The star formation region NGC 6530: Distance, ages and initial mass function

Abstract

Abstract. We present astrometry and BV I photometry, down to V 22, of the very young open cluster NGC 6530, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the V vs. B − V and the V vs. V − I color−magnitude diagrams (CMD) show that the upper main sequence is dominated by very bright cluster stars, while, because of the high obscuration of the giant molecular cloud surrounding the cluster, the blue envelopes of the diagrams at V 14 are limited to the main sequence stars at the distance of NGC 6530. This particular structure of the NGC 6530 CMD allows us to conclude that its distance is about d 1250 pc, significantly lower than the previous determination of d = 1800 pc. We have positionally matched our optical catalog with the list of X-ray sources found in a Chandra-ACIS observation, finding a total of 828 common stars, 90% of which are pre-main sequence stars in NGC 6530. Using evolutionary tracks of Siess et al. (2000), mass and age values are inferred for these stars. The median age of the cluster is about 2.3 Myr; in the mass range (0.6−4.0) M , the Initial Mass Function (IMF) shows a power law index x = 1.22 ± 0.17, consistent with both the Salpeter index (1.35), and with the index derived for other young clusters; towards smaller masses the IMF shows a peak and then it starts to decrease.

Classical T Tauri stars with VPHAS+ – I. H α andu-band accretion rates in the Lagoon Nebula M8

“…Also, the region is known to contain at least one Herbig-Haro object (see Tothill et al 2008), and dense CO clumps suggesting youth, if not younger then at least similar to the age of the Hourglass nebula region. Alternatively, if accretion proceeded north to south as suggested by Prisinzano et al (2005;, the median accretion rate of the north-western region is comparable to the central NGC 6530 cluster located to its south. The north-western region has not been as comprehensively studied as the central Nebula, but Kumar & Anandarao (2010) identify six Class I objects using Spitzer infrared observations near the O-type star HD 164536, and comparable to the number density of Class I objects found in the NGC 6530 cluster.…”

“…We adopt the results of Prisinzano et al (2005), where the authors find a power law index of 1.22±0.17, and a total cluster mass of ∼930 M . The maximum known stellar mass in the region is 20 M (Weidner & Kroupa 2006).…”

“…Lightfoot et al (1984) put forth a similar scenario, where star formation was triggered by the NGC 6530 cluster in the Hourglass nebula region, and cite the presence of peripheral O stars in the Lagoon nebula as evidence for a younger population. Alternatively, Damiani et al (2004); Prisinzano et al (2005) and Prisinzano et al (2012) suggested that star formation progressed from the north to the south, and the north-eastern CTTS were formed initially and have currently shredded their molecular gas, and star formation proceeded towards the west and southwest. Arias et al (2007) reported that the ages of CTTS decreased from the north to the south.…”

“…However, it has been suggested that a previous burst of star formation occurred around ∼ 10 Myr ago by Lada et al (1976) and van den Ancker et al (1997). Lightfoot et al (1984); Damiani et al (2004); Prisinzano et al (2005); Arias et al (2007); Prisinzano et al (2012) have reported sequential age gradients of the PMS members in region.…”

“…This discrepancy is thought to arise because larger determinations rely on fitting only O and early B type stars, rather than the complete blue edge of the distribution of stars in the upper main sequence on the CMD. Prisinzano et al (2005) argue that any background stars lying behind the nebula must be significantly reddened, moving them away from the main sequence. Based on the above argument, Prisinzano et al (2005) suggest that the blue edge of the distribution of stars in the CMD more accurately defines the zero-age main sequence at the distance to the Lagoon Nebula.…”