HM 1

HM 1, also known as Havlen-Moffat 1, is an open cluster located in the constellation of Scorpius, close to the galactic plane. It was first observed by R. J. Havlen and A. F. J. Moffat in 1976.[5] HM 1 is thought to be 9,500 to 12,700 light-years (2,900 to 3,900 parsecs) away from the Earth, beyond the Carina–Sagittarius Arm.[2] It is heavily reddened by interstellar extinction, so although it comprises mostly blue-colored stars, it appears brighter for longer-wavelength passbands.[2] It is projected against the H II region known as RCW 121, and appears to be the source of ionization for the nearby regions RCW 122 and RCW 123.[2]

HM 1 (Havlen-Moffat 1)
Approximate location of HM 1 (circled)
Observation data (J2000 epoch)
Constellation	Scorpius
Right ascension	17^h 18^m 54^s[1]
Declination	−38° 49′ 01″[1]
Distance	9,500–12,700 ly (2,900–3,900 pc[2])
Apparent dimensions (V)	5′[3]
Physical characteristics
Estimated age	1-2 Myr[4] 2-4 Myr[2]
Notable features	Star cluster rich in massive stars
Other designations	C 1715-387

Properties

The extinction value for HM 1 is calculated to be E_B−V = 1.85 magnitudes, and its distance was first estimated to be around 2.9 ± 0.4 kiloparsecs away.[5] Later estimates put the cluster at around 3.3 kiloparsecs away; this is still one of the more closer massive star clusters.[2]

HM 1 is fairly young for an open cluster; it is estimated to be 1 to 2, or 2 to 4 million years old.[6] This is indicated by the presence of Of stars, which have relatively short lives. However, a red supergiant with a bolometric magnitude of −6.6 and a mass of about 20 M_☉ was discovered near the cluster, contradicting the supposed age of the cluster.[7] However, subsequent work showed the star is likely in the foreground.[2]

Composition

HM 1 is noted for its components, including several Wolf–Rayet stars and Of stars. Both types of stars are fairly rare and very massive, and their coexistence in star clusters was unexpected. The evolution of Wolf–Rayet stars and Of stars in star clusters is currently unclear. Very few Wolf–Rayet stars have been found in star clusters, and a possible explanation is that the formation of these cluster Wolf–Rayet stars requires a binary star system containing two Of stars. Alternatively, cluster Wolf–Rayet stars may simply be rare because they represent a short stage in the evolution of Of stars, whether single or binary.[5]

Many of the stellar components are extremely blue-colored O-type stars that are among some of the most luminous stars known. Some of the candidate members listed have masses of over 20 M_☉, making this cluster one of the richest known.[6] Examples include the peculiar star LSS 4067, an O-type supergiant with a spectral type of O4If+.[4] WR 89, another luminous star, is a strong X-ray source and is possibly a colliding-wind binary, based on data from XMM-Newton.[6] In contrast, WR 87 has a high plasma temperature consistent with that of a colliding-wind binary but otherwise appears to be a single star (similar to WR 136).[4]

Properties of HM 1 cluster members

Name[note 1]	Teff	MV	Mass (M☉)	Spectral type	Ref.
HM 1 1 (WR 89)	39800	−7.25	44	WN8h	[8]
HM 1 4 (LSS 4067)	47800	−7.0	120	O4 If+	[4]
HM 1 6 (WR 87)	44800	−6.5	95	WN7h	[4]
HM 1 8	46100	−5.7	68	O5 V	[4]
HM 1 9	37900	−5.3	38		[4]
HM 1 12	41900	−5.5	50	O6 If	[4]
HM 1 13	41000	−5.3	44	O7 V((f))	[4]
HM 1 16	33100	−5.4	32		[4]
HM 1 18	38100	−4.4	29		[4]
HM 1 19	39300	−4.1	28		[4]
HM 1 20	34700	−4.1	22	O9.5 V:	[4]

See also

• Cygnus OB2, an OB association containing many massive stars

References

1. Kharchenko, N. V.; Piskunov, A. E.; Schilbach, E.; Röser, S.; Scholz, R.-D. (2013). "Global survey of star clusters in the Milky Way". Astronomy & Astrophysics. 558: A53. arXiv:1308.5822. Bibcode:2013A&A...558A..53K. doi:10.1051/0004-6361/201322302.
2. Vázquez, R. A.; Baume, G. (2001). "The open cluster Havlen-Moffat No. 1 revisited". Astronomy and Astrophysics. 371 (3): 908–920. Bibcode:2001A&A...371..908V. doi:10.1051/0004-6361:20010410.
3. Morales, Esteban F. E.; Wyrowski, Friedrich; Schuller, Frederic; Menten, Karl M. (2013). "Stellar clusters in the inner Galaxy and their correlation with cold dust emission". Astronomy & Astrophysics. 560: A76. arXiv:1310.2612. Bibcode:2013A&A...560A..76M. doi:10.1051/0004-6361/201321626.
4. Massey, P.; Degioia-Eastwood, K.; Waterhouse, E. (2001). "The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations". The Astronomical Journal. 121 (2): 1050–1070. arXiv:astro-ph/0010654. Bibcode:2001AJ....121.1050M. doi:10.1086/318769.
5. Havlen, R. J.; Moffat, A. F. J. (1977). "A new cluster containing 2 Wolf-Rayet-stars and 2 Of-stars". Astronomy and Astrophysics. 58 (3): 351–356. Bibcode:1977A&A....58..351H.
6. Nazé, Y.; Rauw, G.; Sana, H.; Corcoran, M. F. (2013). "X-ray properties of the young open clusters HM1 and IC 2944/2948". Astronomy & Astrophysics. 555: A83. arXiv:1305.5105. Bibcode:2013A&A...555A..83N. doi:10.1051/0004-6361/201321099.
7. The, P. S.; Arens, M.; van der Hucht, K. A. (1982). "An investigation of the Scorpius open cluster C1715-387 containing two WN7, two Of and one red supergiant members". Astrophysical Letters. 22: 109. Bibcode:1982ApL....22..109T.
8. Hamann, W.-R.; Gräfener, G.; Liermann, A. (2006). "The Galactic WN stars. Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation". Astronomy and Astrophysics. 457 (3): 1015–1031. arXiv:astro-ph/0608078. Bibcode:2006A&A...457.1015H. doi:10.1051/0004-6361:20065052.

Notes

1. These numbers are not the same as the ones used in SIMBAD. For example, HM 1 6 (WR 87) is [NRS2013] HM1 6, not Cl HM 1 6.