Photometry of M57 Field Stars

Photometry of M57 Field Stars
by Brian Skiff

Lowell Observatory
(Updated November 2000)

Introduction

A common goal among amateur deep-sky observers is picking out the central star of the Ring Nebula, Messier 57. With a large telescope (say a 24-inch) this is quite easy to do. But because of physiological effects in the eye, detection with minimal apertures can be problematic (an 8- or 10-inch telescope is required), with the eye sometimes showing a spurious brightening at the proper location even in quite small instruments.

For some time I have wanted to produce a proper photometric sequence around this much-observed deep-sky object. One immediate use is allowing a check on magnitude limits with different telescopes, magnifications, and sky conditions. All published photometric measurements make the central star about V mag. 15.0 or perhaps one to three tenths fainter, but with no demonstrable evidence of variability. This fact, combined with the bright background of the center of the nebula, makes seeing the central star more difficult than would be the case if it were an isolated star. Beyond merely being able to see faint enough, picking up the central star in small telescopes requires both quite high magnification (~500x) and sub-arcsecond seeing.

Photometric Observations

As a start on this problem I made single observations of several brighter stars in the field using the Lowell 53cm photometric telescope in October 1996. I would have preferred to get them on two or three nights each, but lack of time prevented this. I used a 19-arcsecond diaphragm and Str�mgren b and y filters on the photometer. About a dozen primary and secondary standard stars were included in the series each night for calibration to the standard system. The y filter observations are transformed to standard Johnson V magnitudes, which are appropriate for visual use. The uncertainties on the measurements normally range from about 0.01 to 0.03 mag. In several cases below, I have retained my V magnitudes, but adopted B-V from the Tycho-2 catalogue to maintain consistency with a larger CCD dataset. The Tycho colors were corrected to the standard system using the transformations of Bessell (2000).

On two nights in 2000 Arne Henden (USNO-Flagstaff) observed the field using a CCD on the USNO 1-m telescope, which covers a 10'x10' field. The data reach far fainter than I can get with the Lowell telescope, and I've adopted his results here for selected stars within the field, replacing my values except for the brightest stars. Results for stars in common match nearly perfectly. One can expect that zero-point errors are no worse than ±0.02 mag. I have made a selection of stars covering the full range of the reliable data, which now extends to nearly mag. 20, and give below a list with positions and BVR photometry. The fainter stars all lie in the northwest quadrant from the Ring toward the galaxy IC 1296. If you're greedy, Arne's complete file of over 700 stars in the 10'x10' CCD field is located at:

Name:	RA: [J2000]	Dec:	s	V	B-V	V-R	Remarks:
NGC 6720	18 53 35.1	+33 01 45	-	-	-		HD 175353
IC1296	18 53 18.8	+33 03 58	G				GSC 2642-1400
HD 175267*	18 53 08.7	+33 12 36	T	8.74	0.11		spec: A0
GSC 2642-1690*	18 53 04.0	+32 55 33	T	10.53	0.95		spec: G
AG+32 1682	18 53 11.6	+33 03 13	T	11.10	1.31	0.66	spec: K
GSC 2642-1568	18 53 17.3	+32 58 22	T	11.58	1.26	0.66
GSC 2642-2506	18 53 15.0	+33 00 03	G	12.34	0.34	0.18
GSC 2642-0590*	18 53 18.4	+33 02 47	G	12.85	0.96	0.55	Double star
GSC 2642-0433*	18 53 39.8	+33 01 46	G	13.03	0.60	0.35	E edge of nebula
GSC 2642-1934	18 53 48.0	+32 58 58	G	13.36	0.68	0.37
GSC 2642-0426	18 53 24.1	+33 03 43	G	14.12	0.53	0.31
GSC 2642-0239	18 53 42.9	+33 00 47	G	14.13	0.69	0.38
147	18 53 30.9	+33 02 32	A	14.71	0.55	0.33	Brighter of two NW
153a	18 53 37.4	+32 59 31	A	15.27	1.03	0.56
153b	18 53 29.4	+33 02 24	A	15.32	0.75	0.42	Fainter of two NW
154	18 53 35.4	+32 59 33	A	15.39	1.15	0.66
156	18 53 34.6	+33 00 40	A	15.63	1.04	0.58	sup on neb S side
157	18 53 30.8	+33 01 42	A	15.68	0.75	0.43	sup on neb W side
161	18 53 34.3	+33 03 14	A	16.13	1.27	0.76
166	18 53 38.1	+33 00 24	A	16.57	0.70	0.38
168	18 53 32.0	+33 03 21	A	16.79	1.16	0.65
173	18 53 24.4	+33 02 40	A	17.28	0.82	0.45
177	18 53 24.8	+33 02 06	A	17.72	0.46	0.26
181	18 53 24.2	+33 01 26	A	18.07	0.81	0.46
187	18 53 32.5	+33 03 06	A	18.65	0.88	0.55
191	18 53 26.5	+33 02 44	A	19.05	0.82	0.45
194	18 53 27.5	+33 01 08	A	19.40	0.53	0.32
198	18 53 26.0	+33 00 55	A	19.81	0.51	0.35

JPEG from POSS print, with magnitude information, for this sequence. Similar to above.
The image (83Kb) shows the faint outer halo of the nebula as well as the nearby mag. 14 galaxy IC 1296. The brightest star in the list, bluish HD 175267, appears near the northwest (upper-right) corner of the chart, which covers approximately 15' x 20'. For reference, this is the first star northwest of the nebula as plotted on Uranometria chart 117.

The Hubble Guide Star Magnitudes are quite a bit brighter than visual (V), and are not comparable to the V magnitudes for the stars I observed, as given in the table above.

A couple of stars are worth noting specifically. The star on the east side of the nebula, GSC 2642-0433, has been claimed to be as bright as mag. 11, and even thought to vary. I have made several careful measurements of this star in the past, accounting as best I could for the background near the nebula, and the results scatter between V mag. 13.02 and 13.05, all of which can be attributed simply to normal photon-counting errors in the data. Henden's magnitude matches the average of mine exactly, so the brightness of this star is well in hand.

The first reasonably bright star west-northwest of the nebula, GSC 2642-0590, is readily seen on large-scale photographs to be a fairly close pair. It was finally catalogued as such by G. Popovic in 1985, as GP 192. The separation and position angle are from his observations, but appear to be fixed when compared to plates taken at the turn of the Century. Because the component stars are quite faint, a fairly large aperture and high power are required to resolve them visually.

Among the fainter field stars I have selected several in the immediate vicinity of the nebula to answer the questions that continually come up about their brightness. Since they are disturbed by faint nebulosity to varying degrees, Henden's values may not be utterly correct. Some fainter isolated stars slightly further afield complete the list.

Holden found this last comment inexplicable, noting that the central star "has been well seen by Professor Schultz at Upsula [Uppsala, Sweden] with the Steinheil refractor of 9 1/2 inches, and is seen here [Lick Observatory] with the 12-inch equatorial." Alas, no citation to the Schultz observation. Holden's continuing description includes what is apparently the first observation of the companion to the central star, first observed by Schaeberle at Lick. It is noted as being doubtful, since only Schaeberle could see it—meaning merely that Holden was not a good visual observer, whereas Schaeberle was one of the best in his day. These comments make clear that the central star can be seen in modest apertures under good conditions.

Finally, I note that Steve O'Meara has seen the central star using the 9-inch refractor at Harvard (a distinctly urban observing site); see his recent Messier book for details.