M16 and Open Cluster NGC 6611

Figure 1: Objects of Interest in Origin's Capture of the Eagle Nebula M16

Robert Burnham, Jr. devoted several pages of his third volume of Burnham's Celestial Handbook to the Eagle Nebula M16 in the constellation Serpens (Dover Publications, Inc., New York, 1978, pp. 1782-1792). The photograph taken by the 200-inch Palomar Observatory reflector (p. 1786) was stunning for its day. Many of the features revealed by the 200-inch are also seen in my 6-inch Celestron Origin Observatory image shown here. Who would have thought that possible back then!

One of the most beautiful colour photos ever created in M16 was entitled "Pillars of Creation" (POC). Three towering columns of gas and dust were captured by the Hubble Space Telescope (HST) in April 1995 and published in the February 1996 issue of Sky & Telescope, Vol. 91(2), pp. 32-33. Fast forward to the James Webb Space Telescope (JWST) and an extraordinary photograph of the POC revealing more stars and protostars never seen before (see Astronomy, Vol. 51(6), June 2023, p. 19 and Cosmos, Penguin Random House, New York, 2024, pp. 222-223).

NGC 6611, an open cluster of young stars and the main object of interest in subsequent sections of this webpage, has had a long history of research owing to its importance to stellar evolution. Also captured in this Celestron Origin 1.27° x 0.85° field of view is most of the open cluster Trumpler 32 (Tr 32), which has not received as much scientific attention as NGC 6611 (see Sky & Telescope, Vol. 138(2), August 2019, p. 55). Not far from Tr 32 are two variables, the carbon star ES Ser and the Beta Cepheid HD 168015. I will return to study these three lesser-known objects at a later date.

Logbook

Date & Time: October 1, 2024, 8:43 pm
Location: Kempt Shore, Nova Scotia, Canada, 45°N
Conditions: Clear, calm, 12°C
Moon: Waning crescent (1%)
Technical: 10-sec exposures, 40-min integration, battery power, no filter, minor post-processing using my Samsung Galaxy S23 Ultra's built-in photo editing tools

Cluster Membership By M.F. Walker (1961)

Figure 2: First Research Paper on NGC 6611 Cluster Membership

Beginning in the mid-1950s, Merle F. Walker began to study young star clusters at the Lick Observatory. His series of publications included open clusters NGC 2264 in Monoceros (1956), NGC 6530 in Sagittarius (1957), IC 5146 in Cygnus (1959), and NGC 6611 in Serpens (1961). Using my Origin star field image, Walker's list of NGC 6611 cluster members are labelled in blue, and stars he listed as foreground stars are labelled in white. In this first image I have labelled 15 members around the outskirts of the more densely populated central region of the cluster. This gives you an idea of the size of NGC 6611. More of Walker's member stars are added in subsequent sections to avoid label congestion.

All of the member stars are linked to the Simbad Database using action buttons at the bottom of this page. V magnitudes and young stellar object (YSO) status are from Simbad. Also, note that I have included several reference field stars using yellow labels and their Bonner Durchmusterung Catalogue numbers.

Reference:

"Studies of extremely young clusters. IV. NGC 6611", Merle F. Walker, Astrophys. J., 133, 438-456, (1961).

Bonner Durchmusterung Catalogue

In the mid-1880s, Friedrich Wilhelm August Argelander and his team estimated magnitudes and positions of approximately 320,000 stars at the Bonn Observatory. This catalogue is still in use today. Examples: BD+13° 4000 star in Aquila, and BD-13° 4906 star in Serpens.

References:

1. Universe, William J. Kaufmann, III, W.H. Freeman and Company, New York, pp. 19-20.

2. Fundamental Astronomy, Hannu Kartttunen, Pekka Kröger, Heikki Oja, Markku Poutanen, and Karl Johan Donner, Editors, Sixth Edition, Springer-Verlag, Berlin, 2017, pp. 31-34.

Figure 3: 28 More NGC 6611 Cluster Members by Walker

Moving inward toward the densely populated central region of the cluster, we find numerous young stellar objects of spectral types O and B. Various photoelectric and photographic magnitude observations reported by Walker were made with instruments at Lowell, Mount Wilson, and Palomar Observatories. His measured V magnitudes for stars from Figures 2 and 3 are listed below. Also included are the magnitudes from the Simbad Database in parentheses (V) and SkySafari 7 Pro in brackets [V] for comparison. Stellar type is from the Simbad Database. Asterisk * means I can't find the star in SkySafari.

NGC 6611 125 (BD-13 4920, TYC 5689-1005-1), V 10.05 (10.06) [10.05], B1V
NGC 6611 150 (BD-13 4921, TYC 5689-1622-1), V 9.88 (9.91) [9.91], B0.7V
NGC 6611 161 (TYC 5689-0663), V 11.30 (11.22) [10.97], O8IV
NGC 6611 166 (TYC 5689-0998-1), V 10.36 (10.30) [10.39], O9V
NGC 6611 175 (BD-13 4923, TYC 5689-0966-1), V 10.05 (10.08) [10.13], O4V((f+))+O7.5V
NGC 6611 197 (HD 168075, BD-13 4925, TYC 5689-1208-1), V 8.77 (8.77) [8.75], O7V(n)((f))z
NGC 6611 205 (HD 168076, BD-13 4926, TYC 5689-0607-1), V 8.24 (8.25) [8.18], O4III(f)
NGC 6611 222 (GAIA 4146612872133107584), V 13.02 (12.97) [12.99], O7V((f))z
NGC 6611 223 (TYC 5689-1381-1), V 11.22 (11.06) {11.06], B1V
NGC 6611 228 (GAIA 4146612803413634688), V 13.52 (13.42) [13.39], B1.5Vp
NGC 6611 235 (TYC 5689-1411-1), V 11.06 (11.13) [10.97], Be
NGC 6611 239 (TYC 5689-1324-1), V 11.49 (11.47) [11.59], B1V
NGC 6611 246 (BD-13 4927, TYC 5689-0989-1), V 9.60 (9.48) [9.48], O7II(f)
NGC 6611 259 (TYC 5689-1658-1), V 11.60 (11.05) [11.05], B0.5V
NGC 6611 280 (GAIA 4146600678717290624*), V 9.94 (10.04) [*], O9.7IVnn
NGC 6611 290 (TYC 5689-0787-1), V 12.14 (11.69) [11.69], B2.5V
NGC 6611 305 (GAIA 4146612528535774080), V 13.56 (13.56) [13.47], B1V
NGC 6611 314 (BD-13 4929, TYC 5689-0441-1), V 9.86 (10.23) [10.24], O7V+B0.5V
NGC 6611 349 (GAIA 4146600403839348096), V 11.62 (11.61) [11.60], G5III
NGC 6611 351 (TYC 5689-0433-1), V 11.24 (11.16) [11.17], B1V
NGC 6611 367 (BD-13 4930, TYC 5689-0439-1), V 9.43 (9.45) [9.34], O9.7V
NGC 6611 401 (HD 168137, TYC 5689-1097-1), V 8.95 (8.94) [8.93], O8Vz
NGC 6611 412 (HD 168183, TYC 5689-1048-1), V 8.30 (8.28) [8.23], O9.5III+B3-5V/III
NGC 6611 455 (GAIA 4146600090310560896), V 12.06 (12.05) ]12.09], A5II
NGC 6611 468 (BD-13 4934, TYC 5689-0815-1), V 9.48 (9.54) [9.48], B1Vp
NGC 6611 469 (BD-13 4933, TYC 5689-0949-1), V 10.74 (10.63) [10.64], B0.5Vn
NGC 6611 503 (BD-13 4936, TYC 5689-1383-1), V 9.69 (9.76) [9.89], B0/5e
NGC 6611 536 (TYC 5689-1156-1), V 11.43 (11.42) [11.44], B1.5V

Young Stellar Objects

Prototype and pre-main-sequence stars are referred to as young stellar objects (YSOs). They are embedded in surrounding dust and gas which radiates much of their original luminous energies in the far infrared, particularly the former stars. Two references that I recommend from the scholarly and layman literatures are provided below.

References:

1. "Optical and infrared observations of young stellar objects - an informal review", S.E. Strom, Publications of the Astronomical Society of the Pacific, 84(502), December 1972, pp. 745-756.

2. "Deciphering the mysteries of stellar origins", Charles Lada, Sky & Telescope, Vol. 85 (5), May 1993, pp. 18-24.

Figure 4: 21 More NGC 6611 Cluster Members by Walker

Walker's measured V magnitudes for stars from Figures 4 and 5 are listed below. Also included are the magnitudes from the Simbad Database in parentheses (V) and SkySafari 7 Pro in brackets [V] for comparison. Stellar type is from the Simbad Database. Asterisk * means I can't find the star in SkySafari.

NGC 6611 207 (GAIA 4146599162597494528), V 12.14 (12.05) [12.01], B1Ve
NGC 6611 210 (GAIA 4146599162597495680), V 11.41 (11.47) [11.41], B1V
NGC 6611 231 (TYC 5689-0199-1), V 12.72 (11.78) [11.78], B1V
NGC 6611 254 (TYC 5689-0637-1), V 10.81 (11.00) [11.00], B1V
NGC 6611 296 (GAIA 4146600575638051712), V 11.80 (11.72) [11.73], B1.5V
NGC 6611 301 (GAIA 4146600674426037248), V 12.20 (12.16) [12.14], B2V
NGC 6611 311 (*), V 12.66 (12.74) [*], B2.5Ve
NGC 6611 323 (GAIA 4146600403839355392), V 13.45 (13.44) [13.42], B5Ve
NGC 6611 336 ((GAIA 4146600399547025280), V 13.28 (13.29) [13.23], B5III

Emission Line Stars

References:

1. "On the nature of early-type emission line objects in NGC 6611", C. Martayan, M. Floquet, A.-M. Hubert, J. Fabregat, Y. Frémat, D. Baade and C. Neiner, SF2A-2007: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics held in Grenoble, France, July 2-6, 2007, Eds.: J. Bouvier, A. Chalabaev, and C. Charbonnel, pp. 518-521.

2. "Early-type objects in NGC 6611 and the Eagle Nebula", C. Martayan, M. Floquet, A.-M. Hubert, C. Neiner, Y. Frémat, D. Baade and J. Fabregat, Astronomy & Astrophysics, 489, 459-480 (2008).

Figure 5: Last 11 NGC 6611 Cluster Members by Walker

Additional references on the creation of young stars through gravitational collapse of the surrounding gas and dust clouds are given here. Specific material devoted to the NGC 6611 cluster is noted with an asterisk *.

References:

* Universe, William J. Kaufmann, III, W.H. Freeman and Company, New York, 1985. Chapter 20: Birth of Stars, pp. 376-391.
* Astronomy Fundamentals and Frontiers, Robert Jastrow and Malcolm H. Thompson, John Wiley & Sons, New York, 1984. Chapter 8: Stellar Evolution, pp. 191-215.
"Star in the making", Charles J. Lada, Sky & Telescope, Vol. 72(4), October 1986, pp. 334-335.
"Deciphering the mysteries of stellar origins", Charles Lada, Sky & Telescope, Vol. 85(5), May 1993, pp. 18-24.
"Early phases of star formation", Bart J. Bok, Sky & Telescope, Vol. 61(4), April 1981, pp.284-293.

Links to the Simbad Database

There are 75 members of the open cluster NGC 6611 listed by M.F. Walker in his 1961 paper. Links to the Simbad Database for all of them are listed as action buttons at the bottom of this webpage.

A.A. Hoag et Al. (1961)

Figure 6. 18 Stars in the Hoag et al. Study

The data for about 7800 stars were presented as a catalogue of 70 clusters using identification charts and tables for each cluster. On page 432 you will find the chart and table for NGC 6611. There are 24 stars listed in the table and referenced to the chart using an XY Cartesian grid overlay. I was able to identify 18 of these stars. Four stars had coordinates lying outside the grid so were not accessible. The other two stars were located in a congested region so were not easily identified.

In the table below you will find the Hoag number (1-24) cross-referenced to the NGC 6611 identifiers, their V result, and the Walker V magnitudes in parentheses. Six of the Hoag stars were not included in Walker's photoelectric observations Table 1 (they are marked with an asterisk *). There is reasonable agreement between the two sets of measurements.

1. (NGC 6611 205), V 8.19 (8.24)
4. (NGC 6611 401), V 8.94 (8.95)
5. (NGC 6611 367), V 9.46 (9.43)
6. (NGC 6611 468), V 9.50 (9.48)
7. (NGC 6611 246), V 9.59 (9.60)
9. (NGC 6611 314), V 9.82 (9.86)
10. (NGC 6611 150), V 9.89 (9.88)
11. (NGC 6611 125), V 10.07 (10.05)
13. (NGC 6611 469), V 10.67 (10.74)
14. (NGC 6611 161), V 11.22 (11.30)
15. (NGC 6611 489), V 11.54 (*)
16. (NGC 6611 349), V 11.61 (11.62)
17. (NGC 6611 231), V 12.80 (12.72)
20. (NGC 6611 262), V 13.58 (*)
21. (NGC 6611 307), V 14.10 (*)
22. (NGC 6611 171), V 14.24 (*)
23. (NGC 6611 260), V 14.37 (*)
24. (NGC 6611 140), V 14.69 (*)

We can also compare the B-V and U-B measurements for the same stars with Walker's results in parentheses. An asterisk * denotes data not available. Note that there are systematic differences between the results of Walker and Hoag et al.

1. B-V 0.48 (0.44), U-B -0.61 (-0.76)
4. B-V 0.42 (0.39), U-B -0.64 (-0.73)
5. B-V 0.26 (0.24), U-B -0.67 (-0.76)
6. B-V 0.29 (0.27), U-B -0.65 (-0.69)
7. B-V 0.88 (0.86), U-B -0.24 (-0.34)
9. B-V 0.62 (0.58), U-B * (-0.55)
10. B-V 0.48 (0.47), U-B -0.48 (-0.54)
11. B-V 0.50 (0.46), U-B -0.48 (-0.55)
13. B-V 0.43 (0.40), U-B -0.58 (-0.62)
14. B-V 1.01 (1.06), U-B -0.12 (-0.23)
16. B-V 1.40 (1.42), U-B 1.05 (0.97)
17. B-V 0.76 (0.75), U-B -0.21 (-0.32)

Affiliation and References

"Photometry of stars in galactic cluster fields", A.A. Hoag, H.L. Johnson, B. Iriarte, R.I Mitchell, K.L. Hallam, and S. Sharpless, Publications of the U.S. Naval Observatory. 2d ser., Vol. 17(7), Washington, U.S. Govt. Print. Off., 1961, pp. 344-542.

The data for cluster NGC 6611 are from observations made using UBV photoelectric and photographic photometry coordinated by the U.S. Naval and Lowell Observatories under contract with the Office of Naval Research NONR 2570(00). H.L. Johnson was the Principal Investigator.

As a side note, back in the early 2000s I too was a Principal Investigator for an ONR contract worth several million Canadian dollars. Fond memories of those years.

W.A. Hiltner and W.W. Morgan (1969)

Figure 7. 15 Stars in the Hiltner and Morgan Study

Table 1 in the Hiltner and Morgan paper includes V, (B-V) and (U-B) measurements, as well as spectral types. Walker's V magnitudes are shown in parentheses below. Again, as with the Hoag et al. measurements, there are systematic differences in their (B-V) and (U-B) measurements which can be found in Tables 1 of both papers if you are interested (see note below this table). The 15 stars in this work are all main sequence O and B types.

NGC 6611 125, V 10.04 (10.05), B1.5V
NGC 6611 150, V 9.85 (9.88), B0.5V
NGC 6611 166, V 10.35 (10.36), O9V
NGC 6611 175, V 10.08 (10.05), O6
NGC 6611 197, V 8.77 (8.77), O7
NGC 6611 205, V 8.24 (8.24), O5
NGC 6611 210, V 11.45 (11.41), B1.5V
NGC 6611 223, V 11.22 (11.22), B1V
NGC 6611 246, V 9.59 (9.60), O8f
NGC 6611 296, V 11.81 (11.80), B2V
NGC 6611 314, V 9.96 (9.86), O9.5V
NGC 6611 351, V 11.25 (11.24), B1Vne
NGC 6611 367, V 9.44 (9.43), O9.5V
NGC 6611 401, V 8.96 (8.95), O8V
NGC 6611 469, V 10.73 (10.74), B1.5V

Note:

Hiltner and Morgan state that the V magnitude agreement with Walker's results are good, however, "... there is a systematic difference in the values of (B-V) and (U-B) of 0.03 and 0.10, respectively, in the sense that Walker's observations are bluer than those presented here".

Affiliation and References

"UBV Photometry and Spectral Types in NGC 6611", W.A. Hilton and W.W. Morgan, The Astronomical Journal, 74(10), 1969, p. 11.52.

The authors from Yerkes Observatory used the McDonald 82-inch reflector to carry out UBV photometry measurements of 15 stars from Table 1 on page 439 of the Walker paper, "Studies of extremely young clusters. IV. NGC 6611", Merle F. Walker, Astrophys. J., 133, 438-456, (1961). Stellar types for these stars are also listed by Hiltner and Morgan.

L.W. Kamp (1974)

Figure 8. 27 Stars Discussed by Kamp (See Notes Below)

Based on proper motions, Kamp determined that more than 40 stars were cluster members (>50% probabilities) or possible cluster members (10%-50% probabilities). Kamp used Walker's numbering system 1-550, and extended it from 551 to 617. I have listed the higher probability group of stars in the table below. Memberships that agree with Walker are marked with a bold W. Twelve new stars with high membership probabilities based on Kamp's proper motion work are shown on my Origin field with blue labels.

125 (BD-13 4920), P=94% W
150 (BD-13 4921), P=71% W
161 (TYC 5689-0663-1), P=85% W
166 (TYC 5689-0998-1), P=94% W
175 (BD-13 4923), P=92% W
207 (GAIA 4146599162597494528), P=92% W
210 (GAIA 4146599162597495680), P=90% W
223 (TYC 5689-1381-1), P=91% W
227 (), P=92% W
235 (TYC 5689-1411-1), P=78% W
239 (TYC 5689-1324-1), P=88% W
246 (BD-13 4927), P=94% W
254 (TYC 5689-0637-1), P=88% W
259 (TYC 5689-1658-1), P=70% W
280 (BD-13 4928), P=81% W
289 (GAIA 4146598887718503168), P=90% W
296 (GAIA 4146600575638051712), P=57% W
306 (GAIA 4146600777505252608), P=82% W
314 (BD-13 4929), P=89% W
343 (TYC 5689-0133-1), P=83% W
351 (TYC 5689-0433-1), P=93% W
367 (BD-13 4930), P=90% W
411 (NGC 6611 411), P=85%
468 (BD-13 4934), P=89% W
469 (BD-13 4933), P=91% W
483 (BD-13 4935), P=93%
489 (TYC 5689-0125-1), P=91%
500 (TYC 5689-0367-1), P=82%
503 (BD-13 4936), P=91% W
525 (GAIA 4152605111724507648), P=75%
534 (GAIA 4146596070214362368), P=88%
536 (TYC 5689-1156-1), P=92% W
559 (TYC 5689-0825-1), P=83%
587 (GAIA 4146592672898071424), P=92%
599 (TYC 5689-0333-1), P=83%
606 (TYC 5689-0675-1), P=92%
614 (GAIA 4152607821841021952), P=89%
617 (NGC 6611 617), P=73%

Fifteen membership stars determined by Walker, which had very low Kamp membership probabilities, are listed below and labeled in the chart using yellow labels.

90 (TYC 5689-1467-1), P=1%
103 (NGC 6611 103), P=0%
222 (NGC 6611 222), P=0%
231 (TYC 5689-0199-1), P=0%
297 (GAIA 4146600777505249408), P=0%
300 (GAIA 4146600575638052608), P=0%
311 (NGC 6611 311), P=0%
349 (GAIA 4146600403839348096), P=0%
400 (GAIA 4146600472558819200), P=0%
402 (GAIA 4146600919235422720), P=0%
409 (TYC 5689-0337-1), P=0%
455 (GAIA 4146600090310560896), P=0%
472 (GAIA 4146601022314634368), P=0%
504 (GAIA 4146599746707122688), P=0%
520 (GAIA 4146596035854624256), P=7%

Affiliation and References

"Proper motions, cluster membership and reddening in NGC 6611", L.W. Kamp, Astron. Astrophys. Suppl. 16, (1974), pp. 1-23.

Using plates from the Yerkes 40-inch refracting telescope, Kemp estimated cluster membership probabilities based on proper motion. Three plates were used and dated 10 August 1922, 23 June 1968, and 14 July 1969 - a span of over 45 years.

Burnham's Celestial Handbook (1978)

Figure 9. 24 Stars Discussed by Burnham

His table on page 1784 includes the visual magnitudes and spectral types of 24 bright stars in the cluster NGC 6611 (not 25 as his body text says on page 1790). In the table below I include the Walker NGC 6611 XXX designation beside his numbers 1-24, which refer to his finder charts on page 1785. He notes that his star number 2 (Walker 206) is a foreground star (see Figure 2 above).

(205), V 8.24, O5e
(206), V 8.27, K0
(412), V 8.30, B0+O8
(197), V 8.77, O7
(401), V 8.95, O8
(367), V 9.43, O9
(468), V 9.48, B1
(246), V 9.60, O8
(503), V 9.69, B0+B5
(314), V 9.86, O9
(150), V 9.88, B1:
(280), V 9.94, B0
(125), V 10.05, B1
(175), V 10.05, O6
(166), V 10.36, O9
(469), V 10.74, B1
(254), V 10.81, B2
(235), V 11.06, O8?
(223), V 11.22, B1
(351), V 11.24, B1
(161), V 11.30, O8+O5
(210), V 11.41, O9
(259), V 11.60, B0:
(349), V 11.62, K2:

Affiliation and References

Burnham's Celestial Handbook, Volume Three: Pavo Through Vulpecula, Robert Burnham, Jr., Dover Publications, Inc., New York, 1978, pp. 1782-1792.

Robert Burnham was a staff member at the Lowell Observatory in Flagstaff, Arizona. His notes on M16 and NGC 6611 summarize research prior to the volume's publication date. Included are two images of M16 in blue and red light using the Lowell 13-inch telescope (p. 1782). He references both Walker (1961) and Hoag et al. (1961) in his discussion.

H.-J. Tucholke Et Al. (1986)

Figure 10. 25 High Probability Membership Stars

Fifteen plates were used in the Tucholke et al. proper motion study: 4 from 1920, 2 from 1962, 4 from 1968, 1 from 1969, 1 from 1971, and three new ones dated June 27 and July 7, 1984. Twenty-two stars in this work did not have a prior designation so the authors extended the numbers used by Walker and Kamp from NGC 6611 618 to NGC 6611 639. Note that the authors state there are 21 new stars in the main text, however, their Figure 1 finder chart on page 320 shows 22 stars.

At this point in the history of NGC 6611 cluster membership there have been four studies: Walker (1961), van Schewick (1962), Kamp (1974) and Tucholke (1986). I do not have access to the van Schewick paper, so, below is a table summarizing some of the membership conclusions from the other three studies.

Stars with membership agreement from all three research groups are listed below. I have arbitrarily chosen 50% probability or better for the two proper motion studies. The first number is the NGC 6611 id assigned by Walker (1-550), then Kamp in % and Tucholke et al. in %. There are 25 stars with high probability of cluster membership. These stars are labeled in orange in the Origin field above.

125, 94%, 94%
150, 71%, 93%
161, 85%, 94%
166, 94%, 94%
175, 92%, 94%
207, 92%, 88%
223, 91%, 94%
227, 92%, 92%
235, 78%, 94%
239, 88%, 94%
246, 94%, 94%
254, 88%, 94%
259, 70%, 94%
280, 81%, 94%
289, 90%, 91%
296, 57%, 94%
306, 82%, 89%
314, 89%, 92%
343, 83%, 93%
351, 93%, 94%
367, 90%, 92%
468, 89%, 93%
469, 91%, 94%
503, 91%, 94%
536, 92%, 94%

Affiliation and References

"The proper motion and membership of stars in the very young open cluster NGC 6611", H.-J. Tucholke, M. Geffert, and P.S. Thé, Astron. Astrophys. Suppl. Ser. 66, 311-322 (1986).

This paper was a collaborative effort by researchers at the Astronomisches Institut der Westfälischen Wilhelms-Universität, Federal Republic of Germany, Observatories Hoher List der Universitätssternwarte Bonn, Federal Republic of Germany, and Astronomical Institute, University of Amsterdam, The Netherlands, respectively.

The research used plates from the 30-cm double refractor at the Hoher List Observatory in Bonn. These plates covered three epochs spanning 64 years: 1920, 1962-1971, and 1984. Of the 233 stars selected for study, 113 were considered members of the open cluster NGC 6611.

This work addressed cluster membership discrepancies between the van Schewick proper motion study (not covered here - see reference below) and Kamp's proper motion conclusions. The telescope used by van Schewick was also used for the new 1984 measurements in this work.

Reference:

H. van Schewick, Veröff. Univ. Sternw. Bonn, 62 (1962).

Part 1: Summary 1961-1986

NGC 6611 Cluster Membership Research 1961-1986

During this period there were four independent research groups investigating NGC 6611 membership using photoelectric and proper motion techniques. The NGC 6611 identifiers in the Simbad Database were assigned by Walker (1961), Kamp (1974), and Tucholke, Geffert and Thé (1986): 1-550, 551-617, and 618-639, respectively. I will refer to this numbering scheme as WKT. Each study determined the following number of members: 75 (Walker), 35 (Kamp), and 113 (Tucholke et al.). I did not have access to H. van Schewick's paper (1962). All three groups agree on 25 stars with a high probability of membership. These stars are marked with asterisks (*) on the Simbad action buttons at the bottom of this webpage.

M. Stoop et Al. (2023)

Figure 11. 12 NGC 6611 Cluster Members (V Mag 8.500-10.999)

Stoop et al. list 146 member stars using GAIA Early Data Release 3 (EDR3) id numbers in their Appendix C table. EDR3 includes photometry and astrometry data for 1.8 billion objects down to magnitude 21 (see Ref. 1 below). Also included in the table, of interest to this webpage's scope, are spectral types, V magnitudes, color index (B-V) , and the color excess E(B-V). The data in Stoop's table comes from several sources (Refs. 2-7 below). Walker numbers (1-550) are given as Wxxx, Kamp numbers (551-617) as Kxxx, and Tucholke et al. (618-639) as Txxx. For Walker members, an asterisk has been added to the Simbad action buttons at the bottom of this webpage.

GAIA 4146599166888812032, W197, V 8.752, (B-V) 0.54, E(B-V) 0.86, O6.5V((f))+B0-1V, YSO
GAIA 4146599682284818176, W401, V 8.942, (B-V) 0.41, O7V+O8V, YSO
GAIA 4146599476126383872, W367, V 9.368, (B-V) 0.34, E(B-V) 0.65, O9.5Vp, YSO
GAIA 4146594386587183360, W468, V 9.396, (B-V) 0.42, E(B-V) 0.7, B0.5V+B1:, YSO
GAIA 4146598995090108288, W150, V 9.687, (B-V) 0.88, E(B-V) 1.17, B0.5V, YSO
GAIA 4146600781797073920, W314, V 9.803, (B-V) 0.71, O7V+(B0.5V+B0.5V), YSO
GAIA 4146604939324925440, K556, V 9.902, (B-V) 1.41, E(B-V) 1.6, B2.5I, YSO
GAIA 4146612670266023040, W175, V 10.007, (B-V) 0.98, O4V((f+))+O7.5V, YSO
GAIA 4146600678717290624, W280, V 10.044, (B-V) 0.56, E(B-V) 0.87, O9.5V, Be
GAIA 4146599029451913984, W166, V 10.296, (B-V) 0.73, E(B-V) 1.04, O9V, YSO
GAIA 4146599819725862912, W469, V 10.628, (B-V) 0.47, E(B-V) 0.76, B0.5Vn, Be
GAIA 4146594906281066368, K601, V 10.733, (B-V) 0.43, E(B-V) 0.68, B1.5V, YSO

Reference

"GAIA Early Data Release 3", GAIA Collaboration, Astronomy & Astrophysics, 649, A1 (2021).
"NGC 6611: A cluster caught in the act", L.A. Hillenbrand, P. Massey, S.E. Strom, and K.M. Merrill, The Astronomical Journal, 106(5), (1993), 1906-1946, 2188-2189.
"The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755, and NGC 6611", C.J. Evans et al., Astronomy & Astrophysics, 437, 467-482 (2005).
"Early-type objects in NGC 6611 and the Eagle Nebula", C. Martian et al., Astronomy & Astrophysics, 489, 459-480 (2008).
"Search for OB stars running away from young star clusters I. NGC 6611", V.V. Gvaramadze and D.J. Bomons, Astronomy & Astrophysics, 490, 1071-1077 (2008).
"The massive star binary fraction in Young open clusters - II. NGC 6611 (Eagle Nebula)", H. Sana, E. Gosset, and C.J. Evans, Monthly Notices of the Royal Astronomical Society, 400, 1479-1492 (2009).
"The Galactic O-star spectroscopic survey (GOSSS). III. 142 additional O-type systems", J. Maíz Apellániz et al., The Astrophysical Journal Supplement Series, 224:4 (42pp), May 2016.

Affiliations and References

"The early evolution of young massive clusters: The kinematic history of NGC 6611/M16", M. Stoop, L. Kaper, A. de Koter, D. Guo, H.J.G.L.M. Lamers, and S. Rieder, Astronomy & Astrophysics, 670, A108 (2023).

The authors are from the following institutions: Anton Pannekoek Institute for Astronomy - University of Amsterdam, Institute of Astronomy - KU Lauven, Geneva Observatory - University of Geneva, School of Physics and Astronomy - University of Exeter, respectively.

M. Stoop et Al. (2023)

Figure 12. 23 NGC 6611 Cluster Members (V Mag 11.000-12.999)

Walker numbers (1-550) are given as Wxxx, Kamp numbers (551-617) as Kxxx, and Tucholke et al. (618-639) as Txxx. For Walker members, an asterisk has been added to the Simbad action buttons at the bottom of this webpage.

GAIA 4146612494169472256, W246, V 11.114, (B-V) 0.67, O7II(f), YSO
GAIA 4146612395388089344, W223, V 11.122, (B-V) 0.71, E(B-V) 0.99, B1V, YSO
GAIA 4146613632338722432, W161, V 11.215, (B-V) 1.24, E(B-V) 1.55, O8.5V, YSO
GAIA 4146600330822092160, W351, V 11.217, (B-V) 0.46, E(B-V) 0.74, B1V, Emission-line star
GAIA 4146599162597495680, W210, V 11.407, (B-V) 0.61, E(B-V) 0.88, B1V, YSO
GAIA 4146594803201839744, W536, V 11.436, (B-V) 0.31, E(B-V) 0.56, B1.5V
GAIA 4146597105304415488, W239, V 11.474, (B-V) 0.5, E(B-V) 0.76, B1.5V, YSO
GAIA 4146600575638051712, W296, V 11.718, (B-V) 0.63, E(B-V) 0.88, B1.5V, YSO
GAIA 4146600674426016640, W275, V 12.037, (B-V) 0.59, E(B-V) 0.87, B1V
GAIA 4146617819936287488, K584, V 12.052, (B-V) 1.24, E(B-V) 1.55, O9V, YSO
GAIA 4146593183996341120, K597, V 12.262, (B-V) 0.39, E(B-V) 0.61, B2V, YSO
GAIA 4146598887718503168, W289, V 12.529, (B-V) 0.65, E(B-V) 0.83, B3V, YSO
GAIA 4152606172568933504, W444, V 12.594, (B-V) 1.04, E(B-V) 1.29, B1.5V, YSO
GAIA 4146600575638052608, W300, V 12.607, (B-V) 0.67, E(B-V) 0.88, B2V, YSO
GAIA 4146600777505252608, W306, V 12.676, (B-V) 0.81, E(B-V) 1.09, B1V, YSO
GAIA 4146604076033711360, K555, V 12.705, (B-V) 0.84, YSO
GAIA 4146600640059737088, W227, V 12.753, (B-V) 0.73, E(B-V) 0.95, B2V, YSO
GAIA 4146600472558819200, W400, V 12.789, (B-V) 0.7, B9III, YSO
GAIA 4146594768842190592, W409, V 12.793, (B-V) 0.51, E(B-V) 0.71, B2.5V, MZ Ser
GAIA 4146600777505249408, W297, V 12.828, (B-V) 0.81, E(B-V) 1.03, B2Vn, YSO
GAIA 4146601022314634368, W472, V 12.834, (B-V) 0.61, E(B-V) 0.78, B3V+?, YSO
GAIA 4146598818993604480, W313, V 12.866, (B-V) 0.62, E(B-V) 0.78, B5III, YSO
GAIA 4146612872133107584, W222, V 12.968, (B-V) 1.52, E(B-V) 1.84, O7V((f)), YSO

Affiliations and References

M. Stoop et Al. (2023)

Figure 13. 19 NGC 6611 Cluster Members (V Mag 13.000-13.999)

GAIA 4146600678717295104, W267, V 13.038, (B-V) 0.64, E(B-V) 0.82, B3V, YSO
GAIA 4146612425456526208, W251, V 13.259, (B-V) 0.81, E(B-V) 1.02, B2V, YSO
GAIA 4152605214803721216, W541, V 13.285, (B-V) 0.64, E(B-V) 0.85, B1-3V, YSO
GAIA 4146600399547025280, W336, V 13.288, (B-V) 0.51, E(B-V) 0.67, B5III, YSO
GAIA 4146598685852419584, W238, V 13.325, (B-V) 0.79, E(B-V) 0.72, A2IV, YSO
GAIA 4146600541278311680, W374, V 13.347, (B-V) 1.06, Delta Sct Variable
GAIA 4146600536980521344, W371, V 13.379, (B-V) 0.69, E(B-V) 0.85, B4V, YSO
GAIA 4146600403839355392, W323, V 13.392, (B-V) 0.74, E(B-V) 0.9, B4V, YSO
GAIA 4146599544847962624, W364, V 13.403, (B-V) 0.58, E(B-V) 0.71, B7V, YSO
GAIA 4146612803413634688, W228, V 13.435, (B-V) 1.11, E(B-V) 1.32, B2V, YSO
GAIA 4146612528535774080, W305, V 13.443, (B-V) 1.31, E(B-V) 1.59, B1V, YSO
GAIA 4146600609998380800, W322, V 13.552, (B-V) 0.79, E(B-V) 1.59, B8V:e, YSO
GAIA 4146599579205603456, W388, V 13.666, (B-V) 0.62, E(B-V) 0.75, B4V, YSO
GAIA 4146600644357559680, W243, V 13.705, (B-V) 0.75, E(B-V) 0.91, B4V, YSO
GAIA 4146600674426021888, W281, V 13.733, (B-V) 0.68, E(B-V) 0.58, A3III-II, YSO
GAIA 4146598273535559936, W194, V 13.843, (B-V) 0.63, E(B-V) 0.7, B9, YSO
GAIA 4146598166158712576, W112, V 13.844, (B-V) 0.62, Ae, YSO
GAIA 4146610913628015872, NGC 6611 PPM 29, V 13.869, (B-V) 1.01, YSO
GAIA 4146599579205601920, W396, V 13.965, (B-V) 1.34, YSO

Affiliations and References

M. Stoop et Al. (2023)

Figure 14. 17 NGC 6611 Cluster Members (V Mag 14.000-14.999)

GAIA 4146600713077005312, W343, V 14.078, (B-V) 1.02, Eclipsing binary
GAIA 4146600609997796864, W307, V 14.178, (B-V) 1.1, E(B-V) 1.31, B2V, YSO
GAIA 4146600678717298304, W266, V 14.267, (B-V) 1.43, YSO
GAIA 4146612704625753600, W202, V 14.314, (B-V) 1.06, E(B-V) 1.19), B7V, YSO
GAIA 4146594871922064000, NGC 6611 MFH 17, V 14.322, (B-V) 0.62, E(B-V) 0.58, A1V, YSO
GAIA 4146613907216640256, W151, V 14.445, (B-V) 1.69, E(B-V) 1.97, B1:V, YSO
GAIA 4146600335119862912, W360, V 14.53, (B-V) 1.36, YSO
... in progress

M16 and Open Cluster NGC 6611

Figure 1: Objects of Interest in Origin's Capture of the Eagle Nebula M16

Logbook

Cluster Membership By M.F. Walker (1961)

Figure 2: First Research Paper on NGC 6611 Cluster Membership

Bonner Durchmusterung Catalogue

Figure 3: 28 More NGC 6611 Cluster Members by Walker

Young Stellar Objects

Figure 4: 21 More NGC 6611 Cluster Members by Walker

Emission Line Stars

Figure 5: Last 11 NGC 6611 Cluster Members by Walker

Links to the Simbad Database

A.A. Hoag et Al. (1961)

Figure 6. 18 Stars in the Hoag et al. Study

Affiliation and References

W.A. Hiltner and W.W. Morgan (1969)

Figure 7. 15 Stars in the Hiltner and Morgan Study

Affiliation and References

L.W. Kamp (1974)

Figure 8. 27 Stars Discussed by Kamp (See Notes Below)

Affiliation and References

Burnham's Celestial Handbook (1978)

Figure 9. 24 Stars Discussed by Burnham

Affiliation and References

H.-J. Tucholke Et Al. (1986)

Figure 10. 25 High Probability Membership Stars

Affiliation and References

Part 1: Summary 1961-1986

NGC 6611 Cluster Membership Research 1961-1986

M. Stoop et Al. (2023)

Figure 11. 12 NGC 6611 Cluster Members (V Mag 8.500-10.999)

Affiliations and References

M. Stoop et Al. (2023)

Figure 12. 23 NGC 6611 Cluster Members (V Mag 11.000-12.999)

Affiliations and References

M. Stoop et Al. (2023)

Figure 13. 19 NGC 6611 Cluster Members (V Mag 13.000-13.999)

Affiliations and References

M. Stoop et Al. (2023)

Figure 14. 17 NGC 6611 Cluster Members (V Mag 14.000-14.999)

Affiliations and References

NGC 6611 Member Links to The Simbad Database

There are 75 NGC 6611 member stars identified by M.F. Walker

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