44. A PROGRESS REPORT ON THE CARINA SPIRAL FEATURE B. J. BOK, A. A. HINE, and E. W. MILLER Steward Observatory, Tucson, Ariz., U.S.A. Abstract. The existing data on the distribution of O and B stars, of optical and radio Hn sources, of Hi and of cosmic dust have been assembled for the Carina-Centaurus Section of the Milky Way, /" = 265° to 305°. The published data have been supplemented by recent photoelectric UBV data and new photographic material. Two Working Diagrams (Figures 9 and 10) of the Carina Spiral Feature have been prepared. The Feature is sharply bounded at /n = 282° and again at /n = 295° in the range of distance from 1.5 to 6 kpc from the sun. Its outer rim is observed from the sun almost tangentially to a distance of 8 kpc from the sun. The Feature is found to bend at distances greater than 9 to 10 kpc from the sun, a result shown by both radio Hi and radio Hn data. Figure 9 presents our basic data for the stellar, gas and dust components of the Feature. The O and early B stars and the H n Regions are closely associated and within 6 kpc of the sun they are concentrated in the range 285° < /" < 295°. The distribution in longitude of Hi is broader and spills over on both sides of the O and B and the Hn peak distributions. Long period cepheids yield a con­ centration similar to that shown by O and B stars and Hn Regions. The visual interstellar absorption between /n = 282° and 295° is represented by a value Av =0.5 mag kpc1, or less, applicable to distances of 4 to 5 kpc. Much higher absorption is present on the outside of the Carina Spiral Feature, 265° < /" <280°, where total visual absorptions as great as 3.5 mags, are found at distances of the order of 2 kpc. Even heavier absorption is indicated for these longitudes at 4 kpc from the sun, thus suggesting that the heavy obscuration on the outside of the Carina Spiral Feature is a phenomenon of general structural relevance (see Figures 8 and 9). Only small values of Av are found at the inside of the Spiral Feature. The Working Diagram (Figure 10) shows that the O and B star peak and the H n peak have a width of 800 parsecs (12°) at 4 kpc from the sun, whereas the Hi width is at least 1500 parsecs at the same distance. The peak of the O and B star distribution and of the Hn distribution lies at about 600 parsecs (8° at 4 kpc) within the outer edge of the spiral feature. The heaviest interstellar absorption is on the outside of the Feature. 1. Preamble In the early 1950s Baade and Mayall (1951) found from their studies of external galaxies that emission nebulae and O and early B stars are the best tracers of spiral arms. Morgan et al. (1952) accepted the challenge that these same objects should be used to trace the spiral structure of our Galaxy. Thus, the first diagrams of the local spiral structure were obtained. Their work was extended later by Becker and Fenkart (1963), Becker (1963, 1964) and by Schmidt-Kaler (1964). The presence of three local spiral features was suggested: the Perseus Arm, the Orion Arm, and the Sagittarius Arm. The Carina Hn regions and OB stars were provisionally linked to the Sagittarius Arm, but they did not receive the attention they deserved. The pitch angle, the angle between the direction of an arm and the direction of rotation, for the three arms averaged about 25°. The 21 cm line of neutral atomic hydrogen was discovered by Ewen and Purcell (1951) and the results of the first extensive studies of spiral structure based on 21 cm profiles by the Leiden and Sydney observers became available shortly after the publi­ cation of the Morgan-Sharpless-Osterbrock results. They revealed an overlying near-circular spiral pattern of Hi distribution. The Leiden-Sydney hydrogen map by Oort et al. (1958), and more recently the studies by Westerhout (1968), Kerr Becker and Contopoulos (eds.), The Spiral Structure of Our Galaxy, 246-261. All Rights Reserved. Copyright © 1970 by the I.A.U. Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 28 Sep 2021 at 01:45:15, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900000644 A PROGRESS REPORT ON THE CARINA SPIRAL FEATURE 247 (1969b), and Hindman (1969), show the spiral arms to be circular, with pitch angles of only 5° to 6°. These pitch angles are much smaller than those found by Becker and Schmidt-Kaler. To reconcile the optical and radio pictures of the Galaxy, Bok (1959) suggested that the Carina Spiral Feature is part of a spiral arm separate from the Sagittarius Arm, and that this feature extends possibly through the sun to connect with the Cygnus Arm. The Orion Arm is then considered to be a second-class spiral feature, a spiral spur rather than a spiral arm. Bok's emphasis on the Carina Feature goes back to his doctoral thesis (1932). The importance of the Carina Feature was stressed especially by him in 1937 in The Distribution of the Stars in Space (University of Chicago Press), notably in the concluding section of the book. A Carina-Cygnus spiral arm would possess a pitch angle in good agreement with the pitch angles found from 21 cm studies. Recent investigations in Carina have strengthened the importance of the concen­ trations of OB stars and interstellar gas in Carina as a major spiral feature. Becker (1956) and Bok (1956) have stressed the high concentration of O and B stars in Carina - CARINA NEBULA Z\ I I I I I I I I I * I I I I I 1 1 1 1 1 1 1 1-5 300° 295° 290° 285° 280° OPTICAL Hn [RODGERS,CAMPBELL ANO WHITEOAK (i960)] Fig. 1. The distribution of optical H11 regions in Carina. Dotted lines indicate the optical H n regions in the list of Rodgers, et al. (1960a). The coordinates are /n, b11. The positions of the three optically bright objects are indicated: the Carina Nebula, IC 2944, and the first magnitude binary star a Crucis. In order to show the apparent Hu distribution schematically, dimensions of the regions (as given by Rodgers et al. (1960a)) are used as the axes of the ellipses. Relative brightnesses are not shown. The Mt. Stromlo H-Alpha Emission Atlas (Rodgers et al. 1960b) was consulted for confirmation of the position, orientation, and shape of each object. Since the outline of RCW 54 (a large emission region between /n = 289° and 291°) could not be ascertained, the four concentrations within the region, given in the notes to Tables II and III of Rodgers et al. (1960a) are shown in the figure. Many optical Hu regions appear in the longitude range /n = 282° to 295°; there are very few Hu regions outside these limits. Note also that the Hu regions appear generally south of the galactic plane. The single large H ii region north of b11 =0° is RCW 59, which is probably a nearby object because of its relatively high galactic latitude ( + 4°.5) and its radial velocity (—12.2 km s-1) as observed by Courtes (1969); its probable distance is 1.7 kpc. Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 28 Sep 2021 at 01:45:15, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900000644 248 B.J. BOK ET AL. with distances ranging from 1 to 4 kpc. More recent observations by Feinstein (1969) have revealed the presence of OB stars to distances of 6 kpc and Graham (1970) has observed OB stars in Carina to distances of 8-10 kpc. Extensive Hu catalogues by Hoffleit (1953) and by Rodgers et al. (1960a) show the Carina region to be abundant in emission nebulae. Radio studies of Hn are providing additional valuable information. The continuum studies of Mathewson et al. (1962) and of Hill (1968) have revealed the presence of many thermal Hn regions in the Carina-Centaurus section. There have been studies recently by Wilson (1969) by the newly-developed hydrogen 109 a techniques. Radial velocities have been obtained for most Hn regions in this section. From these radial velocities, kinematical distances are found for many optical Hn regions and also for Hn regions which are too distant to be observed optically. The Wilson results clearly show that we are looking tangentially along a spiral feature to distances of 8-9 kpc and that the feature curves at distances of 9-10 kpc. Further support for the presence of a major Carina Spiral Feature is provided by the long-period cepheids. In the most recent paper on the subject, Fernie (1968) has found that the cepheids near /H = 280° to 290° extend to distances of 10 kpc; the Carina Feature is shown beautifully in his Figure 2. -CARINA NEBULA -4- Wo RADIO Hn 1410MHz CONTINUUM [HlLL(l968)J & RADIO Hn 109 ALPHA [WILSON (1969)) Fig. 2. The distribution of radio Hn sources in Carina. Solid lines indicate schematically the 1410 MHz continuum sources observed by Hill (1968). Stars mark the positions of the Hn 109a line observations of Wilson (1969). Sources which have been found to be nonthermal by Mathewson et al.