SMITHSONIAN CONTRIBUTIONS TO
ASTROPHYSICS

NUMBER 13

Cecilia H. Payne-Gaposchkin

The Variable Stars
of the Large
Magellanic Cloud

SMITHSONIAN INSTITUTION PRESS

CITY OF WASHINGTON

1971

Contents

Publications of the Smithsonian Institution Astrophysical Observatory

Cecilia H. Payne-Gaposchkin: The Variable Stars of the Large Magellanic Cloud

Introduction

The Cepheid Variables

The Type II Cepheids

The RR Lyrae Stars

Long-period, Cyclic, and Irregular Variables

R Coronae Borealis Stars

Systematic Corrections for Absorption

References

Abstract

Publications of the Smithsonian Institution Astrophysical Observatory

This series, Smithsonian Contributions to Astrophysics, was inaugurated in 1956 to provide a proper communication for the results of research conducted at the Astrophysical Observatory of the Smithsonian Institution. Its purpose is the “increase and diffusion of knowledge” in the field of astrophysics, with particular emphasis on problems of the sun, the earth, and the solar system. Its pages are open to a limited number of papers by other investigators with whom we have common interests.

Another series, Annals of the Astrophysical Observatory, was started in 1900 by the Observatory’s first director, Samuel P. Langley, and was published about every ten years. These quarto volumes, some of which are still available, record the history of the Observatory’s researches and activities. The last volume (vol. 7) appeared in 1954.

Many technical papers and volumes emanating from the Astrophysical Observatory have appeared in the Smithsonian Miscellaneous Collections. Among these are Smithsonian Physical Tables, Smithsonian Meteorological Tables, and World Weather Records.

Additional information concerning these publications can be obtained from the Smithsonian Institution Press, Smithsonian Institution, Washington, D.C. 20560.

The Observatory publishes in Cambridge, Massachusetts, its series of Special Reports, which provide rapid distribution of materials on satellite tracking, research, and analysis and of preliminary or special results in astrophysics. Those Reports in print and indices are available on request from the Publications Division, Smithsonian Institution Astrophysical Observatory, Cambridge Massachusetts 02138.

FRED L. WHIPPLE, Director

Smithsonian Institution

Astrophysical Observatory

Cambridge, Massachusetts

Official publication date is handstamped in a limited number of initial copies and is recorded in the Institution’s annual report, Smithsonian Year.

UNITED STATES GOVERNMENT PRINTING OFFICE
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Cecilia H. Payne-Gaposchkin

The Variable Stars of the Large Magellanic Cloud*

Introduction

The results of the current study of the variable stars in the Large Magellanic Cloud have been summarized by S. Gaposhkin (1971). The total number of stars measured was 2,184, which included the variables published at Harvard (HV) and elsewhere before 1967 and 45 stars discovered in the course of the work. After measurement, 354 stars were rejected as not appreciably variable. Table 1 summarizes the 1,830 remaining stars by type. Bibliographic data for the variables previously known are given by Hodge and Wright (1967) and need not be repeated here.

The accepted variable stars are arranged in order of X coordinate in Table 2; types are indicated as follows: C = Cepheid; RR = RR Lyrae Star; L = long-period variable; Cyc = cyclic variable; W = W Virginis star or Type II Cepheid; I = irregular; R CrB = R Coronae Borealis star; SS = SS Cygni star; N = nova; v? = doubtful variability. Symbols with question marks indicate probable types for stars whose periods have not been found. Positions are given in Miss Leavitt’s X and Y coordinates, which are expressed in seconds of arc and referred to right ascension (1900) in X and declination (1900) -69° 0′ = +10554″ in Y. As noted by Hodge and Wright (1967), the published coordinates of some of the variables are incorrect. The positions of these stars, of the newly discovered variables, and of the stars whose positions were published in equatorial coordinates have been measured by triangulation, referred where possible to the X and Y coordinates of neighboring stars given by Miss Leavitt (Harvard Annals, volume 60, 1908). Such coordinates are marked with asterisks in Table 2. A few outlying stars were reduced to X and Y coordinates by means of the formulas given by Wesselink (1959). A list of duplicate and alternative names is given in Table 3.

Cecilia H. Payne-Gaposchkhi, Smithsonian Institution Astrophysical Observatory, Cambridge, Massachusetts 02138.

* This work was supported in part by NSF Grant GP-4638.

The accuracy of the coordinates can be judged by comparing the published positions of the 17 stars that have inadvertently been announced twice to be variables, with the result that their positions have been twice measured independently. Table 4 contains the comparison. The original coordinate grid extended from 0″ to +24300″ in X and from 0″ to +20100″ in Y; positions outside these limits were given in right ascension and declination. The average difference of position for the 13 stars within the original grid is which corresponds to 0.325 mm on the plate. The average difference for four stars outside the grid is or 1 mm on the plate. The comparison reinforces the statement made by Miss Leavitt (1906) that the published positions serve for rough orientation only, and that marked charts are essential for positive identification. The difficulty of locating known variables is discussed by Hodge and Wright in their description of the procedure used in making their atlas of variable stars in the Large Cloud. A few variables that eluded even their precautions are indicated in the notes to Table 5.

The Cepheid Variables

The results for 1,110 Cepheid variables are given in Table 5, which is arranged as follows:

Column 1:

HV number.

2:

Number assigned by Gaposhkin (1971).

3, 4:

X and Y coordinates on Miss Leavitt’s system.

5:

Previously published period, if any. If two periods have been published, the later is given.

6:

Period determined in the present study.

7:

Logarithm of the period given in column 6.

8:

Maximal magnitude, corrected for absorption.

9:

Minimal magnitude, corrected for absorption.

10:

Magnitude at mean intensity, corrected for absorption.

11:

Amplitude.

12:

Interval from minimum to following maximum.

13:

Interval from mean increasing magnitude to maximum.

14:

Interval from mean increasing magnitude to mean decreasing magnitude, or width of curve at mean magnitude.

15:

Adopted absorption correction.

16:

Date of one well-observed maximum.

17:

Number of positive observations used in forming mean light curve.

18:

Remarks: S = scatter; S! = large scatter; SM = scatter at maximum; Sm = scatter at minimum; P? = period doubtful; cc = close companion or companions; cr = crowded region.

The units in which each quantity is expressed are given at the heads of the corresponding columns. An asterisk after the HV number calls attention to the notes following the table; these are arranged in order of HV number, and approximate periods are given to facilitate reference.

The notes in the last column of Table 5 indicate the weight that can be assigned to the light curve. All the observed points are reduced with the period that has been deduced. For most of the stars the scatter is no greater than expected from the uncertainty of the estimates. If the scatter is unusually large, an attempt is made to assign the cause. Estimates of stars in crowded regions or with close companions are often thus affected. For other stars the adopted period may be in error or may be variable, or the light curve itself may be variable. A number of variable periods have been found. The periods of other stars, indicated by P? in the final column of Table 5, may not be correct. These periods are probably not grossly in error, and many of them agree rather closely with those previously published. Possibly the light curves are variable for some of the stars for which the scatter is most pronounced at maximum; well-known galactic stars that show this feature are TU Cassiopeiae and U Trianguli Austrinae, whose light curves display beat phenomena. Our material is too scattered in time for the disentangling of beats; such stars would repay more concentrated analysis.

The present study has almost doubled the number of Cepheids of known period in the Large Cloud; 537 new periods have been derived, and 574 were known previously. With the very few exceptions indicated in the notes to Table 5, the periods published earlier were of the right order. The ranges of HV 954, 991, 2489, 5921, 5974, 12000, 12001, 12026, 12424, 12518, and 12652 have probably been underestimated because of close companions or dense background. About a dozen stars previously judged not to be variable have been found to be periodic. A few stars, previously thought to be periodic (HV 2687, 2870, 5767, 5846, 11993, 12060, and 12496), now are considered to be irregular or assigned to the nonvariable list. The period published by Shapley and McKibben (1940) for HV 5767 may perhaps belong to HV 5752; the data given by Shapley and Nail (1953) for the eclipsing star HV 5846 may perhaps refer to HV 5864. The other discrepant stars may have been misidentified.

The Type II Cepheids

The 17 Type II Cepheids are listed in Table 6. They have been assigned to the class on the basis of shape and variability of light curve and of erratic variations of period. For many of them a mean light curve cannot be derived from all the observations, and light curves for well-observed intervals have been used. All are probably members of the Large Cloud. The data for HV 5690 are taken from Hodge and Wright (1968), who derived the period. The light-curve types (cr = crested, fl = flat) are those defined by Kwee (1967).

The RR Lyrae Stars

Data for the 29 RR Lyrae stars are given in Table 7; all are regarded as foreground objects. In the area of the Small Cloud, 31 RR Lyrae stars were assigned to the foreground by Payne-Gaposchkin and Gaposchkin (1966). As the galactic latitudes of the Large and Small Clouds are respectively about —32° and —42°, we should expect to find more RR Lyrae stars per square degree in front of the former, the more so on account of its larger angular diameter. Our Large Cloud list contains more bright RR Lyrae stars, and fewer faint ones, than were found for the Small Cloud. Possibly we underestimated the number of short-period variables that actually are members of the Small Cloud. It may be that the less complete survey of the Large Cloud is also in part responsible for the anomaly.

The two faint RR Lyrae stars, HV 13015 and 13016, were discovered by Gaposhkin in the faint cluster HS 83, listed by Hodge and Sexton (1966). They have nearly the same value of <m>o, 16.64 and 16.56; if we adopt a modulus of 17.1, the cluster is 26 kpc from us, and about 29 kpc from the Large Cloud. Both variables are so close to the cluster that they must be members of it.

Long-period, Cyclic, and Irregular Variables

Tables 8, 9, and 10 enumerate the long-period, cyclic, and irregular variables (range one magnitude and greater). The median maximal magnitudes for the three groups are as follows:

Long-period variables

15.50 ± 0.55

Cyclic variables

14.29 ± 0.78

Irregular variables

15.14 ± 0.72

The long-period variables are arranged in order of period. Those that are certainly foreground stars are marked with two asterisks; those that are probably in the foreground, with one asterisk. Nail (1952) classes HV 2578 and 19984 as irregular.

The cyclic variables are enumerated in Table 9. They are distinguished from the long-period variables by their more irregular behavior, but an average cycle can be discerned. The stars HV 888, 996, 2255, 2700, 12407, 12420, and 12437 have been described as irregular by Nail (1952) or by Nail and Shapley (1955). The variations of HV 12501 are occasionally quite irregular.

Table 10 lists 60 irregular variables with ranges over a magnitude. Most of those for which no color is noted are probably red.

R Coronae Borealis Stars

The four R Coronae Borealis stars are given in Table 11. The brightest, HV 966 (W Mensae), has the characteristic spectrum F8:I p, recorded by Feast et al. (1960).

Systematic Corrections for Absorption

The photographic appearance of the Large Cloud gives evidence of localized absorptions. The systematic deviations of Cepheids in different regions from an average period-luminosity curve point in the same direction.

The gross features of the absorption can be approached by way of counts of galaxies and of deviations from the period-luminosity relation. Absorption for individual stars can be determined from accurate color excesses, measures of interstellar bands, and polarization.

Shapley and Nail (1951b) showed by means of galaxy counts that the transparency in the axis is very low and that few galaxies shine through either the clustered areas or the open regions in the center of the Large Cloud. The transparency increases radially and is essentially normal at the edges. They did not attempt to determine absorption for particular regions because of the uneven distribution of the field galaxies, but they considered that it might in some places amount to as much as a magnitude.

The deviations from the average period-luminosity curve were later used by Shapley and Nail (1952) to obtain quantitative values for the absorption in six regions of the Large Cloud. The reality of the differences was established, though on the basis of a relatively small number of Cepheids, 158 in all.

The same method has been used to determine the corrections for absorption from the whole of the Cepheid material covered by the present paper. The face of the Cloud was divided into regions 2000′ × 2000′ in area. The deviations from the mean period-luminosity relation were determined for each area, and were combined and smoothed for a second approximation. In a circle about from the dynamical center of the Cloud the absorption was adopted as zero. The resulting corrections for absorption are shown in Table 12.

The corrections show many of the same features that had been pointed out by Shapley and Nail. By reference to the diagram given by Shapley and Nail (1951b) we can determine the average absorption given in our table for the five regions in which galaxy counts were made:

Mean absorption

Border

0.24

Open regions

0.42

Clustered regions

0.43

Axis

0.60

As in the results from galaxy counts, the absorption is greatest in the axis, about the same in open and clustered regions, and least in the border regions (which are not, however, yet at the unobscured edges of the Cloud).

A more quantitative comparison can be made with the results derived by Shapley and Nail (1952) from the period-luminosity curve deviations. Their absorptions were expressed as differences from a mean; we have increased them by 0.38, the average difference between our values and theirs. The mean absorptions in various regions follow:

The relation of absorption to region is substantiated. Exact agreement cannot be expected, in view of revisions of the magnitude scales and of the much greater number of stars in our sample. An independent check on our zero point is furnished by the value of +0.07 for the (B-V) color excess of NGC 1866 used by Arp (1967), which gives a value of 0.28 for the photographic absorption, if the conventional value of 4 is used for the ratio of color excess to photographic absorption.

Another test of the plausibility of our corrections for absorption can be made by comparing them with the color excesses determined for individual stars in the same areas. Color excesses for bright stars in the Large Cloud have been obtained by Feast et al. (1960), and for individual clusters and associations by Westerlund (1961). In the following comparison, the color excesses have been multiplied by 4 to obtain photographic absorptions, and the visual absorptions by 4/3. The average photographic absorptions derived from the two sources have been assembled for six values of our photographic absorption in the corresponding areas:

Here again, close numerical agreement would hardly be expected, and the data of Westerlund show that some clusters, quite close together, have color excesses that differ considerably (e.g., NGS 2074, 2081, 2092, 2100). The point to be made is that color excesses lead to absorptions at least as great as those we have used. The average absorption for all stars studied by Feast et al. would be 0.44, and their greatest value is larger than any entry in our table.

The 4430 Å band absorption, studied by Hutchings (1966) in the Magellanic Clouds, contributes additional evidence of absorption within both Clouds. Absorption within the Small Cloud, deduced in the same way as for the Large Cloud, was described by Payne-Gaposchkin and Gaposchkin (1966). In this case also the evidence for the existence of absorption is fortified by the galaxy counts of Wesselink (1961a, b) and to some extent by those of Shapley and Nail (1951b) for the central regions of the Small Cloud.

It must, however, be emphasized that our corrections for absorption are based on gross structure. To correct each variable star individually is beyond our present means.

Table 13 contains a list of the eclipsing stars.

References

Arp, H.

1967. Analysis of Stellar Content and Cepheid Evolution in NGC 1866. Astrophysical Journal, 149: 91–106.

Cannon, A. J.

1936. The Henry Draper Extension. Annals of Harvard College Observatory, 100 (6) :205–226.

Feast, M. W., A. D. Thackeray, and A. J. Wesselink

1960. The Brightest Stars in the Magellanic Clouds. Monthly Notices of the Royal Astronomical Society, 121:337–385.

Gaposhkin, S.

1971. 1834 Variables in the LMC. [In preparation.]

Hodge, P. W., and J. A. Sexton

1966. 457 New Star Clusters of the Large Magellanic Cloud. Astronomical Journal, 71:363–368.

Hodge, P. W., and F. W. Wright

1967. The Large Magellanic Cloud. Smithsonian Publication 4699. Washington, D.C.: Smithsonian Institution Press.

1968. Studies of the Large Magellanic Cloud, X: Photometry of Variable Stars. Astrophysical Journal, supplement, 17:467–495.

Hutchings, J. B.

1966. 4430 Å Band Absorption in the Magellanic Clouds. Monthly Notices of the Royal Astronomical Society, 131:299–305.

Janes, K. A.

1964. Period Changes of the Cepheid Variable HV 953. Astronomical Journal, 69:131–132.

Kwee, K. K.

1967. Investigations on Population II Cepheids, III: Light-curves and Two-colour Diagrams. Bulletin of the Astronomical Institute of the Netherlands, 19:260–274.

Leavitt, H. S.

1906. 1777 Variables in the Magellanic Clouds. Annals of the Harvard College Observatory, 60:87–108.

Nail, V. McGibben

1952. New Variables and Periods in the Large Magellanic Cloud. Harvard College Observatory Bulletin, 921:1–4.

Nail, V. McKibben, and H. Shapley

1955. The Magellanic Clouds, XV: On the Tilt of the Large Cloud. Proceedings of the National Academy of Science, U.S.A., 41:685–690.

Payne-Gaposchkin, C., and S. Gaposchkin

1966. Variable Stars in the Small Magellanic Cloud. Smithsonian Contributions to Astrophysics, 9: 1–205.

Shapley, H.

1931. Notes on the Large Magellanic Cloud, III: A Preliminary Determination of the Period-luminosity Relation. Harvard College Observatory Bulletin, 883:16–18.

Shapley, H., and V. McKibben

1940. A Summary of the Periods and Median Magnitudes of Magellanic Cloud Cepheids. Harvard College Observatory Circular, number 439, 5 pages.

Shapley, H., and V. McKibben

1951a. NGC 1866 and the Magellanic Cloud Variables. Astronomical Journal, 55:249–251.

1951b. Magellanic Clouds, I: Transparency. Proceedings of the National Academy of Science, U.S.A., 37:133–138.

1952. Magellanic Clouds, III: Differential Absorption Within the Large Cloud. Proceedings of the National Academy of Science, U.S.A., 38:281–286.

1953. Magellanic Clouds, V: Fifty Eclipsing Stars. Proceedings of the National Academy of Science. U.S.A., 39:1–5.

1955a. The Magellanic Clouds, XIV: The Bar of the Large Cloud. Proceedings of the National Academy of Science, U.S.A., 41:185–190.

1955b. Magellanic Clouds, XVII: Seven Notes on the Cepheid Variables. Proceedings of the National Academy of Science, U.S.A., 41:829–836.

Wesselink, A. J.

1959 Some Formulae Connecting Harvard x and y Equatorial Coordinates and Standard Coordinates for the Magellanic Clouds. Monthly Notices of the Royal Astronomical Society, 119: 576–579.

1961a. The Dust Content of the Small Magellanic Cloud from Counts of Nebulae. Monthly Notices of the Royal Astronomical Society, 122:503–508.

1961b. Absorption and Reddening in the Magellanic Clouds. Monthly Notices of the Royal Astronomical Society, 122:509–512.

Westerlund, B.

1961. Population I in the Large Magellanic Cloud. Uppsala Astronomic Observatory Annals, 5(1): 58 pages.

Wooley, R. v. d. R., A. R. Sandage, O. J. Eggen, J. B. Alexander, L. Mather, E. Epps, and S. Jones.

1962. Studies in the Magellanic Clouds, IV: Photometry of Cepheids in Variable Field I, Large Magellanic Cloud. Royal Observatory Greenwich-Cape Bulletin, 58: 57 pages.

Abstract

Payne-Gaposchkin, Cecilia H. The Variable Stars of the Large Magellanic Cloud. Smithsonian Contributions to Astrophysics, number 13:41 pages, 1971—The variable stars in the Large Magellanic Cloud have been studied on the basis of estimates made on all available plates at the Harvard College Observatory. Of the 2,184 stars measured, 1,830 have been judged to be variable. About 800,000 estimates were made.

The numerical data derived from a discussion of the material are listed in tabular form.

TABLES 1-13

TABLE 1.—Variable stars in the Large Magellanic Cloud

TABLE 2.—Variable stars arranged in order of x

TABLE 3.—Duplications and alternative names

TABLE 4.—Published coordinates of stars discovered twice

TABLE 5.—Cepheid variables

TABLE 6.—Population II Cepheids

TABLE 7.—RR Lyrae stars

TABLE 8.—Long-period variables

TABLE 9.—Cyclic variables

TABLE 10.—Irregular variables of large range

TABLE 11.—R Coronae Borealis stars

TABLE 12.—Adopted corrections for absorption

TABLE 13.—Eclipsing stars in the Large Magellanic Cloud