INTERSTELLAR MATTER IN THE REGION OF THE PERSEUS II ASSOCIATION* BEVERLY T. LYNDS Steward Observatory University of Arizona. Received, June 19,1969

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1 INTERSTELLAR MATTER IN THE REGION OF THE PERSEUS II ASSOCIATION* BEVERLY T. LYNDS Steward Observatory University of Arizona Received, June 19,1969 Observational data are assembled and discussed in order to determine the distribution of the interstellar gas and dust in the direction of the Per II association. It is concluded that most of the obscuration occurs in dark foreground clouds of variable opacities. Only one or two of the 17 Per II cluster members appear to lie in dark nebulae. It also appears that the interstellar gaseous component may be located in an extensive foreground layer. The emission nebula NGC 1499 is considered to be a rim of a foreground dark nebula which is illuminated by the 07 star Per. The electron densities within NGC 1499 are believed to be greater than 10 and less than 100 cm -3 with the possible exception of the brightest rim within the emission nebula. On the basis of the absence of detectable emission in the immediate vicinity of $ Per, it is argued that the value of the electron density between NGC 1499 and Per must be very low, probably less than 5 cm -3. I. Introduction The Perseus II association was recognized and discussed in detail by Blaauw (1951), who identified 17 OB stars as members, the earliest of which is Persei of spectral type 07 and the brightest of which is Per of spectral type B1 lb. The photometric distance modulus of the association has been found to be 7 54 ± 0.13 (Borg- man and Blaauw 1963) if an extinction ratio of A v IE(B V) = 3.75 is used. Proper motions and radial velocities of the members sug- gest an expansion age of 1.3 X 10 6 years. Blaauw states, "The Per group is situated in a region of the sky rich in nebulosity and dark matter. The evidence of the recent origin of the group makes a study of its relation with the interstellar matter particularly inter- esting. The present paper is an attempt to synthesize all available information on the distribution of the interstellar gas and dust in the region of this young cluster in order to determine the amount of interstellar matter related to the association. ^Presented at the Flagstaff meeting of the Astronomical Society of the Pacific, June 18-21,

2 THE PERSEUS II ASSOCIATION 497 Fig. 1 Schematic representation of the bright and dark nebulosities in the region of the association. The 17 black stars identify the association members. The asterisk shows the location of the open cluster NGC 1342, whose distance modulus is 8 7. The shaded areas depict the dark nebulosities apparent on the Palomar-Schmidt prints while the oudined but unshaded areas represent regions of bright nebulosities detectable on the same prints. Figure 1 is a schematic representation of the bright and dark nebulosities in the region of the association. The 17 black stars identify the association members. The asterisk shows the location of the open cluster NGC 1342, whose distance modulus is 8 7 (Johnson, Hoag, Iriarte, Mitchell, and Hallam 1961). The shaded areas depict the dark nebulosities apparent on the Palomar prints of the National Geographic Society-Mount Palomar Sky Survey while the outlined but unshaded areas represent regions of bright nebulosities detectable on the same prints. Two relatively bright regions apparent in this area of the sky are the California nebula, NGC

3 498 BEVERLY T. LYNDS 1499, which is an H n region presumably excited by the association star Per located just south of the nebula; and the nebulosity associated with the Pleiades (m M = 5 5), a few stars of which are identified by the small black circles at l II= 167, b 11 = 23Ï5. In the immediate vicinity of the Pleiades cluster, the nebulosity is blue; however, the apparent extension of this reflection nebula shows a change in color in such a way as to appear brighter on the red than on the blue print in the more opaque areas. All other bright nebu- losities shown in Figure 1 are very faint regions. The large dotted area may not be real its presence is only suggested on the red Palo- mar print. The diagram in Figure 2 is similar to that of Figure 1 and was published oy Khavtassi (1960), who based his study on the prints of the Ross-Calvert Atlas of the Milky Way. Three stars of the Per II association are identified:,, and o. The clusters NGC 1342 and the Pleiades are also shown, as are the bright diffuse nebulosities of the Pleiades, of NGC 1499, and of the area near o Per. The num- bers within the dark clouds refer to Khavtassi s catalog (1955). The overall pattern of the distribution of the interstellar matter in Figures 1 and 2 is the same; the differences arise primarily because the 48-inch Palomar Schmidt telescope is capable of recording fainter luminous nebulosities, and secondly, because the Ross- Calvert prints seem to suggest the presence of less-opaque dark clouds not seen on the Palomar Schmidt prints. The general impression that one gets from studying Figures 1 and 2 is that the Per II cluster is indeed located in a complex region of bright and dark nebulosities coupled with relatively clear regions. Some of the nebulosity must be nearer to us than is the Per II group; this is assumed because the distance to the Pleiades is well known. However, the facts, that one cluster member ( o Per) is apparently illuminating one edge of a dark cloud and that another cluster member (BD ) has bright nebulosity associated with it, indicate that at least some of the interstellar matter must be located spatially in the immediate vicinity of the association. In order to obtain a more quantitative estimate of the amount of dust and gas near the association, each of the following different approaches to the problem will be discussed. 1. A study of the general obscuration over the area, based on star counts, galaxy counts, and photoelectric photometry.

4 THE PERSEUS II ASSOCIATION 499 Fig. 2 Dark nebulae in the region of the Per II association (from Khavtassi 1960). 2. A study of the general features of the interstellar gaseous com- ponent, as evidenced by interstellar absorption and emission lines. 3. Upper limits on the electron densities within the cluster will be estimated, and a schematic model of the spatial distribution of the dust will be suggested. II. General Obscuration A large region in Perseus has been studied in detail by Heeschen (1951), whose analysis was based on star counts to magnitude 15, color excesses of stars, and Hubbles (1922) galaxy counts. Figuie 3 shows Heeschen s results for an area near the Per II association.

5 500 BEVERLY T. LYNDS A x Fig. 3 Distribution of obscuring material in the region of the Per II stars, indicated by black dots (after Heeschen 1951). The heavy obscuration indicated in the figure has been found by Heeschen to be at a distance of approximately 300 parsecs. A second, less-opaque cloud at a distance of 800 parsecs appears to cover the entire area and to produce an absorption of The contours in Heeschen s diagram agree quite well with the location of the dark nebulae apparent in Figures 1 and 2. With the exception of the southeastern segment, dark nebulae having opacities of two magnitudes or more appear to surround the association. Most of the association members are seen projected on the less-obscured areas of opacity no greater than one magnitude. The dark clouds appear to lie at a distance of parsecs from the sun; the Per II association is believed to be located some 300 to 400 parsecs away.

6 THE PERSEUS II ASSOCIATION 501 Fig. 4 Galaxy counts in the region of the Per II stars. The presence of these dark clouds is also evident in the galaxy counts made within the area. Figure 4 is a representation of the raw galaxy counts made by Shane (these data were kindly supplied by C. A. Wirtanen). Each dot in the diagram represents one galaxy counted within a square grid, ten minutes of arc on a side. Figure 5 contains the total number of galaxies counted per square degree. The counts represent the sum of the dots of Figure 4 but also include the various empirical correction factors necessary to reduce the counts to a homogeneous system, as calibrated by Shane and Wirtanen (1967). According to Shane and Wirtanen, the average number of galaxies over the sky that would have been counted without galactic extinction is 89 per square degree. The 176 square degrees of Figure 5 have been divided into three regions as follows: one relatively clear area to the west of the center of the association, one which defines a dark cloud, and the remaining eastern area which is rather irregular in obscuration. The boundaries of the three regions

7 502 BEVERLY T. LYNDS Fig. 5 Galaxy counts in the region of the Per II stars. The area is divided into three regions of different obscuration. are shown in Figure 5. The average number of galaxies per square degree, N, in each field was found and the mean extinction cal- culated, taking the value of y(m) = d(log N)ldm to be 0.47 corres- ponding to m = 18.7 (Shane and Wirtanen, 1967). Table I lists the results together with a comparison of the extinction results of Heeschen (1951); Seyfert, Hardie, and Grenchik (I960); and Harris (1956). From a comparison of the galaxy-count and star-count extinctions listed in Table I, it is seen that most of the absorption in this area is produced within 800 parsecs of the sun. The largest difference between the Heeschen results and the galaxy counts is found in the region of the dark cloud, where the numbers of galaxies and of stars are quite small and any statistical study is subject to large

8 THE PERSEUS II ASSOCIATION 503 TABLE I Extinction In Region of Per II Association Field Western Cloud Eastern Northern Cloud Galaxy Data N Extinction 1* Av. Ext. in Heeschen Cloud: pc 800 pc 0*5 0* Seyfert et al. 0* Harris Per II Stars only: 0*7 (1) 1.5 (5) 0.9 (10) uncertainties. It is therefore reasonable to conclude that very little absorption is produced at distances beyond the 800 parsec cloud and that most of the total absorption is produced in a region between 200 and 300 parsecs from the sun. This conclusion is also confirmed in the work of FitzGerald (1969), who found that moderately strong amounts of interstellar material producing absorptions between 0 9 and 1 8 lie within parsecs of the sun. Photoelectric photometry of a select number of stars in the region of the Per II association has been reported by Seyfert et al. (1960). Spectral types from objective prism plates were determined for 134 B and A stars, and UBV measurements were made for each star. Values of the absorption are listed by the writers, who used an extinction ratio of 3. The stars in the Seyfert-Hardie-Grenchik list have been divided into the three groups as defined by the three areas of the galaxy-count analysis. In addition, a fourth group of stars with declination of +36 to +37 (in galactic coordinates, the northeastern edge of cluster) is located in another region containing obvious dark clouds. The mean values of absorption were found from the UBV photometry and are listed in Table I. No stars fainter than eleventh magnitude were included in the program, which means that the objects all lie within a distance of 700 parsecs, if they are main-sequence stars. The amount of absorption found within each region varies by several tenths of a magnitude and may arise partly because of uncertainties in the luminosity and spectral classifications of the stars and partly because of local density fluctuations within the absorbing clouds. Again, the largest discrepancy between the star-data and the galaxy counts is in the cloud area. The stars selected by Seyfert et al. (1960) lie in the northernmost edge of the cloud; whereas the galaxy counts represent the mean over the entire cloud.

9 504 BEVERLY T. LYNDS One of the best photoelectric techniques of detecting absorbing clouds makes use of the uvby system developed by Strömgren and Crawford (Strömgren 1962). A recent study of the region of the Per II association has been undertaken by A. E. Rydgren (unpublished) using the four-color method. His preliminary results are that the average amount of absorption along the eastern edge of the cloudregion of the Per II association is about 0^8 and that rather marked variation in the amount of obscuration exists within the cloud. The absorption seems to occur between parsecs of the sun; this is in agreement with Heeschen s conclusion. Because of the large scatter ( ~ 0 5) in the amount of absorption found for stars lying within the few square degrees covered by the cloud, only an extensive photometric program will be able to give more precise information on details of the clumpy nature of the obscuring material within this cloud area. Harris (1956) has published UBV measurements of the Per II members, and his absorption values (using A y = 3E(B V)) are listed in Figure 6. The A y values found by Borgman and Blaauw (1963) are also quoted in Figure 6; these latter values appear in parentheses below Harris measurements for the same star and have been changed, for consistency, to the numerical value appropriate to an extinction ratio of 3. The cluster NGC 1342 (distance = 550 pc) is also shown, and its absorption is quoted from Johnson et al. (1961). The mean of the absorption values in each of the three areas of Figure 5 are listed in Table I; the numbers in parentheses denote the number of Per II stars used in determining the mean value. Absorption values for two association members were not included in Figure 5. These stars are X Per whose magnitude, color index, and spectral type vary; and BD which has a color excess E(B V) of 0 86 and is a member of the small, nebulous cluster IC 348. This open cluster has been studied by Harris, Morgan, and Roman (1954), whose value of the color excess is quoted in the preceeding sentence, and by Herbig (1954) who discovered 16 faint emission Ha stars apparently physically associated with IC 348. The reddening of the brightest members of the cluster, the detection of associated Ha emission-line stars, and the presence of the reflection nebula (Hubble 1922) around the cluster strongly suggests that IC 348 is indeed within the dark cloud. The distance

10 THE PERSEUS II ASSOCIATION 505 Fig. 6 Absorption values A y = 3E(B~V) for the Per II stars, as determined by Harris (1956). The values in parentheses below Harris values are those of Borgman and Blaauw (1963). modulus quoted by Harris et al. (1954) is 7 9, corresponding to a distance of about 380 parsecs. Proper motions of the members of IC 348 have been discussed by Blaauw (1952), who showed that the motion of the cluster is con- sistent with the general expansion of the Per II association. Two stars (Gingrich Nos. 3 and 7) were tentatively identified as fore- ground stars on the basis of their proper motions. According to the data of Harris et al. (1954), these two stars appear to be foreground G dwarfs having relatively little reddening. Exact spectral types

11 506 BEVERLY T. LYNDS are not available for this pair, but the apparent magnitudes of and for these stars suggest a distance modulus of about six magnitudes or a distance of approximately 150 parsecs. Thus it appears that most of the absorption is produced in the single dark cloud beyond this distance. Photometry of the cluster members shows that the color excess varies across the cluster. The most heavily reddened star is BD ; the star Gingrich No. 5 is the least reddened, with an E(B V) of 0^46. This again emphasizes the variability in the amount of absorption within the single cloud. An examination of the Palomar Sky Survey prints of the densest regions of this dark cloud suggests that the size of condensations within this cloud is on a scale of about 3'. Thus it appears that very opaque regions (with extinctions probably much greater than three magnitudes) have formed within the cloud, and their linear scale is about 0.3 parsecs. Such relatively small-scale variations in extinction have been reported by Bok (1956) for a region of the Taurus dark cloud (l u ~ 170, b 11 ~ 15 ). The southernmost Per II association member is the star HD 21483, which was tentatively included in the Per II membership by Blaauw (1952) on the basis of its proper motion, although its radial velocity differs by 25 km/sec from the mean value for the group. This star is the central star of SA 47, and the dark cloud in its vicinity has recently been studied by Schreur (1969), who concludes that the cloud lies within a distance of 500 persecs of the sun and produces a mean absorption of approximately two magnitudes. Schreur suggests that HD 21483, classed as a B3 giant, is located well beyond the cloud and may not be a member of the Per II group. From a comparison of the extinctions listed in Table I, it appears that the Per II association lies adjacent to or just behind a region of variable obscuration which produces an average extinction of about 0^8 over most of the association but which shows up as a dark cloud with an average extinction about two to three magnitudes over the southernmost part of the association. Within this cloud may be dust concentrations on a scale less than one parsec in diameter. One star, HD 21866, lies in the clearer region to the west. Two of the southernmost cluster members (one of which is actually the brightest star in the cluster IC 348) may be located within the dark cloud, but there is no evidence that any additional obscuration is present within the association itself, although galaxy counts and

12 THE PERSEUS II ASSOCIATION 507 Heeschen s data suggest that a small amount of additional extinction exists beyond the association. Furthermore, there is no correlation between spectral type and the amount of extinction for the cluster members. III. Interstellar Gas Table II summarizes the available optical information on the presence of an interstellar gaseous component between the Per II association stars and the sun. For comparison purposes, this table also lists the 21-cm velocities, the strength of the A 4430 band, the amount of interstellar polarization, and the extinction measured for each association member. All velocities discussed in this paper have been corrected for a solar motion of 20 km/sec, A = 271, D = +30 (1900). The Can K-line velocities are the values of Adams (1949) plus a systematic correction of +1.6 km/sec (Routly and Spitzer 1952). The strengths of the K line are those reported by Spitzer, Epstein, and Hen (1950). The 21-cm velocities were published by Howard and Wentzel (1963); but for only the two stars HD and HD 22951, whose \b\ > 15, do the hydrogen line velocities represent the values found for the exact direction of the star. For the other cluster members, whose \b\ < 15, the hydrogen velocities quoted by Howard and Wentzel were taken from the low-velocity peaks of complex 21-cm profiles published by McGee, Murray, and Milton (1963). The polarization data of Table II are from Hall (1958) and are illustrated in Figure 7 (adapted from Hall 1955), where the obscured area of Figure 5 has been outlined. It appears that the southern cloud has affected the polarization of the stars lying in or (in the case of HD 23625) near the edge of the cloud. The position angle of the plane of polarization of the nine other members resembles that of stars located in the same longitudes but lying closer to the galactic plane. The equivalent widths of the A 4430 band were measured from spectrograms kindly loaned to the writer by the Lick and Dominion Astrophysical Observatories. The available spectra of Per and Per were unsuitable for measurement of the A 4430 band, although its presence was evident in both spectra. Figure 8 shows the relation between the equivalent widths of the A 4430 band and the color excess. The crosses represent the present measurements for the Per

13 508 BEVERLY T. LYNDS 3 CO 05 t> cm cd oo q 00 O q ID o 6 h o oi OO'ÍN^ CD 05 G q I 0 0 odooh ö ó L- co CM t> ooo a N!-h CO O t> 05 O XT CD i I i I i t i I CM o o o o o ID CM ^ CD ID co > i cm > i cm cm - i oí oooooo oo 1> 00 ID I O CM OOO ' o pp 05 CD CM CO (M CO CD i i i I 05 tf O CD 1C ID *D i I CM CO CD CD TT 00 ID ID TABLE II Interstellar Data for Per II Stars ^ çqqcmqcotm-qqf-cot^^ ^ i i cd i i o O i i CM ID O 1 1,_ h,_ H * w CM ci u O W o V. c/5 Ó CM CD CD CO CM oo o q q 05 r-h Xfi Tjn ID O CO ID + + ID O cd + + t> ID O CO CO CD CD + + oo ic cd cm i 1 i 1 o cd + ID 05 O CD CM 00 OHO ID ID ID ID t> t> 00 o o CM L" t>" i - 1 r ( ^ CD TJ 3 Ö CD cd > cd + CM ID + Ü t <D *«C/5 5 's P4 O CO CO CM CD CM H i I r ( CM > > i I CM CO O CM "H "H i i i CD + + CO CO 05 CM i-h CD i i + ci -M CD Q X CD s ci z CO 00 i i CM CO CDHOOo ID ID CD 00 h OO < ( 05 CM O CO CO i-h CO i CM CM CM CM < Î-I CD O pp ph O 00 ID r-h O 00 Tf I> CM CO CO CD i~h i I CO ID CO CO Tf rf CM CM CM CM CM CM o 5 ÉP P^ xji ^ O CM ^ i i CD 05 hf TP CM CM s Ph CO CO 05 CO ID t- 00 ID ID ID CM CM CM Underhill s value.

14 THE PERSEUS II ASSOCIATION 509 Fig. 7 Polarization of the Per II association stars (after Hall 1955). II stars; for comparison, equivalent widths of field stars measured by other observers are also plotted as identified in the caption. One can see that there is a correlation between color excess and the strength of the A 4430 band; the observational scatter is illustrated by the vertical lines which connect the equivalent widths measured by different observers for the same star. Analogous diagrams have been discussed by Wampler (1966), who published the central depths, Ac, of the A 4430 band for 99 stars, three of which are listed in Table II. Wamplers A c values are quoted in parentheses below the equivalent width measures for the respective stars. In addition, Wampler observed three other fainter members of the cluster IC 348. No. 12, an A2 III star which is very near BD , has A c = 3.6 and 3E(B V) = 2.5; No. 10, another A2 III star which is slightly farther away from -f , has A c = 1.0 and 3E(B V) = 2.3; and No. 4, an A0V star located some 3' away from BD , has no detectable A 4430 band but has a value of 3E(B~V) ~ 1 4. On the average, a star with 1.4 magnitudes of extinction should have a measurable A 4430 strength. Duke (1951) and Wampler (1966) have suggested that the strength of the A 4430 band is sys-

15 510 BEVERLY T. LYNDS Fig. 8 Equivalent widths in angstroms of the A 4430 band as a function of the extinction, set equal to 3E{B V). The references are as follows: Underhill, A. 1956, Pub. Dominion Astrophs. Obs. 10, 201; Greenstein, J., and Aller, L., 1950, Ap. J. Ill, 328; Seddon, H., Nature 1967, 214, 257; Hudson, K., and Geary, J. (unpublished); Herbig, G. H., 1966, Zs. f. Ap. 64, 512. The straight line defines the one-to-one relation between the equivalent width and 3E(B~V). tematically weakened in stars lying at high galactic latitudes. This phenomenon was not found in the present data. The open symbols in Figure 7 represent stars having \b\ < 6 ; while the Per II stars (crosses in Figure 7) have b > 13, as do the stars represented by the filled symbols. The situation for the K-line strengths is even more ambiguous. The least-reddened star, HD 21856, has the strongest K line (equiva-

16 THE PERSEUS II ASSOCIATION 511 lent width Â), whereas one of the most heavily-reddened stars, Per, has a relatively weak K line (equivalent width Â). Adams (1949) reports, however, that the interstellar absorption spectrum of Per is rich in molecular lines and resembles Ophi- uchi, whose interstellar spectrum has recently been analyzed in de- tail by Herbig (1968). According to Herbig, the equivalent width of the strongest component of the interstellar K line in Oph is Â; the star has a color excess E(B V) = 0.32, which is about the same value as that of Per. Herbig found that Ca i and Ca n may be underabundant by several orders of magnitude in the interstellar cloud producing the 15 km/sec Ca n lines; such an anomaly may be present in the direction of Per also. The interstellar absorption spectrum of X Per is also rich in molecular lines, according to Adams. Zeta and X Per lie in the cloud area of the association; it may be that the strength of the molecular lines is better correlated with the presence of obscura- tion than are the atomic lines. This problem has been discussed by Bates and Spitzer (1951) who based their study on Adams visual estimates of the strengths of the interstellar lines. Of the twelve Per II stars listed by Adams, six have molecular lines present in their spectrum (X Per, Per, 0 Per, HD 22951, HD 24131, and Per), but only three of these lie in the cloud area of Figure 5. Adams also comments that the relative intensities of the molecular lines in Per are quite different from those of X and Per. Clearly, in order to determine the location and physical state of the gaseous com- ponent of the interstellar medium in the region near the Per II association, we need more quantitative and homogeneous data on the interstellar line strengths. The Per II stars seem to isolate a segment of galactic longitude and latitude in the O-B star distribution; this fact was pointed out by Blaauw (1956). The only relatively nearby early-type stars are those belonging to the a Per group. Crawford (1969) has found a true distance modulus of 6 2 ± 0 1 for this group, and he quotes a mean color excess of E(B V) = (Wß. Six of the a Per stars are listed by Adams (1949) as having moderately strong interstellar K lines (with a mean intensity of 6 as compared to the Per II mean of 9 on Adams visually-estimated scale). The radial velocities of the K line in these six stars are all similar to the mean value for the interstellar lines of the Per II stars: +2 km/sec for the a Per group; +5 km/sec for the

17 512 BEVERLY T. LYNDS Per II stars, with a dispersion of ±2 km/sec for each group. The B0.5 V star e Per (V = 2 5, E(B~V) = 0 12) whose position is shown in Figure 2, may be a member of the a Per group. For this star Adams reports two interstellar K-line components; one has a velocity of +2.8 km/sec and an intensity of 4, the other has a velocity of km/sec and an intensity of 3. The Per II star seen closest to is è Per, which is the only association member with a two-component K line having velocities of +4.7 km/sec and km/sec and intensities 10 and 1 respectively. Located a similar angular distance away from the Per II stars but in the opposite direction is the Pleiades cluster. The true distance modulus of this cluster is 5 5 and the average color excess E(B V) is 0^5 (Crawford 1969). Even with so small an amount of redden- ing, the Pleiades stars have weak interstellar K lines according to Adams. For this cluster, the mean K-line velocity is +8 km/sec and the mean intensity is 1 5. Adams reports that the interstellar molec- ular lines are present in the spectra of the Pleiades stars; he does not indicate their presence in the a Per group, in e Per, in 30 Per, or in HD (a B5 field star near NGC 1342, which has an E(B V) of 0 06 (Crawford 1969) and a K-line velocity of +3.2 km/sec with intensity 3). The simplest conclusion to be drawn from this circumstantial evidence is that the interstellar K line is formed in an extensive fore- ground layer capable of producing absorption lines of mean inten- sities 1.5, 6, and 9 at the respective distances of 126, 174, and 400 parsecs. Whatever the mechanism of production of interstellar molecules may be (see for example, Herbig 1969), it appears to operate more efficiently in the vicinity of the Taurus obscuration than in the clearer regions closer to the galactic plane. How the characteristics of the A 4430 band fit into this model is as yet un- determined. Adams (1949) has pointed out that a comparison of the average radial velocity ( + 8 km/sec) of the interstellar lines seen in the spectra of the Pleiades with the mean radial velocity of the cluster stars themselves ( 1 km/sec) suggests that the clouds producing the interstellar lines are moving relative to the stars with a radial velocity of more than 9 km/sec. A similar situation exists for the Per II association; the mean radial velocity of the 17 members of Per II is +16 km/sec, whereas the average K-line velocity for

18 THE PERSEUS II ASSOCIATION 513 eleven of the stars is +5 km/sec and the mean of the 21-cm veloc- ities in the direction of seven of these stars is +4 km/sec. The agreement between the velocity measurements of the interstellar K lines with the 21-cm line suggests that the clouds producing the interstellar absorption are H i regions. However, near Per is the H ii region NGC Courtes, Cruvellier, and Georgelin (1966), using a Fabry-Perot interferometer, report a radial velocity of +0.6 km/sec for the Ha emission line of NGC Miller (1968) quotes a velocity of 2.8 ±5.1 km/sec based on spectra obtained with a slit spectrograph whose grating was set for first-order images of Ha. May all (1953) published velocities for four different regions of the nebula, and his values range from +28 ±18 to +59 ±16 km/sec, based on the mean velocities of A 3727, Hß, Hy, HS, and He. At the writer s request, C. R. Lynds obtained a series of seven spectra centered on the brighter rims in NGC The main purpose in obtaining these spectra was to determine the A 3729/3726 ratio in an effort to estimate the electron densities in the brightest parts of the nebula, but the emission lines of A 3727, He, HS, and He A 3888 were also measured for radial velocities. Unfortunately this spectrographic system is not yet calibrated for accurate radial velocity work, and only preliminary velocities can now be quoted. Slit positions, dou- blet ratios, and velocities obtained on the basis of a single plate of each region are shown in Plate I. The radial velocities range from + 2 km/sec to +28 km/sec with a mean of +14 km/sec and a dis- persion of ± 7 km/sec. Miller (1968) states that he tried to avoid structural features and to select a portion of the nebula which would produce uniform illimination of the slit, whereas May all and Lynds selected approximately the same bright regions within the nebula. Courtes (1960) lists his individual velocity measurements of NGC 1499, although precise positions within the nebula are not given. Two of his fields, apparently centered near the stars BD and BD , have individual velocity measures ranging from to +2.3, and from +8.3 to 2.5 km/sec, respectively. It is probable that turbulent velocities exist within the nebula and that the so-called comet-tail structures exhibit higher expansion veloc- ities than does the main body of the nebula (Osterbrock 1959). These possibilities may explain most of the velocity discrepancies. Until more data are available, a radial velocity of 1 ± 5 km/sec

19 514 BEVERLY T. LYNDS» PLATE I Slit positions and the corresponding values of the A 3729/3726 ratio and the radial velocity. will be adopted as the average radial motion for NGC This velocity is similar to the values derived for the interstellar absorp- tion lines ( + 4 ± 3 km/sec) and it is quite different from the +64 km/sec radial velocity of Per. The nebula s velocity is also appre- ciably different from the mean velocity of +16 km/sec of the Per II members. The conclusion, already drawn by others, is reconfirmed that NGC 1499 is a denser portion of an interstellar cloud which happens to be, at present, illuminated by the 07 star Per. IV. Electron Densities and Space Distribution of Material Plate I contains the values for the À 3729/3726 ratio determined from the Kitt Peak spectra of C. R. Lynds. The dispersion of 40 Â/mm allows good separation of the doublet, and the linear charac- ter of the image tube avoided plate calibration difficulties. The un- certainty in the ratio is estimated to be ± It appears that the

20 THE PERSEUS II ASSOCIATION 515 radiative de-excitation value of 1.50 is indicated by the observations. One exception is a bright condensation with an observed r = 1.33 located east of the star BD , implying an electron density of 160 cm -3 assuming a T e = 10,000 K (Aller and Liller 1968). The spectrum at this point also has the strongest lines of A 3727, He i A 3888, and the Balmer lines. The region appears also to be the brightest portion of the California nebula on the blue Palomar- Schmidt print. It thus appears that the electron density in NGC 1499 is less than or equal to 100 cm -3, with the possible exception of the very brightest rims, in which the densities may be as high as 160, but certainly less than 250 cm -3. Two other estimates of electron densities have been made. Pot- tasch (1960) used measurements made by Shajn, Hase, and Pikel- ner (1954) of the line/continuum emission ratio in order to predict the value of N e on the assumption that the continuum emission arises primarily from the two-quantum transition of the 2s level of hydrogen. The value of N e so determined is 42 cm -3 ; however, Pottasch developed his theory on the basis of a spherically sym- metrical nebula, which is certainly not the case for NGC The second estimate of electron densities was made by John- son (1953) and is based on a determination of the emission measure (EM) of the nebula. Utilizing the B spectrograph (f/0.65 camera) at Yerkes Observatory, Johnson obtained spectra of many dif- fuse nebulae; NGC 1499 was one of them. The strengths of the Ha line were calibrated against exposures of the central (8' X 24') region of IC 405, which has an EM of 6300, according to Strömgren (1951). Johnson s intensities recorded on a three-hour exposure of the bright regions of NGC 1499 correspond to an emission mea- sure of 10,700. The value of the hydrogen density was calculated from N e =(EMI2s) 1^. Johnson took s= r sin (cx/2), where r is the distance to the nebula and o- is its angular extent on the slit. For NGC 1499 Johnson reports the value of 31 cm -3 for N e, assuming a depth of 5 parsecs. Johnson (1968) concludes that, on the average, N e is greater than cm -3 for NGC Thus it appears that the electron density within the California nebula is fairly clearly confined to a range between 10 and 100 cm -3, with the possible exception of the very brightest filaments, which may be denser by a factor of 2 or 3.

21 516 BEVERLY T. LYNDS The numerous but rather indirect arguments presented in this paper also suggest that the portion of the interstellar medium pro- viding the source of the hydrogen lies between 200 and 300 parsecs from the sun. With this spatial arrangement, it is possible to set an upper limit to the electron density in a sphere around Per. John- son states that the faintest Ha emission recorded by his B spectro- graph has a value of 400. The angular extent of Ha on his spectra of NGC 1499 is 5.4, and he describes the nebula as having a very faint extension to the east of the core of the emission usually ob- served. His description of the nebula coincides with the structure seen on the red Palomar-Schmidt photograph, and thus it appears that the 48-inch prints are able to record, in the red, an emis- sion measure of 400 or less, depending on the relative strengths of all emission lines recorded on the red plate. Because no emission is apparent on the part of the Palomar print lying in the western segment of a circle around Per, it is concluded that N e < (40012s) 1/è and if 25 is arbitrarily assumed to be 5 parsecs, then N e < 6 cm -3. This calibration of the red Sky Survey print is a useful number to have in the general study of diffuse nebulae, and a more exten- sive attempt was made to establish the value of N e below which no red emission would be recorded on the 48-inch Schmidt print. For this purpose, the description of the observations made with the 150-foot McDonald nebular spectrograph by Struve and Elvey (1940) was compared with the appearances of the same regions on the Sky Survey prints. For example, Struve and Elvey s region No. 44 is described as having Balmer lines strong in emission, being slightly stronger near the eastern edge of the slit than in the im- mediate vicinity of the guide star 30 Canis Majoris. The observa- tions are confirmed on the Palomar plates, although the A 3727 line strongly recorded by Struve and Elvey does not register on the blue plate. In region No. 40, Struve and Elvey report that a five-hour exposure shows a strong A 3727 line but a rather weak Ha line. The emission is easily detected on the red print but is very weak on the blue. Struve and Elvey s region No. 18 has 8 Cephei as a guide star. A six-hour exposure recorded Ha and A 3727 fairly conspicuously, according to Struve and Elvey. On the Sky Survey prints the field around 8 Cep has quite a bit of emission; however, within a radius of about 30' around 8 Cep no emission is certain. This is an example

22 THE PERSEUS II ASSOCIATION 517 of the slightly greater sensitivity of the McDonald spectrograph. In general however, the red Sky Survey print compares quite favorably with the sensitivity of the McDonald nebular spectrograph; and, to a first approximation, it is assumed that the sensitivities in the red are approximately the same. According to Strömgren, an emission measure of 1000 was about the faintest detectable with the McDonald nebular spectrograph. His calibration indicated that in order for an O star situated within a cloud to have an observable H n region surrounding it, the electron density must exceed 10. Thus, the region of space around Per must have an electron density less than 10, and probably less than 6 cm -3 because the upper limit for the emission measure in this region is very likely below 400. Furthermore, the temperatures used by Strömgren for the early-type stars may be too high, in which case N e should be reduced even more for an optically thin nebula. For example, Strömgren used 50,000 K for an 07 star and found an H ii sphere of 87IN e ^ parsecs. The 35,000 K temperature recently suggested for an 07 star (Morton and Adams 1968) reduces the radius of the Strömgren sphere to 50IN e^ (Williams 1969). Thus the upper limit for Ne would be reduced to about 5 cm -3. It therefore seems reasonable to conclude that the hydrogen density in the immediate vicinity of é Per is less than 5 ions/cm -3 and the corresponding upper limit to the radius of the H n region around Per is 17 parsecs which corresponds to an angular size of about 3 (which is approximately the angular separation of Per and HD 25833). NGC 1499 is contained within this sphere. The projected width of the California nebula is approximately 3/4 which corresponds to a linear width of about 5 parsecs if the nebula s distance is assumed to be 350 parsecs. The hydrogen density within the nebula is probably at least 30 and may be as high as 100 ions/cm 3, thus the ionizing radiation could not penetrate more than a distance of 5 parsecs even under the most favorable conditions (no radiation loss before reaching the surface of the nebula). It seems more reasonable to conclude that NGC 1499 is an illuminated rim of the dark cloud. The Tar side of the cloud is ionized by i Per; the stellar radiation penetrates to a small depth, probably only one or two parsecs. The apparent width of the nebula arises because it lies between us and Per and we are viewing it obliquely as sketched in Figure 9.

23 518 BEVERLY T. LYNDS Fig. 9 Schematic sketch of the illumination of NGC 1499 by star è Per; (a) is the cross section perpendicular to the galactic plane, (b) represents the projection onto the galactic plane. The dashed vertical shading represents the suggested location of dark nebulosities. The solid black line defines the location of NGC If this model is correct, the small dark nebulae in NGC 1499 would probably represent condensations seen projected against the H ii region but located this side of the radiation-bounded ionized zone. This hypothesis is consistent with the fact that there seem to be no bright rims associated with these opaque areas. If such condensations are a general characteristic of the dark clouds in this region, it is understandable why large variations in extinction exist between relatively nearby stars in this area. Blaauw has suggested that the point of origin of the Per II association is located near the star o Per. There is no doubt that the dark nebulae near o Per are associated with at least one of the Per II members. There also appears to be little evidence of any other interstellar material being directly related to this group. In fact, it appears that the Per II stars represent an association located near the outer edge of our local spiral arm and that there are some obvious dark clouds on the inner side of the association, but that very little interstellar material exists in or beyond the association.

24 THE PERSEUS II ASSOCIATION 519 The writer would like to express her thanks to Dr. C. R. Lynds for obtaining the image-tube spectra of NGC Acknowledgment also goes to Drs. A. B. Underhill and G. H. Herbig for selecting the D.A.O. and Lick plates to be loaned to the writer during the course of this investigation, and to Mr. John Geary and Mrs. Katherine Hudson for the plate reductions. The writer has profited by several discussions with Drs. D. L. Crawford and R. E. Williams and is also grateful to these astronomers and to Dr. R. E. White for the critical reading of the manuscript. This study was begun under the ONR contract N C REFERENCES Adams, W. S. 1949, Ap.J 109,354. Aller, L. A., and Liller, W. 1968, in Nebulae and Interstellar Matter, B. M. Middlehurst and L. H. Aller, eds. (Chicago: University of Chicago Press), p Bates, D. R., and Spitzer, L., Jr. 1951, Ap.J. 113,441. Blaauw, A. 1952, B.A.N. 11, , Ap.J. 123, 408. Bok, BJ. 1956, A.J. 61, 309. Borgman, J., and Blaauw, A. 1963, B.A.N. 17, 358. Courtes, G. 1960, Ann. d Ap. 23, 118; 195. Courtes, G., Cruvellier, P., and Georgelin, Y. 1966, Jour. d. Obs. 49, 329. Crawford, D. L. 1969, I.A.U. Symposium No. 38 (in preparation). Duke, D. 1951, Ap.J. 113, 100. FitzGerald, M. Pim 1969, A.J. 73, 983. Hall, J. S. 1955, Liege Symposium. 1958, Pub. U. S. Naval Obs. 7, 6. Harris, D. L., Ill 1956, Ap.J. 123, 371. Harris, D. L., Morgan, W. W., and Roman, N. G. 1954, Ap.J. 119, 622. Heeschen, D. S. 1951, Ap.J. 114, 132. Herbig, G. H. 1954, Pub. A.S.P.66, , Zs.f.Ap. 68, , Liege Symposium. Howard, W. E., Ill, and Wentzel D. G. 1963, Ap.J. 138, 988. Hubble, E. 1922, Ap.J. 56,162. lohnson, H. L., Hoag, A. A., Iriarte, B., Mitchell, R. I., and Hallam, K. L. 1961, Lowell Obs. Bull. 5, 133. lohnson, H. M. 1953, Ap.J. 118, , in Nebulae and Interstellar Matter, B. M. Middlehurst and L. H. Aller, eds. (Chicago: University of Chicago Press), p. 65. Khavtassi, J. 1955, Bull. Abastumani Obs., No , Atlas of Galactic Dark Nebulae, Abastumani Astrophysical Obs. Mayall, N. U. 1953, Pub. A.S.P. 65, 152. McGee, R. X., Murray, J. D., and Milton, J. A. 1963, Aust. J. Phys. 16, 136. Miller, J. S. 1968, Ap.J. 151,473. Morton, D. C., and Adams, T. F. 1968, Ap.J. 151, 611. Osterbrock, D. E. 1959, Pub. A.S.P. 71, 23.

25 520 BEVERLY T. LYNDS Pottasch, S. R. 1960, Ann. d Ap. 23, 749. Routly, P. M., and Spitzer, L. 1952, Ap.J. 130, 227. Schreur, J (in press). Seyfert, C. K., Hardie, R. H., and Grenchik, R. T. 1960, Ap.J. 132, 58. Shajn, G. A., Hase, V. T., and Pickeiner, S. B. 1954, Liege Symposium. Shane, C. D., and Wirtanen, C. A. 1967, Pub. Lick Obs. 22, Pt. 1. Spitzer, L., Epstein, I., and Li Hen 1950, Ann. d Ap. 13, 147. Strömgren, B. 1951, Problems in Cosmic Aerodynamics (Dayton, Ohio: Central Air Document Office 7). 1962, in Interstellar Matter in Galaxies, L. Woltjer, ed. (New York: W. A. Benjamin, Inc.). Struve, O., and Elvey, C. T. 1940, Ap.J. 89, 517. Wampler, E. J. 1966, Ap.J. 144,921. Williams, R. E. 1969, (private communication).