NGC 7419 As a Template for Red Supergiant Clusters
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Astronomy & Astrophysics manuscript no. amarco7419˙v4˙astroph c ESO 2018 October 30, 2018 NGC 7419 as a template for red supergiant clusters⋆ A. Marco1 and I. Negueruela1 Departamento de F´ısica, Ingenier´ıa de Sistemas y Teor´ıa de la Se˜nal. Escuela Polit´ecnica Superior. University of Alicante. Apdo.99 E-03080. Alicante. (Spain) e-mail: [email protected] Received; accepted ABSTRACT Context. The open cluster NGC 7419 is known to contain five red supergiants and a very high number of Be stars. However, there are conflicting reports about its age and distance that prevent a useful comparison with other clusters. Aims. We intend to obtain more accurate parameters for NGC 7419, using techniques different from those of previous authors, so that it may be used as a calibrator for more obscured clusters. Methods. We obtained Str¨omgren photometry of the open cluster NGC 7419, as well as classification spectroscopy of ∼ 20 stars in the area. We then applied standard analysis and classification techniques. Results. We find a distance of 4 ± 0.4 kpc and an age of 14 ± 2 Myr for NGC 7419. The main-sequence turn-off is found at spectral type B1, in excellent agreement. We identify 179 B-type members, implying that there are more than 1200 M⊙ in B stars at present. Extrapolating this to lower masses indicates an initial cluster mass of between 7000 and 10000 M⊙, depending on the shape of the Initial Mass Function. We find a very high fraction (≈ 40%) of Be stars around the turn-off, but very few Be stars at lower masses. We also report for the first time a strong variability in the emission characteristics of Be stars. We verified that the parameters of the red supergiant members can be used to obtain accurate cluster parameters. Conclusions. NGC 7419 is sufficiently massive to serve as a testbed for theoretical predictions and as a template to compare more obscured clusters. The distribution of stars above the main-sequence turn-off is difficult to accommodate with current evolutionary tracks. Though the presence of five red supergiants is marginally consistent with theoretical expectations, the high number of Be stars and very low number of luminous evolved B stars hint at some unknown physical factor that is not considered in current synthesis models. Key words. open cluster and associations: individual: NGC 7419 - stars: Hertzsprung-Russell (HR) and C-M diagrams, early-type, evolution, emission-line, Be, supergiant - techniques:photometric and spectroscopic 1. Introduction large systematic differences in (U − B) in the study of the nearby open cluster Mrk 50. Open clusters are excellent laboratories for the study of stellar More recently, Subramanian et al. (2006) also presented evolution, where we can find rare evolutionary phases in their UBV photometry of the cluster, finding an age of 25 ± 5 Myr natural environment. In addition, most massive stars are found and a distance of 2.9 kpc. Joshi et al. (2008) estimated, from in open clusters, because their short lifetimes prevent them from UBVRI photometry, an age of 23 ± 3 Myr and a distance of drifting into the field. Unfortunately, the most massive (and in- 3.3 kpc. Unfortunately, there are very large systematic differ- ff teresting) clusters are usually a ected by very heavy foreground ences between the photometric values reported in these three ffi extinction, makes it di cult and very expensive in telescope works. Joshi et al. (2008) and Subramanian et al. (2006) present time to study them. Very detailed study of accessible open clus- substantial differences in (U − B) and (B − V) with respect ters can provide us with valuable information to interpret the to Beauchamp et al. (1994), which may have some dependence much poorer datasets available for very heavily reddened clus- on the observed colours. There are, moreover, important differ- ters. ences between Joshi et al. (2008) and Subramanian et al. (2006), arXiv:1302.5649v1 [astro-ph.SR] 22 Feb 2013 NGC 7419 is an open cluster located in Cepheus, near the which do not seem to depend on the observed colours, but reach = ◦ = ◦. Galactic plane (l 109 , b 1 14). Its spatial extent is very well ∼ 0.2 magin (U − B) for many stars. The reasons for these dif- defined, with a strong concentration towards the centre and little ferences are unclear. They may be related to the high reddening foreground contamination (Joshi et al. 2008). Beauchamp et al. towards the cluster, perhaps combined with the choice of cali- (1994) estimated its distance, age, and mass function, based on bration. Indeed, of the three works, Joshi et al. (2008) is the only UBV CCD photometry and a comparison with the isochrones of one to calibrate the photometry with Landolt standards. Maeder (1990). They obtained a distance of 2.3 kpc, an age of Based on photometric searching techniques, 14 ± 2 Myr, and a mass function that satisfies the Salpeter form. Pigulski& Kopacki (2000) determined the fraction of Be Their photometry was not calibrated with standard stars, but stars to total number of B-type stars to be 36±7% among cluster with the photometric sequence of the nearby cluster NGC 7510. members brighter than R = 16.1 mag. Such a high fraction of Turner et al. (1983) had noted that the same procedurehad led to C Be stars places NGC 7419 in the small group of clusters very ⋆ Partially based on observations collected at the Nordic Optical rich in Be stars, such as NGC 663 in the Milky Way, NGC 330 Telescope and the William Herschel Telescope (La Palma) and at the in the Small Magellanic Cloud (SMC), and NGC 1818A in the 1.93-m telescope at Observatoire de Haute Provence (CNRS), France. Large Magellanic Cloud (LMC). This high fraction of Be stars 1 A. Marco and I. Negueruela: NGC 7419 and red supergiants has been confirmed by Subramanian et al. (2006) using slitless run to take photometry and spectroscopy with the 2.6-m Nordic spectroscopy and by Joshi et al. (2008) using Hα photometry. Optical Telescope (NOT, La Palma, Spain). The telescope was Malchenko & Tarasov (2011) obtained intermediate-resolution equipped with the imager and spectrograph ALFOSC. On this spectroscopy around the Hα line for most of the candidate Be occasion, the weather conditions were not photometric and we stars, confirming their nature. could only take slit spectra of the brightest stars and slitless im- A second peculiarity of NGC 7419 is the presence of ages to detect emission-line stars. In 2005, we had another run at five red supergiants (RSGs) as confirmed radial velocity mem- the same telescope. On that occasion, weather conditions were bers of the cluster (Beauchampet al. 1994). Until recently, excellent and we could take photometric observations. In addi- this was the highest number of RSGs in a Galactic cluster 1. tion, we took more slit spectra of bright members. Spectra of the Beauchamp et al. (1994) and Caron et al. (2003) noticed thatthis red supergiant members of the cluster were taken in 2007 and high number of RSGs was not accompanied by any blue super- 2009 with the William Herschel Telescope and ISIS. In 2008 and giant. This proportion is at odds with what is generally found in 2011, we obtained slitless images again to detect emission-line Galactic open clusters, where blue supergiants outnumber RSGs stars with the NOT, and in 2010 we took a few more slit spec- by a factor ∼ 3 (Eggenberger et al. 2002). tra. With this large collection of data, we are able to confirm the In recent years, several clusters of RSGs have been discov- extremely high fraction of Be stars, find evidence of variability ered towards the Scutum Arm tangent (Figer et al. 2006, Davies in many of them, and determine spectral types for the brightest 2007, Clark et al. 2009a). These clusters are so heavily obscured members. that their population of intrinsically blue stars have not been found yet, and only the RSGs have been observed as bright sources in the near-infrared. Population synthesis models in- 2.1. Photometry dicate that, given the rarity of RSGs, the clusters must con- tain very large stellar populations. Simulations by Clark et al. We obtained Str¨omgren photometry of the open cluster (2009b) predict a stellar mass of about 104 for each 1–3 RSGs NGC 7419 using ALFOSC on the Nordic Optical Telescope at at 10 Myr and 3–5 RSGs at 20 Myr. NGC 7419 offers the op- the Roque de los Muchachos Observatory (La Palma, Spain) on portunity of studying a cluster with a moderately high number the night of October 3, 2005. ALFOSC allows observations in of RSGs whose population of B-type stars can be characterised different modes. In imaging mode the camera covers a field of in detail. 6.′5 × 6.′5 and has a pixel scale of 0′′. 19/ pixel. For each frame, In this paper, we determine the fundamental parameters we obtained two series of different exposure times in each fil- of NGC 7419 using techniques different and more accurate ter to achieve accurate photometry for a broad magnitude range. than those of previous authors: Str¨omgren photometry and The central position for the cluster and the exposure times are intermediate-resolution spectroscopy in the classification region. presented in Table 1. In view of the very discrepant photometric measurements re- ported in previous studies, we have decided to try intermediate- Standard stars were observed the same night in the clusters band Str¨omgren photometry which, if correctly calibrated, may NGC 869, NGC 884, NGC 6910, and NGC 1502 using exposure allow easy individual dereddening of stellar colours and give ac- times suitable to obtain good photometric values for all stars se- curate photometric spectral types.