Spitzer /IRAC‐MIPS Survey of NGC 2244: Protostellar Disk Survival in the Vicinity of Hot Stars (original) (raw)

Spitzer observations of NGC 2264: the nature of the disk population

Astronomy & Astrophysics, 2012

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.

Spitzer Observations of NGC 2547: The Disk Population at 25 Million Years

The Astrophysical Journal Supplement Series, 2004

We present Spitzer observations of the young cluster NGC 2547, obtaining photometry at 3. 6, 4.5, 5.8, 8.0, and 24 m, reaching significantly fainter infrared sensitivities than previous studies. With these observations, we investigate the disk frequency in this cluster. A total of 3770, 2408, 1988, 1238, and 1123 sources were detected in the common region of the five respective bands. The detection limits were 14.9, 14.0, 13.5, 13.3, and 12.0 mag, respectively. The large majority of sources are associated with 2MASS objects. From this large ensemble, we utilize the criteria of Naylor et al. to identify 184 likely members of the cluster. The analyses in this paper are focused on these likely members. Of the 184 candidates, 162 were detected in at least one Infrared Array Camera (IRAC) band. These objects form a well-defined family in the J À K; K À½3:6 color-color diagram. There is no evidence for excess emission at 3.6 m for cluster candidates with J À K < 0:8, corresponding to spectral types earlier than late K. For later type stars, only 12 have evidence for a 3.6 m excess. Hence, we derive a 3.6 m emitting disk fraction of less than 7%. The lack of excess for the more massive stars may indicate a difference in the disk dissipation timescales for different mass stars. At 24 m, 32 of the sources are detected. Most of them have photospheric K À½3:6 and K À½24 colors, but approximately a quarter show an excess at 24 m. This observation may be evidence for cool disks with central holes.

Infrared Spectrograph Characterization of a Debris Disk Around an M-Type Star in NGC 2547

The Astrophysical Journal, 2009

We present 5 to 15 µm Spitzer Infrared Spectrograph (IRS) low resolution spectral data of a candidate debris disk around an M4.5 star identified as a likely member of the ∼40 Myr old cluster NGC 2547. The IRS spectrum shows a silicate emission feature, indicating the presence of warm, small, (sub)micron-sized dust grains in the disk. Of the fifteen previously known candidate debris disks around M-type stars, the one we discuss in this paper is the first to have an observed mid-infrared spectrum and is also the first to have measured silicate emission. We combined the IRS data with ancillary data (optical, JHK s , and Spitzer InfraRed Array Camera and 24 µm data) to build the spectral energy distribution (SED) of the source. Monte Carlo radiation transfer modeling of the SED characterized the dust disk as being very flat (h 100 =2 AU) and extending inward within at least 0.13 AU of the central star. Our analysis shows that the disk is collisionally dominated and is likely a debris disk.

A Spitzer Study of Dusty Disks around Nearby, Young Stars

The Astrophysical Journal, 2005

We have obtained Spitzer Space Telescope MIPS (Multiband Imaging Photometer for Spitzer) observations of 39 A-through M-type dwarfs, with estimated ages between 12 and 600 Myr; IRAC observations for a subset of 11 stars; and follow-up CSO SHARC II 350 m observations for a subset of two stars. None of the objects observed with IRAC possess infrared excesses at 3.6-8.0 m; however, seven objects observed with MIPS possess 24 and/or 70 m excesses. Four objects ( Phe, HD 92945, HD 119124, and AU Mic), with estimated ages 12-200 Myr, possess strong 70 m excesses, !100% larger than their predicted photospheres, and no 24 m excesses, suggesting that the dust grains in these systems are cold. One object (HD 112429) possesses moderate 24 and 70 m excesses with a color temperature, T gr ¼ 100 K. Two objects ( 1 Lib and HD 177724) possess such strong 24 m excesses that their 12, 24, and 70 m fluxes cannot be self-consistently modeled using a modified blackbody despite a 70 m excess >2 times greater than the photosphere around 1 Lib. The strong 24 m excesses may be the result of emission in spectral features, as observed toward the Hale-Bopp star HD 69830.

New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope

The Astrophysical Journal, 2009

We present 24 µm and 70 µm Multiband Imaging Photometer for Spitzer (MIPS) observations of 70 A through M-type dwarfs with estimated ages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed time program, including a re-analysis of some previously published source photometry. Our sample is selected from stars with common youth indicators such as lithium abundance, X-ray activity, chromospheric activity, and rapid rotation. We compare our MIPS observations to empirically derived K s -[24] colors as a function of the stellar effective temperature to identify 24 µm and 70 µm excesses. We place constraints or upper limits on dust temperatures and fractional infrared luminosities with a simple blackbody dust model. We confirm the previously published 70 µm excesses for HD 92945, HD 112429, and AU Mic, and provide updated flux density measurements for these sources. We present the discovery of 70 µm excesses for five stars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. HD 135599 is also a known Spitzer IRS (InfraRed Spectrograph) excess source, and we confirm the excess at 24 µm. We also present the detection of 24 µm excesses for 10 stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD 123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find that large 70 µm excesses are less common around stars with effective temperatures of less than 5000 K (3.7 +7.6 −1.1 %) than around stars with effective temperatures between 5000 K and 6000 K (21.4 +9.5 −5.7 %), despite the cooler stars having a younger median age in our sample (12 Myr vs. 340 Myr). We find that the previously reported excess for TWA 13A at 70 µm is due to a nearby background galaxy, and the previously reported excess for HD 177724 is due to saturation of the near-infrared photometry used to predict the mid-infrared stellar flux contribution. In the Appendix, we present an updated analysis of dust grain removal timescales due to grain-grain collisions and radiation pressure, Poynting-Robertson (P-R) drag, stellar wind drag, and planet-dust dynamical interaction. We find that drag forces can be important for disk dynamics relative to grain-grain collisions for L IR /L * < 10 −4 , and that stellar wind drag is more important than P-R drag for K and M dwarfs, and possibly for young (<1 Gyr) G dwarfs as well.

IRS Characterization of a Debris Disk around an M-type star in NGC2547

2009

We present 5 to 15 micron Spitzer Infrared Spectrograph (IRS) low resolution spectral data of a candidate debris disk around an M4.5 star identified as a likely member of the ~40 Myr old cluster NGC2547. The IRS spectrum shows a silicate emission feature, indicating the presence of warm, small, (sub)micron-sized dust grains in the disk. Of the fifteen previously known candidate debris disks around M-type stars, the one we discuss in this paper is the first to have an observed mid-infrared spectrum and is also the first to have measured silicate emission. We combined the IRS data with ancillary data (optical, JHKs, and Spitzer InfraRed Array Camera and 24 micron data) to build the spectral energy distribution (SED) of the source. Monte Carlo radiation transfer modeling of the SED characterized the dust disk as being very flat (h100=2AU) and extending inward within at least 0.13AU of the central star. Our analysis shows that the disk is collisionally dominated and is likely a debris disk.

Young stars and protostellar cores near NGC 2023

Astronomy and Astrophysics, 2009

Context. We investigate the young (proto)stellar population in NGC 2023 and the L 1630 molecular cloud bordering the h ii region IC 434, using Spitzer IRAC and MIPS archive data, JCMT SCUBA imaging and spectroscopy as well as targeted BIMA observations of one of the Class 0 protostars, NGC 2023 MM 1. Aims. We study the distribution of gas, dust and young stars in this region to see where stars are forming, whether the expansion of the h ii region has triggered star formation, and whether dense cold cores have already formed stars. Methods. We have performed photometry of all IRAC and MIPS images, and used color-color diagrams to identify and classify all young stars seen within a 22 × 26 field along the boundary between IC 434 and L 1630. For some stars, which have sufficient optical, IR, and/or sub-millimeter data we have also used the online SED fitting tool for a large 2D archive of axisymmetric radiative transfer models to perform more detailed modeling of the observed SEDs. We identify 5 sub-millimeter cores in our 850 and 450 μm SCUBA images, two of which have embedded class 0 or I protostars. Observations with BIMA are used to refine the position and characteristics of the Class 0 source NGC 2023 MM 1. These observations show that it is embedded in a very cold cloud core, which is strongly enhanced in NH 2 D. Results. We find that HD 37903 is the most massive member of a cluster with 20-30 PMS stars. We also find smaller groups of PMS stars formed from the Horsehead nebula and another elephant trunk structure to the north of the Horsehead. Star formation is also occurring in the dark lane seen in IRAC images and in the sub-millimeter continuum. We refine the spectral classification of HD 37903 to B2 Ve. We find that the star has a clear IR excess, and therefore it is a young Herbig Be star. Conclusions. Our study shows that the expansion of the IC 434 h ii region has triggered star formation in some of the dense elephant trunk structures and compressed gas inside the L 1630 molecular cloud. This pre-shock region is seen as a sub-millimeter ridge in which stars have already formed. The cluster associated with NGC 2023 is very young, and has a large fraction of Class I sources.

Spitzer 24 mum Survey for Dust Disks around Hot White Dwarfs

Astron J, 2011

Two types of dust disks around white dwarfs (WDs) have been reported: small dust disks around cool metal-rich WDs consisting of tidally disrupted asteroids and a large dust disk around the hot central WD of the Helix planetary nebula (PN) possibly produced by collisions among Kuiper-Belt-like objects. To search for more dust disks of the latter type, we have conducted a Spitzer MIPS 24 μm survey of 71 hot WDs or pre-WDs, among which 35 are central stars of PNe (CSPNs). Nine of these evolved stars are detected and their 24 μm flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against the detection of distant objects, the 24 μm detection rate for the sample is gsim15%. It is striking that seven, or ~20%, of the WD and pre-WDs in known PNe exhibit 24 μm excesses, while two, or 5%-6%, of the WDs not in PNe show 24 μm excesses and they have the lowest 24 μm flux densities. We have obtained follow-up Spitzer Infrared Spectrograph spectra for five objects. Four show clear continuum emission at 24 μm, and one is overwhelmed by a bright neighboring star but still shows a hint of continuum emission. In the cases of WD 0950+139 and CSPN K 1-22, a late-type companion is present, making it difficult to determine whether the excess 24 μm emission is associated with the WD or its red companion. High-resolution images in the mid-infrared are needed to establish unambiguously the stars responsible for the 24 μm excesses.

Spitzer and Magellan Observations of NGC 2264: A Remarkable Star‐forming Core near IRS 2

The Astrophysical Journal, 2006

We analyze Spitzer and Magellan observations of a star forming core near IRS-2 in the young cluster NGC 2264. The submillimeter source IRAS 12 S1, previously believed to be an intermediate mass Class 0 object is shown to be a dense collection of embedded, low mass stars. We argue that this group of stars represents the fragmenting collapse of a dense, turbulent core, based on a number of indicators of extreme youth. With reasonable estimates for the velocity dispersion in the group, we estimate a dynamical lifetime of only a few x 10 4 years. Spectral energy distributions of stars in the core are consistent with Class I or Class 0 assignments. We present observations of an extensive system of molecular hydrogen emission knots. The luminosity of the objects in the core region are consistent with roughly solar mass protostars.

A Spitzer space telescope study of disks in the young σ Orionis cluster

The Astrophysical …, 2007

We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (∼ 3 Myr) σ Orionis cluster. The populous nature of this cluster makes it a good target for statistically-significant studies of disk emission as a function of mass. We identify 336 stars as members of the cluster using optical and near-infrared color magnitude diagrams. Using the spectral energy distribution (SED) slopes in the IRAC spectral range, we place objects in several classes: non-excess stars, stars with optically thick disks (like classical T Tauri stars), class I (protostellar) candidates, and stars with "evolved disks"; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but non-flared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify 7 transition disk candidates and 3 possible debris disk systems. There appears to be a spatial extension of infrared excess sources to the northeast , which may be associated with the young (< 1 Myr) embedded cluster NGC 2024. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ∼10%