Publications of the Astronomical Society Of

PUBLICATIONS OF THE

ASTRONOMICAL SOCIETY OF THE PACIFIC

Vol. 55 S;m Francisco, California, December 1943 No. 327

SURVEY OF THE YEAR'S WORK AT MOUNT WILSON*

W. S. Adams

Seven members of the scientific staff of the Observatory have been on leave of absence throughout the year to engage in research upon military problems and two members of the operat- ing staff are in the armed services. In addition numerous others, while remaining at the Observatory, have devoted much time to assisting in the solution of specific optical and instrumental ques- tions brought to their attention. Essentially the entire activities of the instrument and optical shops have been devoted to con- tracts with the Army and the Office of Scientific Research and Development. The astronomical work of the Observatory, but slightly reduced in amount, has been maintained chiefly by the older members of the staff. The essential solar observations, in which continuity of records is especially important, have been carried on without interruption, and the 60-inch and 100-inch telescopes have been used regularly throughout the year. An interesting result of the dimout regulations in the valley area including Pasadena and Los Angeles has been a darkening of the sky to the south and southwest of Mount Wilson which has made possible a doubling of exposure times on very faint objects without increase of fog on the plates. This has been especially valuable in the study of faint variables and other stars in extragalactic nebulae. The year was characterized by exceptionally favorable ob-

* For a complete report covering the period July 1, 1942—June 30, 1943, see Carnegie Institution of Washington Year Book, No. 42, 1943.

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serving conditions, solar observations being made on 327 days and stellar observations on 294 nights. Although the precipita- tion during the winter season was unusually high, amounting to nearly 66 inches, it was almost wholly in the form of rain largely concentrated in three heavy storms. A mild winter appears to favor good astronomical definition. Sunspot activity decreased materially during the year and is now at a very low stage. As the minimum is approached, special interest attaches to the discovery of the first spot of the new cycle. This was found on December 20, 1942, in north latitude 32°. On May 16, 1943 a group appeared in south latitude 41° which was the largest group ever photographed in latitudes higher than 40°. Polarity observations indicated the expected reversal of sign as compared with spots of the previous cycle. In the course of an extensive study of solar prominences, Pettit has found clear evidence of an interchange of gases between interactive prominences. He found also that interactive prominences may become eruptive. In general the angle of ejec- tion of eruptive prominences shows but a small inclination to the solar radius, although exceptions occur. Two-thirds of the 39 trajectories available show an angle of less than 20°. Eruptive prominences move great distances without change of velocity, distances of 200,000 km or more having been observed for 18 prominences, with two exceeding a solar radius. The frequency of eruptive prominences is found to be of the order of 400 per year at sunspot maximum and 25 to 50 per year at sunspot minimum. This investigation has also dealt with motions in prominences of the tornado type and with the structure of quiescent prominences. A palisade-like form seems to be characteristic of the quiescent type with as many as 20 or 30 columns or withes, 2000 km wide, and as much as 50,000 km high. Spectroscopic observations of the sun have included studies of the extreirfe ultraviolet spectrum of the disk and spots, photographs of a portion of the green region with a Lummer plate for measurements of the general magnetic field and the solar rota- tion, and an investigation of the presence of additional com- pounds in disk and spot spectra. This last investigation, carried

on by Babcock and Mrs. Coffeen, has resulted in the identification of O 2 in the sun where it appears in several faint extensions of the Schumann-Runge band system, and the probable recog- nition of NH in two low states of electronic excitation. The presence of O 2 in the solar atmosphere, even if greatly diluted, must very profoundly reduce the outflow of solar radiation in the region λ 670-λ 2000 A because of its enormous absorptive power. A study still in progress is a comparison by Richardson of the Η and Κ lines at times of violent magnetic storms and of magnetic calm to test the suggestion by Chapman that a cloud containing ionized calcium atoms moving toward the earth during a magnetic storm might be detected by the presence of a faint absorption component on the violet side of the lines. A satisfac- tory method of determining the intensities of the lines has been devised and the measures are being collected. A search for the asteroid Adonis was made by Nicholson on photographs with the 100-inch telescope covering a range of 14 days in the time of perihelion passage, but no definite evidence of the presence of the asteroid was found. Nicholson has also reobserved the positions of the fainter satellites of Jupiter and has recalculated and improved the orbit of Satellite IX. Cometary observations have included photographs by Baade of periodic comet Wolf I, which was found close to the predicted position, and of Whipple's comet (1942/). The latter showed remarkably rapid changes in the structure of the tail in the course of a few hours. Spectrograms of Whipple's comet by Minkowski enabled him to identify several previously unidentified features in the visual region with bands of Ν Η 2. In the broad field of stellar investigation van Maanen has completed measurements of parallax for 14 faint stars of large proper motion, and has increased his second-epoch material for the determination of proper motions of Cepheid variables to a total of 91 objects. The proper motions of about 800 stars in the distant open cluster h Persei have been measured on two pairs of photographs separated by intervals of 27 and 17 years, respectively. In spite of the small motion of but 0^004 for the cluster, the accuracy of measurement is such as to make it pos-

sible to separate to a very large extent the cluster stars from the field stars. An interesting study of the physical characteristics of the companion of small mass in the system of 61 Cygni has been made by Dr. Russell. The use of theories applicable to normal lucid stars for an object of such mass necessarily involves extra- polation and may result in considerable uncertainties. Calcula- tions of the radius of the star based upon the mass given by Strand yield wide possible limits, but the assumption of the rea- sonable intermediate value of three-fourths the radius of the sun indicates a central temperature of 1,600,000° K, a surface temperature below self-luminosity, and a radiation which could be supplied by contraction alone for a period of 5 X 109 years at a cost of but 1/400 of its radius. Several photometric investigations have been completed during the year. In one of these Dr. Stebbins has used a combination of a photoelectric cell with suitable filters to isolate six spectral regions between λ 3500 and λ 10,000. About one hundred fifty stars of all types have been measured, most of which are found to radiate much like black bodies. The chief devia- tions are due to strong hydrogen absorption in the ultraviolet region in A-type stars, and to the strong red and infrared bands in M-type stars. The color temperatures range from about 24,000° Κ for stars of types O and early Β to 2,000° Κ for the reddest M-type stars. The value for type A is 11,000° K. The completion of the investigations of stars in the north polar region by Dr. Seares and Miss Joyner has confirmed the provisional results for the spectrum-color relationship and the color temperatures of the stars. The general uniformity of the obscuring cloud over the polar cap 20° in diameter seems to be well established, the solar system lying close to if not actually within the cloud. The color excess increases at a nearly linear rate to a value of 0.27 magnitude at about 450 parsecs where the cloud apparently ends. The constancy of the zero point of the color indices of the Polar Catalogue is well shown by an intercomparison of the results for small individual fields within the polar cap. An important astrophysical result of this study is the evi-

dence for the small dispersion in color index for stars of the same spectral type, the upper limit being 0.035 magnitude for types KO and earlier, and about 0.1 magnitude for types K5-M. The accidental error in the various systems of spectral classification may also be derived from the results of the investigation. Visual observations of the light-curve of Nova Puppis (1942) were made by Pettit over a period of about six months with a modified form of wedge photometer. The form of the curve was that characteristic of rapidly changing novae but no such large oscillations of light were observed as were seen in some other bright novae. The maximum brightness measured was +0.35 magnitude on November 11 ; on May 6 the magnitude was 8.35. In the field of stellar spectroscopy the identification of faint members of the Taurus cluster through measurement of their radial velocities has been continued by R. E. Wilson. A total of 217 stars with proper rpotions reasonably close to those of the brighter stars of the cluster has been investigated. Of these 150 are cluster members, 20 are probable, 25 doubtful, and 22 certainly not members. Thirty-five stars, mainly of magnitudes 9.0 and 11.0, have not yet been observed. Practically all the 9th-magnitude stars of A-type are not members of the cluster. Within the range A5 to K5, the cluster stars, with the exception of the few known giants, fall closely upon the main type-luminosity curve. About 250 early-type stars with emission lines have been added by Merrill and Miss Burwell to their previously published Catalogue. Of these, 166 were discovered at Mount Wilson. Some remarkably interesting spectra were found including the star HD 242257 in which absorption components of the hydrogen lines show velocities of approach of 530 km/sec while dark lines of iron are undisplaced. Several stars with combination spectra have been observed. Among these Β F Cygni shows remarkably rapid changes in the intensity of the nebular lines. A study by Joy of the radial velocities, spectral types, and absolute magnitudes of about 120 stars with proper motions exceeding 0^35 annually is nearing completion. Many of these stars show emission lines of calcium and a few show emission

lines of hydrogen. Some very faint dwarf stars of especial interest have been observed by Humason. Forty-eight faint stars found by Dr. Zwicky to have a very small color index, 33 of which are near the north galactic pole and the remainder near the Hyades cluster, have also been observed by Humason, pri- marily in a search for white dwarfs. The spectral types are found to be mainly BO to Β5. Numerous classes of variable stars have been investigated by Joy and Merrill, and some of the spectra are found to show extraordinary variations. An illustration is one of the components of the double variable star UZ Tauri in which with increase of brightness, an emission spectrum characteristic of Τ Tauri variables develops and overshadows the dwarf Me spectrum previously seen. The stars observed by Joy include, 20 variables of high luminosity in globular clusters. The radial velocities of about 100 stars of spectral types Ν and R have been determined by Sanford, several of them from observations with the coudé spectrograph. The brightest of these stars, 19 Piscium, has been photographed on an Eastman IV Ν plate with the 114-inch camera throughout the region λ 7000- λ 8700. The large-scale coudé spectrograms have been used for a wide variety of investigations. In a study of M-type stars. Miss Davis has identified numerous lines including the rubidium pair in the red region of the spectrum of β Pegasi. Measures of dif- ferential displacements in giant M stars indicate that resonance lines of Can and Al ι are displaced to the violet more than those of other elements, and that the displacements are small for lines arising from levels with excitation potentials between 0.5 and 1.6 volts. Where the resonance absorption lines are double, as in aOrionis and several other stars, the components of the Ti lines are less widely separated than those of Fe, Cr, and Mn. Observations by Adams indicate that the components in the spectrum of α Orionis are subject to appreciable variations in relative intensity. The emission components of Η and Κ in late-type giant stars show wide variations, in relative intensity and width from star to star and may vary at different times in the same star. Miss

Davis has found considerable evidence for a correlation between separation of the emission components and absolute magnitude, separation increasing with luminosity. Between dwarfs and giants the effect is very marked, the distance between the components being 0.17 A in 611 Cygni and 1.38 A in α Orionis. Three bright early-type stars of exceptional interest have also been studied with very high dispersion. The emission Ha line in β Orionis has been found by Sanford to vary greatly with no definite periodicity. Emission components may be present on both sides of the absorption line, may be completely absent, or so strong as nearly to mask the absorption. The chief feature of interest in the spectrum of Ρ Cygni, observed by Adams, is the variability of the sharp violet absorption component of the hydrogen lines which had a displacement corresponding to a radial velocity of —240 km/sec. It disappeared completely between July 1942 and April 1943. The total widths and displacements of the lines of various elements in the spectrum of τ Scor- pii have been determined by Merrill and Adams. The profiles and displacements of the helium lines offer strong evidence of the Stark effect in the star's atmosphere. Investigations dealing with interstellar lines have included observations by Sanford of B-type stars in open clusters at different distances and a study of the broad unidentified interstellar line near λ 4430. Four additional stars of types Ν and R have been found which show interstellar D lines separated from the stellar D lines. The structure of the interstellar Η and Κ lines has been studied by Adams on spectrograms taken with the 114-inch camera of the coudé spectrograph. In a few stars as many as four or five separate components have been observed, doubtless corresponding to the same numbers of discrete gaseous clouds in the line of sight. Complexity of the lines is most frequent in the regions of Orion, Sagittarius, and Cygnus and least frequent in Perseus and Scorpius. " Marked differences in relative intensity have been observed between the lines of CH ι and of CH n as well as between the lines of the diatomic molecules and the atomic lines. Eight of the nine stars observed in the Pleiades show faint

interstellar lines. In Pleione they are probably masked by thé strong stellar absorption lines. An interesting result is the discovery that in two stars, Asterope and Merope, interstellar lines of CH il occur without interstellar H and K. Galactic nebulae and novae have received considerable study during the year. A spectroscopic survey by Minkowski of 63 objects found by W. C. Miller to show Ha in emission indicates that 32 are planetary nebulae and seven are diffuse nebulae. The remaining objects are mainly stars of types O and Β with strong emission lines, but several have peculiar spectra. One of these, an irregular variable, shows absorption lines of hydrogen con- sisting of several components which vary rapidly in intensity and . are displaced by amounts corresponding to velocities of from —500 km/sec to —2200 km/sec. These values are comparable with those found in novae. An extensive investigation of three of the nearer novae, Nova Herculis (1934), Nova Persei (1901), and Nova Τ Aurigae (1891), and of the variable star RAquarii has been made by Baade and Humason. Some of the spectroscopic exposures were as much as 20 hours in length. Direct photographs of Nova Herculis taken in red light at the Cassegrain focus of the 100-inch telescope in the summer of 1942 showed Jhat the red emission was restricted to a wide band along the minor axis and to two faint clouds symmetrically placed at the ends of the major axis; while the N1 and N2 emis- sions of forbidden O m were uniformly distributed throughout the shell. Spectrograms showed that the localized emission was due to the two lines of forbidden Ν ii at λλ 6548 and 6584. The two components observed visually by Kuiper in 1942 could not be seen on direct photographs taken in light of the N1 and N2 lines but appeared on the spectrograms because these are more favorable to the detection of faint condensations. These components are outstanding features in the N1 and N2 lines when the slit of the spectrograph is along thé major axis of the en- velope. No additional condensations have been found. Measures of about a dozen well-defined condensations in the shell around Nova Persei show that practically all the ejected matter is contained in the rear part of the southwest quadrant.

and that the motion of this main fragment of the shell appears to have been uniform since the outburst. The thickness of the shell is 0.22 of its outer radius ; the distance of the nova derived from the expansion data is 470 zb 28 parsecs ; and the luminosity at maximum was M — —8.4. An elliptical shell about 12^ in diameter has been discovered by Baade around Nova Τ Aurigae. A calculation based upon the diameter and the velocities observed at the time of outburst gives a distance of approximately 800 parsecs and a maximum luminosity οί M = —5.3. Τ Aurigae like Nova Herculis is a nova of rather low luminosity. The results for this star tend to confirm the indications that slowly fading novae of this type are several magnitudes less luminous than rapidly fading novae of the Nova Aquilae type. A recent photograph by Baade of the nebulosity surrounding R Aquarii confirms Hubble's previous observation that the outer nebulosity is expanding. Evidently it was ejected during a major outburst of the blue companion' of R Aquarii which has a Ρ Cygni type of spectrum. Numerous spectroscopic observations of Nova Cygni (1942) and Nova Puppis (1942) have been made by Sanford. A shell absorption spectrum in the ultraviolet region of Nova Τ Coronae Borealis was found by Minkowski in February 1943. It disappeared one month later and the spectrum returned to, that observed in 1942. To aid in the intercomparison of magnitudes between members of the local group of extragalactic nebulae, Baade is transferring the photometric scale of Selected Area 68 to the Andromeda nebula and Messier 33, and that of S.A. 136 to NGC 6822. The discovery of two faint Cepheid variables in the Sextans system provides a means of fixing the upper limit of luminosity in a dwarf galaxy of low stellar content. As previously stated the increased darkness of the sky due to the dimout in the San Gabriel Valley has been of great assistance in these difficult observations. Spectroscopic observations of 36 extragalactic nebulae have been completed by Humason during the year. Altogether 406 nebulae have been observed at Mount Wilson, and it is planned

to continue the investigation until a total of 500 is reached. Many of those not yet observed are very faint nebulae of later types. In the physical laboratory A. S. King has made an extensive study of the spectrum of dysprosium in the arc, spark, and electric furnace. More than 500 of the stronger lines of Dy n have been identified and 57 of those which have low atomic-level characteristics have been found by Mrs. Sitterly to be present in the solar spectrum. A temperature classification of the Dy ι spectrum is in progress. The spectra of neutral and ionized gadolinium, observed extensively by King in past years, are being analyzed according to energy levels by Dr. Russell, and several hundred lines have been assembled according to terms. Through the co-operative work of Mrs. Sitterly and King the identification of lines of ionized thorium and neutral gold in the solar spectrum has been established with considerable certainty. The observation by Babcock and Mrs. Coffeen that the gold line at λ 3123 is strengthened in sunspot spectra provides additional evidence for this element. The ruling machines have been used occasionally in con- nection with certain military problems, and in addition special microscopic study has been given to ruled surfaces to aid in the control of intensity distribution, particularly to intensify the lower orders. The application of such gratings to stellar spectroscopy is rapidly becoming of great importance.

Carnegie Institution of Washington Mount Wilson Observatory