Spitzerobservations of NGC 2264: the nature of the disk population

Abstract

Aims. NGC 2264 is a young cluster with a rich circumstellar disk population which makes it an ideal target for studying the evolution of stellar clusters. Our goal is to study the star formation history of NGC 2264 and to analyse the primordial disk evolution of its members. Methods. The study presented is based on data obtained with the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS) on board the Spitzer Space Telescope, combined with deep near-infrared (NIR) ground-based FLAMINGOS imaging and previously published optical data. Results. We build NIR dust extinction maps of the molecular cloud associated with the cluster, and determine it to have a mass of 2.1 × 10 3 M above an A V of 7 mag. Using a differential K s -band luminosity function (KLF) of the cluster, we estimate the size of the population of NGC 2264, within the area observed by FLAMINGOS, to be 1436 ± 242 members. The star formation efficiency is ≥∼25%. We identify the disk population and divide it into 3 groups based on their spectral energy distribution slopes from 3.6 μm to 8 μm and on the 24 μm excess emission: (i) optically thick inner disks, (ii) anaemic inner disks, and (iii) disks with inner holes, or transition disks. We analyse the spatial distribution of these sources and find that sources with thick disks segregate into subclusterings, whereas sources with anaemic disks do not. Furthermore, sources with anaemic disks are found to be unembedded (i.e., with A V < 3 mag), whereas the clustered sources with thick disks are still embedded within the parental cloud. Conclusions. NGC 2264 has undergone more than one star-forming event, where the anaemic and extincted thick disk population appear to have formed in separate episodes: the sources with anaemic disks are more evolved and have had time to disperse and populate a halo of the cluster. We also find tentative evidence of triggered star-formation in the Fox Fur Nebula. In terms of disk evolution, our findings support the emerging disk evolution paradigm of two distinct evolutionary paths for primordial optically thick disks: a homologous one where the disk emission decreases uniformly at NIR and mid-infrared (MIR) wavelengths, and a radially differential one where the emission from the inner region of the disk decreases more rapidly than from the outer region (forming transition disks).

Circumstellar discs in Galactic centre clusters: Disc-bearing B-type stars in the Quintuplet and Arches clusters

“…We compare our findings to studies of transition discs with near-to mid-infrared emission recently discovered by the Spitzer and AKARI surveys (e.g. Fujiwara et al 2013;Maaskant et al 2013;Espaillat et al 2010Espaillat et al , 2011Muzerolle et al 2010;Teixeira et al 2012;Furlan et al 2009) to investigate the evolutionary state of the disc candidates in the Arches and Quintuplet clusters. The problem of the disc lifetime is addressed, and secondary disc formation will be proposed as a possible scenario to explain the Arches and Quintuplet nearinfrared excess sources and their apparent expanded lifetime compared to primordial discs around Herbig Be stars.…”

Mapping accretion and its variability in the young open cluster NGC 2264: a study based onu-band photometry

“…Figure 16 shows the mass accretion rate distribution inferred for a population of 236 high-confidence NGC 2264 members, as a function of stellar mass, whose range extends down to ∼0.1 M and up to ∼1.5 M . These objects have been classified as CTTS, i.e., accreting objects, as opposed to WTTS among the PMS population analyzed so far, based on spectroscopic criteria (H α EW, H α width at 10% intensity) and/or photometric criteria (significant UV excess from the CFHT sample, α IRAC value consistent with Class II sources from the data of Teixeira et al 2012, large variability from the CFHT photometry; see Venuti et al, in prep., for this last point). The accretion parameters for these objects are reported in Table 4.…”

Stellar models simulating the disk-locking mechanism and the evolutionary history of the Orion Nebula cluster and NGC 2264

“…Solid curves refer to masses and ages obtained with our models, dotted curves refer to masses and ages obtained with the models of Siess et al (2000). we therefore used Spitzer IRAC data kindly provided by Paula Teixeira (Teixeira et al 2012), who classified the circumstellar disks of NGC 2264 stars based on their spectral energy distribution slope between 3.6 μm and 8 μm, defined as α IRAC . These IRAC data were obtained in two epochs, March and October 2004, and can help us to distinguish the stars with a disk from diskless objects.…”

“…To relate this Hα index to some physical property, Lamm (2003) and Lamm et al (2004) (2003), for ctts and wtts the Hα-index values agree well with the W λ (Hα) measurements by Rebull et al (2002) and Dahm & Simon (2005 Unfortunately, the only observational disk indicator for which we have measurements for all stars is Δ[R c − Hα], and this indicator is inconclusive for more than half of our sample stars, classifying 210 objects as intermediate cases. To distinguish these stars as objects with and without disks, we also used the α IRAC index reported by Teixeira et al (2012). Of the 405 stars of our sample, 170 have α IRAC measurements, 75 have α IRAC > −2.56 (indicating that they have dust in the inner disk) and 95 have α IRAC ≤ −2.56 (indicating that they have naked photospheres).…”