1. TIME LINE FOR HELEN'S RESEARCH ACTIVITIES AT DAO

1931:

On an afternoon in late August 1931, Helen and Frank arrived at the DAO

(H. Sawyer Hogg, 1988, JRASC, 82, 328)

[Not a research activity but worth noting the date.]

1931: September 7-9, 1931

- The 46th AAS meeting was held in Delaware, Ohio. Helen submitted an abstract, but it appears that she did not attend the meeting:

[Note by CC: Up to about 1950, the AAS published photographs and lists of its meeting attendees. From inspecting these, I ascertained whether or not Helen actually attended the meetings. It appears that, in the 1930s, members could present papers without attending the meeting. Presumably they arranged for a colleague to read their paper.]

THE PERIODS OF 36 VARIABLE STARS IN FOUR GLOBULAR CLUSTERS

Observations of classical Cepheids in Omega Centauri, NGC 362, M4 and NGC 6397. The bearing of the results on the period- luminosity relation was discussed.

The abstract was published in 1933 (PAAS Volume 7, page 35). The formal writeup of her results was published in 1931 (Harvard Circular, 366) which was submitted on May 1, 1931 before she left New England. I discussed this paper in my file hsh_harv.txt.

1931: September 22

- On this date, Helen started her observations with the DAO 72-inch telescope. She observed globular clusters on 2 nights in September and 3 in October that year.

1931: December 28-30 1931

- The 47th AAS meeting was held in Washington, D. C. Helen submitted an abstract, but it appears that she did not attend the meeting:

VARIABLE STARS IN THE NORTHERN EDGE OF THE SMALL MAGELLANIC CLOUD

This was based on data from Harvard plates. In her earlier investigation of NGC 362 (1931, HC 366), she found three classical Cepheids with periods ranging from 2 to 4 days, but they seemed too faint to belong to the cluster. Since NGC 362 is located near the SMC in the sky, she decided to search for more variables in the surrounding regions to ascertain whether the period-luminosity relation is invalid for short periods or whether the three faint classical Cepheids were actually members of the SMC. She discovered and derived periods and magnitudes for 14 Cepheids with periods greater than a day. She found that they defined a period-luminosity relation and that the three classical Cepheids in NGC 362 also fell closely on the curve. She therefore concluded that the three "NGC 362" Cepheids belonged to the SMC and not the cluster.

[I assume she did the plate work while still at Harvard and the period derivation after she arrived at the DAO.]

The abstract was published in 1933 (PAAS Volume 7, page 100). The formal writeup of the results was published in 1932 (Harvard Circ. 374), a paper she submitted on May 2, 1932 (see below).

1932: February 1st,

Harvard Bulletin 886, page 15

By Helen B Sawyer: Note on Luyten's Nova in the Large Magellanic Cloud. This was based on a measurement she made on a Harvard plate. Presumably this was a note she submitted before she left New England.

1932: February

Edwin Hubble submitted a paper on globular clusters in Messier 31 (the Andromeda Galaxy). It was published in 1932 (ApJ, 76, 46).

I cite this paper because it was the first investigation to call into question the assumption that globular cluster Cepheids follow the same period luminosity relation as Classical Cepheids, an assumption we now know to be invalid.

Helen's work with Shapley when she was at Harvard played a role in Hubble's result.

Hubble found that, according to Shapley's distances and magnitudes for the Milky Way globular clusters, the objects in M31 were systematically fainter than the ones in our Milky Way Galaxy. There were two possibilities for this discrepancy. The difference in brightness might have been real or it might be that Classical Cepheids and globular clusters do not follow the same period-luminosity relation.

Hubble's distance to the Andromeda galaxy was derived from Classical Cepheids while the distances to the Milky Way globular clusters were derived from RR Lyrae variables and globular cluster Cepheids.

The brightnesses of the Milky Way globular clusters were from data reported by Shapley in his book, Star Clusters and Helen was involved in the production of this book.

The original papers were by Sawyer and Shapley (Photographic magnitudes of 95 Globular Clusters, published in 1927, Harv. Bulletin, 848, 1) and by Shapley and Sawyer (The Distances of Ninety-three Globular Star Clusters, published in 1929, Harv. Bulletin, 869, 1).

Hubble's result was discussed later by Baade in the lecture he gave when he received the the Bruce Gold Medal in 1955 (1956, PASP, 68, 5 - see page 8).

By that time, it was recognized that the globular cluster Cepheids were fainter than Classical Cepheids by about 1.5 magnitudes. Consequently there was no difference in the brightness of the clusters in the two systems.

1932: May 2, 1932

Helen submitted the paper, PERIODS AND LIGHT CURVES OF TWENTY TWO VARIABLE STARS IN THE NORTHERN BORDER OF THE SMALL MAGELLANIC CLOUD which was published in 1932 (Harvard Circ. 374).

This was the formal writeup for her AAS abstract submitted for the meeting in Washington in December 1931.

1932: June 15-18, 1932

A meeting of the Astronomical Society of the Pacific held in Pullman, Washington. Frank and Helen presented a paper:

A TEST OF THE CONSTANCY OF LIGHT OF BRIGHT STARS IN MESSIER 4.

They estimated magnitudes for 250 stars on each of 10 plates obtained with the Mt. Wilson 60-inch telescope. Each plate was measured twice. From the deviations in magnitudes, they recovered 30 of the 33 known variables and discovered 4 new, 6 probably and 8 possible variables. No further information about the new variables was given and they were not mentioned in her 1939 catalogue of variable stars in globular clusters.

This paper by F. S. Hogg and H. B. Sawyer was published in 1932 (PASP, Vol 44, Page 258)

[Note: IT IS UNLIKELY THEY REALLY ATTEND THIS MEETING BECAUSE SALLY WAS BORN 2 DAYS LATER.]

1932: June 20

- daughter Sally was born [not a research activity! But it is worth noting the date.]

1932: July 26

- Helen resumed her telescope observing and observed on Jul 26, Aug 1, 3, 4, 5, 10, 24, 26

1932: December 27-29, 1932

- The 49th AAS meeting was held in Atlantic City, NJ. Helen submitted an abstract, but it appears that she did not attend the meeting:

NEW VARIABLE STARS IN GLOBULAR CLUSTERS

She reported on her searches for variable stars based on the observations she had made at the DAO the previous summer. She investigated four globular clusters for which there was no record of previous searches: M10, M12, M14 and NGC 6934. The search confirmed facts already known, that the frequency of variable stars in globular clusters is exceedingly irregular. Only one or two variables were discovered in M10 and M12, but M14 and NGC 6934 had many more. She also reported that, although M2 had 11 previously known variables, her new observations showed that there were several more.

The abstract was published in 1933 (PAAS Volume 7, page 185)

1933: December 28-30, 1933

- The 51st AAS meeting was held in Cambridge, Mass. Helen submitted an abstract, but it appears that she did not attend the meeting:

PERIODS OF VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 2

She reported on her DAO observations of 11 previously known variables and 4 new ones discovered from an extensive search of the DAO plates. Eleven of the variables were cluster type Cepheids whose average period was longer than in some other clusters. The other four variables were brighter and appeared to be Cepheids, but their periods had not yet been derived because the cluster had not been observed on enough nights. The proportion of Cepheids with periods greater than a day is higher than in any other cluster.

The abstract was published in 1936 (PAAS Volume 8, page 20).

1935: March 6, 1935

Helen submitted the paper, PERIODS AND LIGHT CURVES OF THE VARIABLE STARS IN THE GLOBULAR CLUSTER M2. It was published in 1935 (Pub. DAO, Vol. 6, No. 14).

This was the formal writeup for her AAS abstract submitted for the meeting in Cambridge in December 1933.

In her 1933 presentation she reported that M2 had four bright variables that appeared to be long period Cepheids, but she did not have enough observations to derive their periods. Her observations during the 1934 observing season were more broadly distributed in time so she was able to derive their periods. She also discovered 2 more cluster type Cepheids. She plotted a period-luminosity relation for the 17 (13 cluster-type and 4 long period Cepheids) variables in M2 and found that it fit the P-L relation that Shapley had derived in his 1930 book on Star Clusters.

She drew attention to the fact that the some of the cluster-type Cepheids in M2 had unusually long periods, illustrating that the period frequency for cluster-type Cepheids differs radically from cluster to cluster.

She also noted that M2 was the second richest cluster (after Omega Centauri) in long-period Cepheids.

1935: September 10-12, 1935

- The 54th AAS meeting was held in Toronto. Helen presented a paper:

VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 6934

She reported on her four years of DAO observations and noted that she was continuing to observe the cluster at DDO. She announced the discovery of approximately 50 variables. Since they all had approximately the same magnitude, she assumed they were all cluster type Cepheids (RR Lyrae variables). She had already determined periods for 12.

The abstract was published in 1936 (PAAS, Volume 8, page 149)

By this time Helen had moved to Toronto, but she was reporting on results obtained at DAO.

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2. Helen's DAO Globular Cluster observations (1931-1934)

[I obtained this information from the astro library webpage on the website of the David A. Dunlap Department of Astronomy and Astrophysics at the University of Toronto. There is a list of all the plates in the University of Toronto Globular Cluster Plate Collection.]

https://www.astro.utoronto.ca/AALibrary/glbclstr.html

[Note: Some of the globular cluster photos included a photo of a Kapteyn area as well. In those days this was the method for setting up a sequence of stars with standard magnitudes in the cluster. The Kapteyn areas were fields in which the apparent magnitudes of the stars had been established on a standard system.]