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Register a new userSpitzer Observations of the Lambda Orionis cluster I: the frequency of young debris disks at 5 Myr
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Jesus Hernandez PhD
Publication date
2009
fields
Physics
and research's language is
English
Authors
Jesus Hernandez
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We present IRAC/MIPS Spitzer observations of intermediate-mass stars in the 5 Myr old Lambda Orionis cluster. In a representative sample of stars earlier than F5 (29 stars), we find a population of 9 stars with a varying degree of moderate 24um excess comparable to those produced by debris disks in older stellar groups. As expected in debris disks systems, those stars do not exhibit emission lines in their optical spectra. We also include in our study the star HD 245185, a known Herbig Ae object which displays excesses in all Spitzer bands and shows emission lines in its spectrum. We compare the disk population in the Lambda Orionis cluster with the disk census in other stellar groups studied using similar methods to detect and characterize their disks and spanning a range of ages from 3 Myr to 10 Myr. We find that for stellar groups of 5 Myr or older the observed disk frequency in intermediate mass stars (with spectral types from late B to early F) is higher than in low mass stars (with spectral types K and M). This is in contradiction with the observed trend for primordial disks evolution, in which stars with higher stellar masses dissipate their primordial disks faster. At 3 Myr the observed disk frequency in intermediate mass stars is still lower than for low mass stars indicating that second generation dusty disks start to dominate the disk population at 5 Myr for intermediate mass stars. This result agrees with recent models of evolution of solids in the region of the disk where icy objects form (>30 AU), which suggest that at 5-10 Myr collisions start to produce large amount of dust during the transition from runaway to oligarchic growth (reaching sizes of ~500 km) and then dust production peaks at 10-30 Myr, when objects reach their maximum sizes (>1000 km)
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Fluxes and upper limits in the wavelength range from 3.6 to 70 microns from the Spitzer Space Telescope are provided for twenty solar-mass Pleiades members. One of these stars shows a probable mid-IR excess and two others have possible excesses, presumably due to circumstellar debris disks. For the star with the largest, most secure excess flux at MIPS wavelengths, HII1101, we derive Log(L[dust]/L[Sun]) ~ -3.8 and an estimated debris disk mass of 4.2 x 10^-5 M(Earth) for an assumed uniform dust grain size of 10 microns If the stars with detected excesses are interpreted as stars with relatively recent, large collision events producing a transient excess of small dust particles, the frequency of such disk transients is about ~ 10 % for our ~ 100 Myr, Pleiades G dwarf sample. For the stars without detected 24-70 micron excesses, the upper limits to their fluxes correspond to approximate 3 sigma upper limits to their disk masses of 6 x 10^-6 M(Earth) using the MIPS 24 micron upper limit, or 2 x 10^-4 M(Earth) using the MIPS 70 micron limit. These upper limit disk masses (for warm and cold dust, respectively) are roughly consistent, but somewhat lower than, predictions of a heuristic model for the evolution of an average solar-mass stars debris disk based on extrapolation backwards in time from current properties of the Suns Kuiper belt.
By collecting optical and infrared photometry and low resolution spectroscopy, we have identified a large number of low mass stars and brown dwarf candidates belonging to the young cluster (~5 Myr) associated with the binary star lambda Orionis. The lowest mass object found is a M8.5 with an estimated mass of 0.02 Msun (~0.01 Msun for objects without spectroscopic confirmation). For those objects with spectroscopy, the measured strength of the Halpha emission line follows a distribution similar to other clusters with the same age range, with larger equivalent widths for cooler spectral types. Three of the brown dwarfs have Halpha emission equivalent widths of order 100 AA, suggestive that they may have accretion disks and thus are the substellar equivalent of Classical T Tauri stars. We have derived the Initial Mass Function for the cluster. For the substellar regime, the index of the mass spectrum is alpha=0.60$+-0.06, very similar to other young associations.
We report Spitzer Space Telescope IRAC 3.6, 4.5, 5.8 and 8 um and MIPS 24 and 70 um observations of the 32 Ori Group, a recently discovered nearby stellar association situated towards northern Orion. The proximity of the group (~93 pc) has enabled a sensitive search for circumstellar dust around group members, and its age (~20 Myr) corresponds roughly to an epoch thought to be important for terrestrial planet formation in our own solar system. We quantify infrared excess emission due to circumstellar dust among group members, utilizing available optical (e.g. Hipparcos, Tycho) and near-IR (2MASS) photometry in addition to the Spitzer IR photometry. We report 4 out of the 14 objects which exhibit 24 um excess emission more than 4sigma above the stellar photosphere (>20%) though lacking excess emission at shorter wavelengths: HD 35656 (A0Vn), HD 36338 (F4.5), RX J0520.5+0616 (K3), and HD 35499 (F4). Two objects (HD 35656 and RX J0520.0+0612) have 70 um excesses, although the latter lacks 24 um excess emission. The 24 um disk fraction of this group is 29(+14,-9%), which is similar to previous findings for groups of comparable ages and places 32 Ori as the young stellar group with the 2nd most abundant 24 um excesses among groups lacking accreting T Tauri stars (behind only the approximately coeval Beta Pic Moving Group). We also model the infrared excess emission using circumstellar dust disk models, placing constraints on disk parameters including L_IR/L_*, T_disk, characteristic grain distance, and emitting area. The L_IR/L_* values for all the stars can be reasonably explained by steady state disk evolution.
This is the first paper of a series devoted to the Lambda Orionis star-forming region, from the X-ray perspective, which will provide a comprehensive view of this complex region. In this paper we focus in uncovering the population of the central, young cluster Collinder 69 (C69), and in particular those diskless members not identified by previous near- and mid-infrared surveys, and to establish the X-ray luminosity function for the association. We have combined two exposures taken with the XMM-Newton satellite with an exhaustive data set of optical, near- and mid-infrared photometry to assess the membership of the X-ray sources based on color-color and color-magnitude diagrams, as well as other properties, such as effective temperatures, masses and bolometric luminosities. We detected a total of 164 X-ray sources, of which 66 are probable and possible cluster members. A total of 16 are newly identified probable members. The two XMM-Newton pointings east and west of the cluster center have allowed us to verify the heterogeneous spatial distribution of young stars, probably related to the large scale structure of the region. The disk fraction of the X-ray detected cluster sample is very low, close to 10%, in remarkable contrast to the low-mass stellar and substellar population (mostly undetected in X-rays) where the disk fraction reaches about 50%. The X-ray luminosity function of C69 provides support for an age of several Myr when compared with other well known young associations. With our improved cluster census we confirm previous reports on the untypically low disk fraction compared to other clusters of several Myr age. The different disk fractions of X-ray detected (essentially solar-like) and undetected (mostly low-mass stars and brown dwarfs) members can be understood as a consequence of a mass-dependence of the time-scale for disk evolution.
Over the past years, the lambda-Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function and other properties of low mass cluster members. In the continuity of our previous studies of young associations cores, we search for ultracool members and new multiple systems within the central 5.3 (~0.6pc) of the cluster. We obtained deep seeing limited J, Ks-band images of the 5.3 central part of the cluster with NTT/SofI and H-band images with CAHA/Omega2000. These images were complemented by multi-conjugate adaptive optics (MCAO) H and Ks images of the 1.5 central region of the lambda-Orionis cluster obtained with the prototype MCAO facility MAD at the VLT. The direct vicinity of the massive lambda-Ori O8III-star was probed using NACO/SDI at the VLT. Finally, we also retrieved Spitzer IRAC images of the same area and used archival Subaru Suprime-Cam and CFHT CFHT12K i-band images. We report the detection of 9 new member candidates selected from optical and near-IR color-color and color-magnitude diagrams and 7 previously known members. The high spatial resolution images resolve 3 new visual multiple systems. Two of them are most likely not members of the association. The third one is made of a brown dwarf candidate companion to the F8V star HD36861C. The simultaneous differential images allow us to rule out the presence of visual companions more massive than M>0.07Msun in the range 1-2.5, and M>0.25Msun in the range 0.5-2.5
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