Astrometric Membership Tests for the Zinn–Newell–Gibson UV-bright Stars in Galactic Globular Clusters

In color–magnitude diagrams (CMDs) of globular clusters (GCs), the vast majority of stars in post-main-sequence evolutionary stages lie on the subgiant branch, red giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB). There are, however, rare stars in transient phases of rapid evolution that lie above the HB and to the blue of the AGB/RGB in cluster CMDs. These objects include stars that have departed the HB and are evolving toward the AGB, post-HB stars that reached the AGB but then left it before arriving at the AGB tip (post-early-AGB, or PEAGB, stars), and objects that reached the tip of the AGB and are now evolving rapidly toward higher temperatures (post-AGB, or PAGB, stars). See Moehler et al. (2019, hereafter M+19; their Figure 7) for examples of theoretical post-HB evolutionary tracks that produce stars above the HB in GC CMDs. It is also possible that binary interactions can generate stars lying above the HB.

A group at Pennsylvania State University is conducting an observational survey aimed at creating a complete census of above-horizontal-branch (AHB) stars in the Galactic GC system. This survey is based on photometry in the uBVI system (Bond 2005), which is optimized for the detection of low-gravity evolved stars with large Balmer discontinuities in their spectral-energy distributions. With astrometry from Gaia now available, including the recent Early Data Release 3 (EDR3; Gaia Collaboration et al. 2020), one can further test cluster membership using precise parallaxes and proper motions (PMs). Previous uBVI work led to discoveries of luminous yellow PAGB stars in M79 (Bond et al. 2016, hereafter BCS16) and M19 (Bond et al. 2021, hereafter B+21). In the case of M19, we also discovered a hotter, and equally luminous, blue PAGB star belonging to the cluster. Full details of the uBVI survey for AHB stars will be presented in a forthcoming paper (B. D. Davis et al. 2021, in preparation, hereafter D+21). Subsequent publications will explore the utility of yellow PAGB stars—which are the visually brightest stars in old populations—as standard candles for determining extragalactic distances, as I have advocated (Bond 1997a, 1997b).

The two luminous PAGB stars that B+21 found in M19 are among the brightest and most conspicuous members known in any GC. In spite of this, they had not, to my knowledge, been recognized as cluster members by previous investigators of this massive GC. However, when I researched the literature, I found that both of the M19 objects had been identified as UV-bright candidates in the classical survey of Zinn et al. (1972, hereafter ZNG).

The ZNG team had blinked photographs of 27 GCs obtained in the U and V bands, and had identified 156 candidates that were the brightest objects in the clusters in the U band. The term "UV-bright" was a bit of a misnomer. Many of the ZNG objects are indeed hot post-AGB stars, including the prototypical luminous blue stars Barnard 29 in M13, and von Zeipel 1128 in M3. However, many other ZNG candidates are even cooler than the yellow Type II Cepheids in GCs, but nevertheless brighter than most of the cluster members in the U band—including objects that are actually unrelated bright foreground stars.

ZNG presented finding charts for their candidates, but did not provide celestial coordinates. There have been several follow-up studies of individual ZNG objects (e.g., Zinn 1974; Harris et al. 1983; de Boer 1985; Moehler 2001, 2010; Jasniewicz et al. 2004; M+19; and references therein), and several of these stars have proven to be of great astrophysical interest. These include, for example, several now well-known hot PAGB stars (see the lists in M+19). However, there has never, to my knowledge, been a published list of the celestial coordinates of all of the ZNG stars.

As an adjunct to the D+21 study, I determined the coordinates of all of the ZNG stars, and I present them here. I then used astrometry from Gaia EDR3 to test the cluster memberships of the candidates. Lastly, I give an indication of the evolutionary status of the stars that appear to be cluster members, based on their locations in the CMDs, and I point out several unstudied objects of interest.

I first identified each ZNG star in my uBVI CCD GC survey CCD images. These frames had been obtained with 0.9, 1.5, and 4 m telescopes at Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory; they are described in detail by B+21 and D+21. I blinked the u and V frames, while comparing them with the ZNG finding charts. In nearly every case, the stars were identified unambiguously; only a handful of objects near the cluster centers were less certain. Approximate coordinates were then determined using images from the Space Telescope Science Institute Digitized Sky Survey, ⁴ images from the PanSTARRS-1 sky survey, ⁵ and/or in a few cases images from the Hubble Space Telescope. ⁶ With all of these tools, coordinates can be obtained by placing a cursor on the stellar images.

Finally, I identified each object in the Gaia EDR3 catalog. ⁷ Table 1 lists the 156 ZNG candidates, along with their J2000 coordinates, Gaia parallaxes and PMs, and the Gaia apparent G magnitudes and BP − RP colors. The notes to the table give information on the few instances of uncertain identifications, as well as other information on the objects, but are not intended to be a complete literature survey.

Table 1. The ZNG UV-bright Candidates and Gaia Astrometry and Photometry

Cluster	ZNG	R.A.	Decl.	Parallax	μα	μδ	G	BP − RP	Cluster	Classification ii	Remarks iii
NGC/M	No.	(J2000)	(J2000)	(mas)	(mas yr−1)	(mas yr−1)	(mag)	(mag)	Member? i
2419	1	07 38 11.156	+38 52 27.74	0.5929 ± 0.0633	4.740 ± 0.057	− 8.559 ± 0.036	16.318	0.987	no
	2	07 38 09.234	+38 53 12.77	0.2301 ± 0.1527	− 0.257 ± 0.135	− 0.414 ± 0.090	17.395	0.305	yes	AHB
4147	1	12 09 58.479	+18 34 08.78	1.0151 ± 0.0248	− 20.213 ± 0.026	− 8.615 ± 0.023	14.326	0.823	no
	2	12 10 05.661	+18 32 45.69	0.1253 ± 0.1069	− 1.858 ± 0.105	− 2.430 ± 0.099	16.856	0.250	yes	HB	−49
4590/	1	12 39 40.099	−26 43 02.62	0.3911 ± 0.0289	− 11.104 ± 0.030	− 5.431 ± 0.021	14.300	0.498	no
M68	2	12 39 22.513	−26 45 11.87	0.0916 ± 0.0160	− 2.792 ± 0.016	>1.943 ± 0.012	12.258	1.451	yes	AGB	−48
5024/	1	13 12 56.879	+18 10 53.17	4.6060 ± 0.0146	− 78.011 ± 0.019	− 29.825 ± 0.016	12.388	1.013	no
M53	2	13 12 47.541	+18 10 23.15	1.1056 ± 0.0206	− 17.135 ± 0.026	− 4.531 ± 0.024	14.395	0.903	no
	3	13 12 45.206	+18 11 57.44	0.0211 ± 0.0269	− 0.165 ± 0.036	− 1.351 ± 0.033	14.850	0.192	yes	AHB	−47
	4	13 12 47.406	+18 07 10.77	0.0519 ± 0.0505	− 0.401 ± 0.065	− 1.422 ± 0.062	15.701	0.297	yes	AHB
	5	13 13 09.478	+18 10 58.70	0.9811 ± 0.0241	− 15.057 ± 0.032	− 24.435 ± 0.031	14.741	0.770	no

Notes.

ⁱ Indication of cluster membership, based on parallax and PM. A few uncertain cases are labeled with question marks. ⁱⁱ The evolutionary classification scheme is explained in the text (Section 6) and the caption of Figure 2, based on the position in the Gaia CMDs. Abbreviations are: AGB (asymptotic giant branch); RGB (red giant branch); HB (horizontal branch); rPAGB, yPAGB, bPAGB, and vPAGB (red, yellow, blue, and violet post-AGB stars); AHB (above-horizontal branch); and CV (cataclysmic variable). ⁱⁱⁱ Remarks: (1) 4147 ZNG 2: the HST images show the object to be a blend of five cluster members of similar brightness; the brightest one is listed. (2) 4590 ZNG 2: at the tip of the AGB/RGB; see Schaeuble et al. (2015) and references therein. (3) 5024 ZNG 3: considered a nonmember by Boberg et al. (2016). (4) 5024 ZNG 7: close nearly equal pair, with a separation of 09. Astrometry has large uncertainties, but both components are consistent with membership. The brighter component is listed. It is a nonmember according to Boberg et al. (2016). (5) 5024 ZNG 8: astrometry has large uncertainties. It is a member according to Boberg et al. (2016). (6) 5024 ZNG 9: it is a member according to Boberg et al. (2016) and is blended with a fainter AGB star 20 away. (7) 5024 ZNG 14: the identification is uncertain. Astrometry has large uncertainties, but membership is unlikely. (8) 5272 ZNG 1: M3 von Zeipel 1128. (9) 5272 ZNG 3: RR Lyr variable V10. (10) 5272 ZNG 4: blend of three stars where the bluest one is listed. RR Lyr variable V165. (11) 5272 ZNG 5: RR Lyr variable V90. (12) 5466 ZNG 5: extreme HB star; see Geier et al. (2019). (13) 5466 ZNG 6: AGB star; see Shetrone et al. (2010). (14) 5897 ZNG 2: luminous red post-AGB star. The Gaia radial velocity is consistent with membership. (15) 5904 ZNG 1: the PM is consistent with membership, and the parallax is discordant; it is an AGB or RGB star if it is a member. (16) 5904 ZNG 2: the PM is consistent with membership, and the parallax is discordant; it is an AGB or RGB tip star if it is a member. (17) 5904 ZNG 6: RR Lyr variable V41. (18) 5904 ZNG 7: RR Lyr variable V25. (19) 6093 ZNG 1: the PM is consistent with membership, and the parallax is discordant; it is an RGB star if it is a member. (20) 6093 ZNG 3: EDR3 does not give a parallax or PM for this source; there is a bright blue object at this position in the Hubble Space Telescope images, but the Gaia BP − RP color is red. The position is close to that of the X-ray source CX3 (Heinke et al. 2003). (21) 6121 ZNG 2: RR Lyr variable V19. (22) 6121 ZNG 4: RR Lyr variable V10. (23) 6121 ZNG 5: this object is a blend of two RR Lyr variables, separated by 16 (Clementini et al. 2019). The brighter component is listed in the table. (24) 6121 ZNG 6: candidate RR Lyr, V58, but it is a nonvariable according to Safonova et al. (2016). (25) 6121 ZNG 7: very bright star at tip of the RGB. The Gaia radial velocity is consistent with membership. (26) 6121 ZNG 9: RR Lyr variable V28. (27) 6205 ZNG 1: Barnard 29 (see M+19). (28) 6205 ZNG 2: the PM is consistent with membership, but it has large uncertainties and a discordant parallax; it is suspected to be a low-amplitude variable (Deras et al. 2019). (29) 6205 ZNG 3: variable star V51, uncertain type (Servillat et al. 2011). (30) 6205 ZNG 4: spectroscopic analysis by Ambika et al. (2004). (31) 6218 ZNG 7: the Gaia radial velocity is consistent with membership. (32) 6218 ZNG 8: the Gaia radial velocity is consistent with membership. (33) 6254 ZNG 1: blue PAGB star (see M+19). (34) 6273 ZNG 2: blue PAGB star (see B+21). (35) 6273 ZNG 3: Type II Cepheid, V1 (Clement & Sawyer Hogg 1978); the PM is consistent with membership, and the parallax is discordant. (36) 6273 ZNG 4: yellow PAGB star (see B+21). (37) 6333 ZNG 1: the PM is consistent with membership; the parallax is slightly discrepant. It is on the boundary between the yellow and blue PAGB stars. (38) 6333 ZNG 4: astrometry consistent with membership, but the uncertainties are large. (39) 6341 ZNG 4: the PM is consistent with membership, but the parallax is discordant. Considered a member by Zinn (1974), but not by Roederer & Sneden (2011). (40) 6356 ZNG 2: the PM is marginally consistent with membership. (41) 6402 ZNG 1: blue PAGB star (star 33 in Schiavon et al. 2012). (42) 6402 ZNG 5: AHB (star 202 in Schiavon et al. 2012). (43) 6402 ZNG 14: Type II Cepheid, V167 (Contreras Peña et al. 2018). (44) 6626 ZNG 5: RV Tauri variable V2342 Sgr = V17. (45) 6656 ZNG 14: the Gaia radial velocity is consistent with membership. (46) 6712 ZNG 1: blue PAGB star (see M+19). (47) 6712 ZNG 4: luminous star at the AGB tip; Mira variable AP Sct = V2. (48) 6779 ZNG 2: violet PAGB star (see M+19). (49) 7078 ZNG 1: hot post-HB or violet PAGB star (see M+19). (50) 7078 ZNG 4: the identification is difficult and uncertain. The listed star is brighter than the close 14 pair. (51) 7078 ZNG 5: the identification is difficult and uncertain. The listed star is brighter of close 14 pair. (52) 7078 ZNG 6: the PM is consistent with membership, but the parallax is discordant. If it is a member, it is a bright star near the AGB tip. (53) 7078 ZNG 7: the astrometry has large uncertainties; Gebhardt et al. (1997) give a radial velocity consistent with membership.

Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.

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Using the EDR3 astrometry, I tested each ZNG candidate for cluster membership. Two criteria were applied to each star, PM and parallax, and in a few instances a third one, radial velocity (RV). First, I determined the mean PMs of each cluster in R.A. and decl. and their dispersions, based on a selection of cluster members from the EDR3 catalog; then I required that each candidate have a PM consistent with that of the host cluster. In a large majority of cases, the PM criterion alone was sufficient to exclude the nonmembers. Next the parallax was considered. I adopted a nominal parallax for each cluster, based on the reciprocals of the distances given in the Harris (2010, hereafter H10) catalog of cluster parameters. ⁸ The parallax of each candidate was generally required to be consistent, within three times its stated uncertainty, with the nominal cluster parallax. There was, however, a handful of cases where the star's PM was in accord with membership, but the parallax was larger (or occasionally smaller) than that of the cluster, by more than the nominal three times the uncertainty. It is plausible that their EDR3 parallaxes may have been affected by source crowding. These ambiguous cases are discussed in the notes of Table 1. Finally, in a handful of instances for the brightest objects, EDR3 also gives a RV, another valuable membership criterion, via comparison with the cluster's RV given in H10. The few cases where the Gaia RV helped confirm membership are indicated in the notes of Table 1.

Figure 1 illustrates the PM and parallax membership tests, for the case of the cluster M22. I selected stars from Gaia EDR3 lying within $5^{\prime}$ of the cluster center, and brighter than G magnitude 13. The PMs of these stars are plotted as black and red points: black for stars with a parallax less than 0.6 mas (which is about twice the nominal cluster parallax), and red for the few stars with a parallax of more than 0.6 mas (likely foreground stars). The EDR3 PMs are so precise that the cluster members form a tight distribution. The small number of field stars and foreground objects have a much wider dispersion and are offset from the cluster mean.

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The larger filled circles in Figure 1 plot the PMs of 16 of the 18 ZNG candidates. (Two of them have PMs so large they are outside the frame.) The ZNG stars plotted in blue have parallaxes less than 0.6 mas, and the ones with larger parallaxes are plotted in green. Four of the ZNG objects have PMs consistent with cluster memberships, as well as small parallax values, making them highly probable members. One of them, as noted in Table 1, also has an RV consistent with membership. The remaining ZNG candidates in the figure have PMs clearly inconsistent with that of the cluster, and most of them also have parallaxes too large for cluster membership. ⁹

The final membership classifications are given in column 10 of Table 1, with a question mark for the few ambiguous cases; these are discussed in the notes of the table.

For the ZNG candidates that I considered to be likely cluster members, I estimated their evolutionary statuses based on their locations in the cluster CMDs. The classification scheme that I adopted is based on cluster CMDs in which the Gaia apparent magnitude G is plotted against the color index BP − RP. Figure 2 provides examples. Here the Gaia data are presented for three ZNG GCs: M13, M53, and NGC 5897. Also included is M79, which is not one of the ZNG clusters, but contains the luminous yellow PAGB star discovered by BCS16. All four clusters are only lightly reddened (E(B − V) = 0.02, 0.02, 0.09, and 0.01, respectively, from H10). For each cluster, I selected a sample of likely members from the EDR3 catalog, with parallaxes and PMs consistent with membership.

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In each panel of Figure 2, the region of the CMD above the HB and to the blue of the AGB is divided schematically into several boxes. These are based on post-HB evolutionary tracks, such as those in M+19. The luminous PAGB sequence is divided on the basis of color into red, yellow, blue, and violet objects—rPAGB, yPAGB, bPAGB, and vPAGB, respectively. The rPAGB stars have unreddened BP − RP colors lying between 0.8 and up to within 0.1 mag of the AGB, and G magnitudes at least 3.5 mag brighter than that of the HB at BP − RP = 0.8. The yPAGB stars have 0 < (BP − RP) < 0.8, and lie at least 3.5 mag brighter than the HB at the same color. bPAGB objects are in the color range of −0.25 < (BP − RP) < 0, and are at least 3.5 mag brighter than the HB at the same color. The hottest PAGB stars are those bluer than BP − RP = −0.25, and either brighter than the brightness of the HB at a color of zero (vPAGB) or fainter (vPAGB/AGB-manqué). AGB-manqué objects evolve directly from the extremely blue HB to the white-dwarf (WD) sequence, without ever becoming cool stars. As it happens, however, the ZNG sample does not contain any such stars. These divisions are somewhat arbitrary, but give a useful indication of the approximate temperatures. The scheme should be considered preliminary, and will be revised in our forthcoming paper (D+21).

Stars lying below the PAGB sequence, but at least 0.5 mag above the HB or AGB, are called AHB objects. They are a mixture of stars evolving off of the blue HB to cooler temperatures, along with PEAGB stars returning from the AGB toward the top of the WD cooling sequence.

Using the plotting tools at Vizier (see footnote 7) to generate CMDs derived from Gaia EDR3 data for each GC, I applied the classification scheme described above to all of the ZNG objects. The classification grid was shifted in color and magnitude according to the reddening and distance of each cluster. The classification results are given in column 11 of Table 1.

Out of the 156 candidates, 86 (55%) are nonmembers or likely nonmembers, and nearly all of them are bright stars that clearly lie in the cluster foregrounds. Although ZNG excluded stars known to be variable at the time of their survey, the list contains several objects subsequently found to vary. These include 2 Type II Cepheids and, more surprisingly, 10 RR Lyrae variables. I speculate that the latter were selected by ZNG because they happened to be near maximum light at the epoch of the U-band exposures.

There are several cases of objects that are blends of two or more stars, which would account for them appearing unusually bright in ground-based images. More surprising is that there are quite a few candidates that appear to be unexceptional stars lying on the RGB, AGB, or HB sequences. However, a few of them are of interest because they are extremely bright, lying close to the tips of the RGB or AGB. One of the most remarkable of these is NGC 6712 ZNG 4, a very luminous star at the cluster's AGB tip; it is the known Mira variable AP Sct, which is NGC 6712 V2 in the Clement et al. (2001) catalog ¹⁰ of variable stars in GCs. One blue object, NGC 6093 (M80) ZNG 3, lies close to the position of a known X-ray source, CX 3.

Among the bright PAGB stars, there are two rPAGB objects: NGC 5897 ZNG 2 and NGC 6626 (M28) ZNG 5. The former star has not, to my knowledge, been recognized previously as a luminous PAGB star; the latter is a known RV Tauri variable, as described in the notes of Table 1. There is only one yPAGB star, the one in the cluster M19 that was recently pointed out for the first time by B+21.

The table contains six luminous bPAGB and two vPAGB stars. Of these, several are already well known (see M+19): NGC 5272 (M3) ZNG 1 (von Zeipel 1128), NGC 6205 (M13) ZNG 1 (Barnard 29), NGC 6254 (M10) ZNG 1, NGC 6712 ZNG 1, and NGC 6779 (M56) ZNG 2. The bPAGB star NGC 6273 (M19) ZNG 2 was only noted for the first time in the recent B+21 study. NGC 6402 (M14) ZNG 1 was identified as a hot PAGB star based on Galaxy Evolution Explorer (GALEX) imaging by Schiavon et al. (2012), but has not otherwise been studied. The bPAGB object NGC 6333 (M9) ZNG 1 has not, to my knowledge, been recognized previously.

Lastly, Table 1 contains 27 AHB objects. As noted above, these are likely to be a mixture of stars evolving off the hot HB toward the AGB, and stars that reached the AGB and are now evolving back toward higher effective temperatures. Like the PAGB stars, they are worth further study.

The ZNG survey was an important early observational investigation that added to the understanding of late stellar evolution. Even after nearly five decades, there are still unexplored objects of considerable interest in the ZNG catalog. My group's uBVI survey, to be reported by B+21, will follow in the path illuminated by this classical investigation.

I thank the Penn State post-AGB group—Robin Ciardullo, Brian Davis, and Michael Siegel—for discussions of evolved stars in globular clusters.

My uBVI observations were partially supported by NASA grant NAG 5-6821 under the UV, Visible, and Gravitational Astrophysics Research and Analysis program, and by the Director's Discretionary Research Fund at STScI. I also thank the staff at Cerro Tololo and Kitt Peak for their support over many years.

This research has made use of the VizieR catalog access tool, CDS, Strasbourg, France (DOI:10.26093/cds/vizier). The original description of the VizieR service was published by Ochsenbein et al. (2000).

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions.

The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the PanSTARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation.

Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

Facilities: CTIO 0.9 m - , 1.5 m; KPNO 0.9 m - , 4 m; Gaia - , Swift - , HST. -