Peremennye Zvezdy (Variable Stars) 27, No. 3, 2007 Received 20 April; accepted 28 April.

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ROSS VARIABLES IDENTIFIED WITH MINOR PLANETS

Brian G. Marsden

Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, U.S.A.;
e-mail: bmarsden@cfa.harvard.edu

Bedient (2003) identified six of the "New Variable Stars" listed by Ross (1925, 1926a, 1926b, 1927a, 1927b, 1928a, 1928b, 1929, 1930, 1931) as minor planets. These were cases where F.E. Ross' comparison of plates taken with the Bruce photographic telescope at the Yerkes Observatory by E.E. Barnard during 1904-1915 and those taken by Ross himself with the same instrument during 1924-1930 showed the "variable" to be present only on a single night. Photographic defects had (presumably) been eliminated, since each observation consisted of a pair of duplicate images with 10-inch and 6-inch lenses. In the preamble to his third list Ross (1926b) had remarked that "while a certain small percentage of the stars listed as variables in the present survey may actually be asteroids [sic], to prove that such would be the case for any individual star would be an exceedingly difficult matter".

The majority of the 379 objects have either been fully recognized as variables or been shown not to vary, but as many as 97 remain in the New Catalogue of Suspected Variable Stars (Kukarkin et al., 1982), and of the latter, the 51 objects collected in Table 1 are single-night detections. In such cases the non-detection was indicated either by a limiting magnitude (invariably 15 for the cases of interest) or - for the third through seventh lists - by the notation "n.s." (not seen), where it is also reasonable to presume a limiting magnitude of 15.

Following the recent publication (Samus and Antipin, 2007) of the identity of DU Aqr with (57) Mnemosyne, Samus (2007) suggested to the present author that Ross 168 was likely to be a minor planet and requested assistance in identifying it. In response, and unaware of the work by Bedient (2003), the present author quickly identified Ross 168 and, further, checked to see which of the 50 other objects listed in Table 1 were detections of identifiable minor planets. The work was simplified by the availability of a new on-line version of the NSV catalogue that includes equinox 2000.0 versions of the equinox 1875.0 positions originally provided by Ross.

The other prerequisite was the availability of computer files at the Minor Planet Center containing osculating J2000.0 orbital elements of all 155,368 numbered minor planets for epochs at 200-day intervals back in time to well before Barnard exposed his plates.

This of course rendered it an "exceedingly easy matter" to conclude that just 15 of the 51 entries in Table 1 refer in fact to minor planets. Seven of the minor planets were recorded during 1904-1915, seven during 1925 or 1927 and - intriguingly - one in 1925 and 1927. The observations of these minor planets are listed in Table 2.

Here the equinox 2000.0 positions in the on-line NSV catalogue have been rounded to the precision of the 1875.0 positions tabulated by Ross. The observed magnitude is that for the positive observation supplied by Ross; it should be noted that the NSV catalogue deliberately gives a number 10 fainter. Since the precise time of observation is unknown, the UT date given corresponds to when (to 01) the computed position (for which just the end figures in R.A. and Decl. are given) was closest to the observed position of the indicated minor planet. These UT dates are one day later than the local dates provided by Ross. To start the local date at noon was standard astronomical practice during 1904-1915, and it appears that Ross continued this practice following the actual introduction of UT in 1925. The computed magnitudes for the minor planets are in the system.

It should be noted that all these minor-planet identifications are absolutely sure, and that no remotely questionable cases were found among the other 36 Ross objects. For six of the minor planets, (26), (59), (115), (389), (449) and (451), astrometric positions are available from other observatories (mainly Heidelberg, but also Greenwich, Marseilles and Algiers) at the same opposition. The 1909 observation of (714) Ulula, the minor planet with the highest number in the whole set, is in fact the earliest known for that object, which was discovered from Heidelberg in May 1911. By curious chance, (24) Themis was observed both as Ross 38 on 1925 Mar. 31 and as Ross 329 on 1927 Oct. 18. That Ross 39 should turn out to be (39) Laetitia is also fortuitous.

In no case was the computed brightness more than 18 different from the observed magnitude given by Ross. The mean brightness discordance was 06 in the sense that the computed magnitude was systematically fainter than the Ross magnitude. Given that the Ross observations are photographic, it would be reasonable to use the computed magnitude, which is typically 08 fainter than . The mean discordance would therefore be 14. Although the brightness discordance is obviously affected by the low precision of the Ross values, the computed magnitudes do support the decision by the compilers of the NSV catalogue consistently to add 10 to the Ross figures.

The six cases previously identified by Bedient (2003) are indicated in Table 2 by means of asterisks following the Ross number. Ross 352 was also independently identified by Schmeer (2003). However, four more of the objects, Ross 40, 168 (the object that inspired this investigation), 230 and 329, were incorrectly characterized by Bedient in his Table 2 as "not to be solar system objects". They are most certainly (162) Laurentia, (106) Dyone, (389) Industria and (24) Themis, respectively. The present author also disagrees with the notation in Bedient's Table 3 that Ross 231 and 344 "had ambiguous search results" and "cannot be characterized". The present work clearly shows that they are (242) Kriemhild and (59) Elpis, respectively.

References:

Bedient, J.R., 2003, Inform. Bull. Var. Stars, No. 5478

Kukarkin, B. V., Kholopov, P. N., Artyukhina, N. M., et al., 1982, New Catalogue of Suspected Variable Stars, Moscow: Nauka

Ross, F.E., 1925, Astron. J., 36, 99

Ross, F.E., 1926a, Astron. J., 36, 122

Ross, F.E., 1926b, Astron. J., 36, 167

Ross, F.E., 1927a, Astron. J., 37, 91

Ross, F.E., 1927b, Astron. J., 37, 155

Ross, F.E., 1928a, Astron. J., 38, 99

Ross, F.E., 1928b, Astron. J., 38, 144

Ross, F.E., 1929, Astron. J., 39, 140

Ross, F.E., 1930, Astron. J., 40, 34

Ross, F.E., 1931, Astron. J., 41, 88

Samus, N.N., 2007, personal communication

Samus, N.N. and Antipin, S.V., 2007, Peremennye Zvezdy (Prilozhenie), 7, No. 10

Schmeer, P., 2003, personal communication to N.N. Samus

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