nebulae per square degree of dark cloud. The mean value is, as already mentioned, 0.22 nebulae per square degree of dark cloud. But the range in the separate plate values is large: from zero to 1.44. In two diametrically opposite regions (see Table III), one in Taurus, other in Scorpius and Ophiuchus, the values in the last column in Table II differ systematically.

The difference is probably real. It may be the result of a smaller average absorption in the Taurus region as compared to that of Scorpius-Ophiuchus (see Bok¹) and the more ragged appearance of the dark clouds in Taurus than in the region of ρ Ophiuchi.

Table 1 (edited)

Column [2] NGC or IC number; [3] Cederblad (1946); [4] Struve & Straka (1962) [7,8] apparent dimensions, in arcminutes, on the blue and red plates; [9.10] coordinates precessed to 2000.0

Notes to Table I

15. Northeast of Barnard 92 (=B10). See also H.M. Johnson, Pub. A.S.P., 72, 10, 1960

37. Variable of RW Aur type, as is T Tau, but not T Tau sub-class.

51. VY Mon is either objects 392 or 393 in the Gonzalez and Gonzalez list. See Plate III, top. VY Mon is (13.7-15.9), RW Aur type, with emission in Hα

53. Object 397 in the Gonzalez and Gonzalez list.

56. In dark area of IC 4604. See also the text of the present article.

57. In dark area of IC 4604.

58 = CD -24° 12698. In dark area of IC 4604. See also the text of the present article.

References and lists used for the preparation of Table I:

Aitken, Double Star Catalogue.

Barnard, Atlas of Selected Regions of the Milky Way.

Boss, General Catalogue.

Dreyer, New General Catalogue

T. E. Espin, M.N.R.A.S., 63, 172, 1903; ibid, 82, 188, 1922

G. Gonzalez and G. Gonzalez, Bol. Tonantzintla y Tacubaya Obs., 14, 19, 1956.

G. Haro, B. Iriarte, and E. Chavira, ibid, 8, 3, 1953.

G. H. Herbig, in Advances in Astronomy and Astrophysics, Vol. 1, Z. Kopal, ed., (New York: Academic Press, 1962), p. 47.

P. N. Kholopov, The Variable Stars, 8, No. 2, 83, 1951.

B. V. Kukarkin, P. P. Parenago, Yu. I. Efremov, and P. N. Kholopov, General Catalog of Variable Stars (Moscow: Academy of Sciences, 1958).

S. Sharpless, Ap. J. Supplements, 4, 257, 1959 (No. 41).

O. Struve and M. Rudkjobing, Ap. J., 109, 92, 1949.

The authors are grateful to the many persons who helped with this investigation, and especially to Mr. N. Divine, who chose the regions to be searched and coordinated the work that led to the construction of Table I; to Mr. Ben L. Thompson, who made the diagram and the charts accompanying the figures; and to the director and staff of the Observatories, who made their facilities available. All the photographs are copyrighted by the National Geographic Society-Palomar Observatory Sky Suvery.

1 B.J. Bok, A.J., 61, 309, 1956.
2 O. Struve, C.T. Elvey and F.E. Roach, Ap.J., 84, 219, 1936.
3 O. Struve, G. Van Biesbroeck, and C.T. Elvey, Ap.J., 87, 559, 1938.
4 E. Hertzsprung, A.N., 195, 449, 1913.

5 O. Struve and H. Story, Ap.J., 84, 203, 1936.
6 L. G. Henyey, Ap.J., 85, 85, 107, 1937.
7 O. Struve, Ann.d'Ap., 1, 11, 1938.
8 O. Struve, Ap.J., 86, 94, 1937.
9 V.F. Hase and G.A. Shajn, Izvestia Crimean Astrophysical Obs., 9, 60, 1952.
10 ibid. 10, 208, 1953.
11 Ol Struve, J. Sahade, and V. Zebergs, Ap.J., 133, 509, 1961.
12 O. Struve and V. Zebergs, Ap.J., 135, 652, 1962.
13 C. Stableford and K.D. Abhyankar, Ap.J., 130. 811, 1959.