NGC 7419 As a Template for Red Supergiant Clusters⋆⋆⋆⋆⋆⋆
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A&A 552, A92 (2013) Astronomy DOI: 10.1051/0004-6361/201220750 & c ESO 2013 Astrophysics NGC 7419 as a template for red supergiant clusters,, A. Marco and I. Negueruela Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal. Escuela Politécnica Superior, University of Alicante, Apdo. 99, 03080 Alicante, Spain e-mail: [email protected] Received 16 November 2012 / Accepted 14 February 2013 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 Strömgren 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 10 000 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 clusters and associations: individual: NGC 7419 – Hertzsprung-Russell and C-M diagrams – stars: early-type – stars: emission-line, Be – supergiants 1. Introduction of Maeder (1990). They obtained a distance of 2.3 kpc, an age of 14 ± 2 Myr, and a mass function that satisfies the Salpeter form. Open clusters are excellent laboratories for the study of stellar Their photometry was not calibrated with standard stars, but evolution, where we can find rare evolutionary phases in their with the photometric sequence of the nearby cluster NGC 7510. natural environment. In addition, most massive stars are found Turner et al. (1983) had noted that the same procedure had led to in open clusters, because their short lifetimes prevent them from large systematic differences in (U − B) in the study of the nearby drifting into the field. Unfortunately, the most massive (and in- open cluster Mrk 50. teresting) clusters are usually affected by very heavy foreground extinction, which makes it difficult and very expensive in tele- More recently, Subramanian et al. (2006)alsopresented UBV ± scope time to study them. A very detailed study of accessible photometry of the cluster, finding an age of 25 5Myr open clusters can provide us with valuable information to inter- and a distance of 2.9 kpc. Joshi et al. (2008) estimated an age of 23 ± 3 Myr and a distance of 3.3 kpc from UBVRI pho- pret the much poorer datasets available for very heavily reddened ff clusters. tometry. Unfortunately, there are very large systematic di er- ences between the photometric values reported in these three NGC 7419 is an open cluster located in Cepheus, near the works. Joshi et al. (2008)andSubramanian et al. (2006)present l = ◦ b = ◦. Galactic plane ( 109 , 1 14). Its spatial extent is very substantial differences in (U − B)and(B − V) with respect to well defined, with a strong concentration towards the centre and Beauchamp et al. (1994), which may have some dependence little foreground contamination (Joshi et al. 2008). Beauchamp on the observed colours. There are, moreover, important differ- et al. (1994) estimated its distance, age, and mass function, based ences between Joshi et al. (2008)andSubramanian et al. (2006), UBV on CCD photometry and a comparison with the isochrones which do not seem to depend on the observed colours, but reach ∼ . − 0 2magin(U B) for many stars. The reasons for these dif- Partially based on observations collected at the Nordic Optical ferences are unclear. They may be related to the high reddening Telescope and the William Herschel Telescope (La Palma) and at the towards the cluster, perhaps combined with the choice of cali- 1.93-m telescope at Observatoire de Haute Provence (CNRS), France. bration. Indeed, of the three works, Joshi et al. (2008) is the only Tables 1 and 2 are available in electronic form at http://www.aanda.org one to calibrate the photometry with Landolt standards. Tables 3, 4 and 7 are only available at the CDS via anonymous ftp Based on photometric searching techniques, Pigulski & to cdsarc.u-strasbg.fr (130.79.128.5)orvia Kopacki (2000) determined the fraction of Be stars to total http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/552/A92 number of B-type stars to be 36 ± 7% among cluster members Article published by EDP Sciences A92, page 1 of 16 A&A 552, A92 (2013) brighter than RC = 16.1 mag. Such a high fraction of Be stars over the past ten years. We began in 2001 by taking slit spectra places NGC 7419 in the small group of clusters very rich in of stars catalogued as Be by Pigulski & Kopacki (2000) with the Be stars, such as NGC 663 in the Milky Way, NGC 330 in 1.93-m telescope at the Haute Provence Observatory (France) the Small Magellanic Cloud (SMC), and NGC 1818A in the and the Carelec spectrograph. In 2004, we had an observation Large Magellanic Cloud (LMC). This high fraction of Be stars run to take photometry and spectroscopy with the 2.6-m Nordic has been confirmed by Subramanian et al. (2006) using slitless Optical Telescope (NOT, La Palma, Spain). The telescope was spectroscopy and by Joshi et al. (2008)usingHα photometry. equipped with the imager and spectrograph ALFOSC. On this Malchenko & Tarasov (2011) obtained intermediate-resolution occasion, the weather conditions were not photometric and we spectroscopy around the Hα line for most of the candidate could only take slit spectra of the brightest stars and slitless im- Be stars, confirming their nature. ages to detect emission-line stars. In 2005, we had another run at A second peculiarity of NGC 7419 is the presence of five the same telescope. On that occasion, weather conditions were red supergiants (RSGs) as confirmed radial velocity members of excellent and we could take photometric observations. In addi- the cluster (Beauchamp et al. 1994). Until recently, this was the tion, we took more slit spectra of bright members. Spectra of the highest number of RSGs in a Galactic cluster1. Beauchamp et al. red supergiant members of the cluster were taken in 2007 and (1994)andCaron et al. (2003) noticed that this high number of 2009 with the William Herschel Telescope and ISIS. In 2008 and RSGs was not accompanied by any blue supergiant. This pro- 2011, we obtained slitless images again to detect emission-line portion is at odds with what is generally found in Galactic open stars with the NOT, and in 2010 we took a few more slit spec- clusters, where blue supergiants outnumber RSGs by a factor ∼3 tra. With this large collection of data, we are able to confirm the (Eggenberger et al. 2002). extremely high fraction of Be stars, find evidence of variability In recent years, several clusters of RSGs have been discov- in many of them, and determine spectral types for the brightest ered towards the Scutum Arm tangent (Figer et al. 2006; Davies members. 2007; Clark et al. 2009a). These clusters are so heavily ob- scured that their population of intrinsically blue stars have not been found yet, and only the RSGs have been observed as bright 2.1. Photometry sources in the near-infrared. Population synthesis models indi- We obtained Strömgren photometry of the open cluster cate that, given the rarity of RSGs, the clusters must contain very NGC 7419 using ALFOSC on the Nordic Optical Telescope at large stellar populations. Simulations by Clark et al. (2009b)pre- the Roque de los Muchachos Observatory (La Palma, Spain) on dict a stellar mass of about 104 for each 1–3 RSGs at 10 Myr and − ff the night of October 3, 2005. ALFOSC allows observations in 3 5 RSGs at 20 Myr. NGC 7419 o ers the opportunity of study- different modes. In imaging mode the camera covers a field of ing a cluster with a moderately high number of RSGs whose 6.5 × 6.5 and has a pixel scale of 0. 19/pixel. For each frame, population of B-type stars can be characterised in detail. we obtained two series of different exposure times in each fil- In this paper, we determine the fundamental parameters ff ter to achieve accurate photometry for a broad magnitude range. of NGC 7419 using techniques di erent and more accurate The central position for the cluster and the exposure times are than those of previous authors: Strömgren photometry and presented in Table 1.