We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula clu... more We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula cluster whose light curves show narrow (few day timescale) flux dips. Based on comparison to the only previously well-documented member of this class (AA Tau), we believe these flux dips are due to 'clouds' in the inner circumstellar disk of these stars that pass through our line of sight. Our 2009 data suggest that most YSO disks have similar structures and that those which exhibit these flux dips are simply those whose disks are close to edge-on to our line of sight. We propose here to obtain IRAC time series data of much higher cadence and significantly better RMS noise for twelve members of this class over a 10 day timespan. These data will allow us to constrain better the size distribution of the occulting bodies, their grain properties and their internal density structure. These data will therefore provide quantitative inputs to realistic models of circumstellar disk evolution and planet formation/migration.
IRS 14-38 um spectra have provided strong evidence for extremely small silicate dust grains in th... more IRS 14-38 um spectra have provided strong evidence for extremely small silicate dust grains in the vicinity of B-type field stars. The signature of these very small (less than 1 attogram) rocks is an unusually high temperature, given their projected distance from the exciting star. We presently have 1-dimensional images (i.e. along the LL slit) of these attorock clouds. This data indicates a temperature which is independent of projected distance from the exciting star. This is one of the key signatures of stochastic heating by single photons. If stochastic heating, rather than equilibrium heating, is at play, then the precise size of the dust grains can be deduced from detailed modeling. We propose to make a 50x200 arcsecond map of the attorock clouds around the three most prominent attorock sources. We will convert this, via our modeling, to a map of silicate dust temperature, which will definitively confirm (or deny) stochastic heating. In addition, we propose to get SL data at the position 20' W of HR 1415, where we have the strongest evidence for attorocks, to look carefully for any PAH emission, which is surprisingly absent in the SL data we do have, i.e. along the SL slit perpendicular to the LL slit.
Spitzer/IRAC in the warm mission is the only facility now existing or planned capable of carrying... more Spitzer/IRAC in the warm mission is the only facility now existing or planned capable of carrying out an extensive, accurate time series photometric monitoring survey of star-forming regions in the thermal infrared. The demonstrated sensitivity and stability of IRAC allows measurement of the relative fluxes of YSO's down to the substellar mass limit to 1-2% accuracy in star-forming regions out to >500 pc. We propose a time series monitoring exploration science survey of the Orion Nebula Cluster and 11 very young, populous embedded star-forming cores which will provide >D 80 epochs of data for > 1500 YSO's. We will complement these observations with contemporaneous optical and near-IR monitoring data in order to allow comparison of the phase, amplitude and light-curve shape as a function of wavelength. These data will allow us to: (a) provide otherwise unobtainable constraints on the structure of the inner disks in Class I and II YSOs - and hence, perhaps, provide clues to the formation and migration of planets at young ages; (b) measure the short and long-term stability of hot spots on the surfaces of YSO's of all evolutionary stages; and (c) determine rotational periods for the largest sample to date of Class I YSO's and hence obtain the best measure of the initial angular momentum distribution of young stars.
We propose to re-observe and enhance the original GLIMPSE survey region by following the Galactic... more We propose to re-observe and enhance the original GLIMPSE survey region by following the Galactic warp at a Galactocentric distance of 13 kpc to survey the far outer Galaxy. The new survey will cover longitudes of 15 < l < 65, -15 > l > -95, with a latitude width of 2.5 degrees; 55% of the survey will overlap with GLIMPSE, and 45% will cover new territory. The survey strategy of 3 HDR visits on each sky position will increase the dynamic range over GLIMPSE by a factor of 13 on the faint end and 3 on the bright end. The survey will be enhanced by several complementary surveys at near-IR, mid-IR, far-IR, submm, and radio wavelengths. Deep GLIMPSE will allow us to map stellar Galactic structure out to the edge of the stellar disk; map star formation and H II regions in the far outer Galaxy; improve our estimate of the star formation rate (SFR) of the Galaxy and study the SFR as a function of Galactocentric distance; study low-mass as well as high-mass star formation in the Galactic plane and determine stellar mass functions and evolutionary timescales of YSOs as a function of stellar mass and environment; continue to catalog PAH bubbles, IRDCs, YSO outflows, stellar clusters, external galaxies, brown dwarfs, and more; and search for dwarf galaxies hidden by the Galactic midplane.
We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a sample of 41 young lo... more We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a sample of 41 young low-mass stars and brown dwarfs, to study the characteristics of their disks and to search for evidence of ongoing planet formation. We also propose to use IRAC to observe an additional 43 newly-identified brown dwarfs in Taurus and Chamaeleon, for future IRS spectroscopic followup. This guaranteed-time observing program has a total of 50.1 hours, with 14 hours provided by IRAC PI Giovanni Fazio, and 36.1 hours by IRS PI Jim Houck.
We present multi-band photometry of 185 type-Ia supernovae (SN Ia), with over 11500 observations.... more We present multi-band photometry of 185 type-Ia supernovae (SN Ia), with over 11500 observations. These were acquired between 2001 and 2008 at the F. L. Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics (CfA). This sample contains the largest number of homogeneously-observed and reduced nearby SN Ia (z < 0.08) published to date. It more than doubles the nearby sample, bringing SN Ia cosmology to the point where systematic uncertainties dominate. Our natural system photometry has a precision of 0.02 mag or better in BVRIr'i' and roughly 0.04 mag in U for points brighter than 17.5 mag. We also estimate a systematic uncertainty of 0.03 mag in our SN Ia standard system BVRIr'i' photometry and 0.07 mag for U. Comparisons of our standard system photometry with published SN Ia light curves and comparison stars, where available for the same SN, reveal agreement at the level of a few hundredths mag in most cases. We find that 1991bg-like SN Ia are sufficiently distinct from other SN Ia in their color and light-curve-shape/luminosity relation that they should be treated separately in light-curve/distance fitter training samples. The CfA3 sample will contribute to the development of better light-curve/distance fitters, particularly in the few dozen cases where near-infrared photometry has been obtained and, together, can help disentangle host-galaxy reddening from intrinsic supernova color, reducing the systematic uncertainty in SN Ia distances due to dust.
We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in... more We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 micron data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (Ic) and/or near-infrared (JKs) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence (PMS) eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 & 4.5 micron variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009, and we highlight our light curves for AA-Tau analogs - YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.
We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a large sample of young... more We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a large sample of young stellar objects with protoplanetary disks in the Orion A and B molecular cloud complexes. The aim is to study the influence of the wide range of star-formation conditions in the Orion clouds on the structure, composition and evolution of protoplanetary disks, and on the conditions and occurrence of planet formation in these disks. This guaranteed-time observing program has a total of 100.9 hours, with 20 hours provided by IRAC PI Giovanni Fazio, 20 hours by MIPS PI George Rieke, and 60.9 hours by IRS PI Jim Houck.
We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the youn... more We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (~ 3 Myr) sigma Orionis cluster. 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. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% in the T Tauri mass range (1-0.1 msun). We find that the disk fraction does not decrease significantly toward the brown dwarf candidates (<0.1 msun). The IRAC infrared excesses found in stellar clusters and associations with and without central high mass stars are similar, suggesting that external photoevaporation is not very important in many clusters. Finally, we find no correlation between the X-ray luminosity and the disk infrared excess, suggesting that the X-rays are not strongly affected by disk accretion.
Spitzer and Caltech Submillimeter Observatory (CSO) images and spectrophotometry of epsilon Erida... more Spitzer and Caltech Submillimeter Observatory (CSO) images and spectrophotometry of epsilon Eridani at wavelengths from 3.5 to 350 um reveal new details of its bright debris disk. The 350 um map confirms the presence of a ring at r = 11-28 arcsec (35-90 AU) observed previously at longer sub-mm wavelengths. The Spitzer mid- and far-IR images do not show the ring, but rather a featureless disk extending from within a few arcsec of the star across the ring to r ~ 34 arcsec (110 AU). The spectral energy distribution (SED) of the debris system implies a complex structure. A model constrained by the surface brightness profiles and the SED indicates that the sub-mm ring emission is primarily from large (a ~ 135 um) grains, with smaller (a ~ 15 um) grains also present in and beyond the ring. The Spitzer IRS and MIPS SED-mode spectrophotometry data clearly show the presence of spatially compact excess emission at lambda > 15 um that requires the presence of two additional narrow belts of dust within the sub-mm ring's central void. The innermost belt at r ~ 3 AU is composed of silicate dust. A simple dynamical model suggests that dust produced collisionally by a population of about 11 M_Earth of planetesimals in the sub-mm ring could be the source of the emission from both in and beyond the sub-mm ring. Maintaining the inner belts and the inner edge to the sub-mm ring may require the presence of three planets in this system including the candidate radial velocity object.
Publications of The Astronomical Society of The Pacific, 2006
This paper summarizes contributions and suggestions as presented at the Chandra Workshop Star For... more This paper summarizes contributions and suggestions as presented at the Chandra Workshop Star Formation in the Era of Three Great Observatories conducted in July 2005. One of the declared goals of the workshop was to raise recognition within the star formation research community about the sensible future utilization of the space observatories Spitzer, Hubble, and Chandra in their remaining years of operation to tackle imminent questions of our understanding of stellar formation and the early evolution of stars. A white paper was generated to support the continuous and simultaneous usage of observatory time for star formation research. The contents of this paper have been presented and discussed at several other meetings during the course of 2005 and January 2006.
We propose a Spitzer Legacy Program to perform a uniform, unbiased survey of the Cygnus-X complex... more We propose a Spitzer Legacy Program to perform a uniform, unbiased survey of the Cygnus-X complex (~6x6 deg area) with IRAC and MIPS. This region contains the richest known concentration of massive protostars and the largest OB associations in the nearest 2 kpc. With this survey we can 1) analyze the evolution of high mass protostars with a large and statistically robust sample at a single distance, 2) study the role of clustering in high mass star formation, 3) study low mass star formation in a massive molecular cloud complex dominated by the energetics of ~100 O-stars, 4) assess what fraction of all young low mass stars in the nearest 2 kpc are forming in this one massive complex, and 5) provide an unbiased survey of the region and produce a legacy data set which can be used in conjunction with future studies of this region (e.g., with Herschel and JWST). The Cygnus-X survey will be an important step in constructing one of Spitzer's greatest legacies: surveying with high sensitivity and spatial resolution a representative sample of galactic star forming regions, from Bok globules to complexes containing millions of solar masses of gas and hundreds of O-stars.
The origin and evolution of the X-rays in very young stellar objects (YSOs) are not yet well unde... more The origin and evolution of the X-rays in very young stellar objects (YSOs) are not yet well understood since it is very hard to observe YSOs in the protostellar phase. We study the X-ray properties of Class 0-I objects in the Orion Nebula Cluster (ONC) and compare them with those of the more evolved Class II and III members. Using Chandra Orion Ultradeep Project (COUP) data, we study the X-ray properties of stars in different evolutionary classes: luminosities, NH, temperatures and time variability are compared in order to understand if the interaction between the circumstellar material and the central object can influence the X-ray emission. We have assembled the deepest and most complete photometric catalog of objects in the ONC region from the UV to 8 microns using data from HST, WFI@2.2m ESO and ISPI@4m CTIO telescopes, and Spitzer IRAC. We select high probability candidate Class 0-I protostars, distinguishing between those having a spectral energy distribution which rises from K up to 8 microns (Class 0-Ia) from those where the SED rises from K up to 4.5 microns and decreasing afterwards (Class 0-Ib). We select a sample of bona fide Class II stars and a set of Class III stars with IR emission consistent with normal photospheres. Our principal result is that Class 0-Ia objects are significantly less luminous in X-rays, both in the total and hard bands, than the more evolved Class II stars with mass larger than 0.5 Msun; these latter show X-ray luminosities similar to those of Class 0-Ib stars. This result supports the hypothesis that the onset of X-ray emission occurs at a very early stage of star formation. Temporal variability and spectral properties of Class 0-I stars are similar to those of the more evolved Class II and III objects, except for a larger absorption likely due to gas in the circumstellar material.
We present multi-wavelength optical and infrared photometry of 170 previously known low mass star... more We present multi-wavelength optical and infrared photometry of 170 previously known low mass stars and brown dwarfs of the 5 Myr Collinder 69 cluster (Lambda Orionis). The new photometry supports cluster membership for most of them, with less than 15% of the previous candidates identified as probable non-members. The near infrared photometry allows us to identify stars with IR excesses, and we find that the Class II population is very large, around 25% for stars (in the spectral range M0 - M6.5) and 40% for brown dwarfs, down to 0.04 Msun, despite the fact that the H(alpha) equivalent width is low for a significant fraction of them. In addition, there are a number of substellar objects, classified as Class III, that have optically thin disks. The Class II members are distributed in an inhomogeneous way, lying preferentially in a filament running toward the south-east. The IR excesses for the Collinder 69 members range from pure Class II (flat or nearly flat spectra longward of 1 micron), to transition disks with no near-IR excess but excesses beginning within the IRAC wavelength range, to two stars with excess only detected at 24 micron. Collinder 69 thus appears to be at an age where it provides a natural laboratory for the study of primordial disks and their dissipation.
We will conduct a survey of the Orion A (L1641) and Orion B (l1630) Molecular Clouds in all four ... more We will conduct a survey of the Orion A (L1641) and Orion B (l1630) Molecular Clouds in all four IRAC bands. From this survey we can achieve the following: 1.) probe the spatial distribution of young stars in the Orion clouds, comparing the number of clustered and isolated stars, 2.) study the evolution of circumstellar disks and envelopes during the first few millions years of pre--main sequence evolution, and 3.) measure the Orion cloud IMF down to 10-20 Mjupiter.
The Workshop “12 Questions on Star and Massive Star Cluster Formation” was held in Garching from ... more The Workshop “12 Questions on Star and Massive Star Cluster Formation” was held in Garching from 3 to 6 July 2007. The programme was set up to allow long (and fruitful) discussions around several questions connecting the formation of stars and star clusters. Here we summarise some of the discussions, and encourage interest- ed readers to download the contributions from http://www.eso.org/star07.
We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula clu... more We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula cluster whose light curves show narrow (few day timescale) flux dips. Based on comparison to the only previously well-documented member of this class (AA Tau), we believe these flux dips are due to 'clouds' in the inner circumstellar disk of these stars that pass through our line of sight. Our 2009 data suggest that most YSO disks have similar structures and that those which exhibit these flux dips are simply those whose disks are close to edge-on to our line of sight. We propose here to obtain IRAC time series data of much higher cadence and significantly better RMS noise for twelve members of this class over a 10 day timespan. These data will allow us to constrain better the size distribution of the occulting bodies, their grain properties and their internal density structure. These data will therefore provide quantitative inputs to realistic models of circumstellar disk evolution and planet formation/migration.
IRS 14-38 um spectra have provided strong evidence for extremely small silicate dust grains in th... more IRS 14-38 um spectra have provided strong evidence for extremely small silicate dust grains in the vicinity of B-type field stars. The signature of these very small (less than 1 attogram) rocks is an unusually high temperature, given their projected distance from the exciting star. We presently have 1-dimensional images (i.e. along the LL slit) of these attorock clouds. This data indicates a temperature which is independent of projected distance from the exciting star. This is one of the key signatures of stochastic heating by single photons. If stochastic heating, rather than equilibrium heating, is at play, then the precise size of the dust grains can be deduced from detailed modeling. We propose to make a 50x200 arcsecond map of the attorock clouds around the three most prominent attorock sources. We will convert this, via our modeling, to a map of silicate dust temperature, which will definitively confirm (or deny) stochastic heating. In addition, we propose to get SL data at the position 20' W of HR 1415, where we have the strongest evidence for attorocks, to look carefully for any PAH emission, which is surprisingly absent in the SL data we do have, i.e. along the SL slit perpendicular to the LL slit.
Spitzer/IRAC in the warm mission is the only facility now existing or planned capable of carrying... more Spitzer/IRAC in the warm mission is the only facility now existing or planned capable of carrying out an extensive, accurate time series photometric monitoring survey of star-forming regions in the thermal infrared. The demonstrated sensitivity and stability of IRAC allows measurement of the relative fluxes of YSO's down to the substellar mass limit to 1-2% accuracy in star-forming regions out to >500 pc. We propose a time series monitoring exploration science survey of the Orion Nebula Cluster and 11 very young, populous embedded star-forming cores which will provide >D 80 epochs of data for > 1500 YSO's. We will complement these observations with contemporaneous optical and near-IR monitoring data in order to allow comparison of the phase, amplitude and light-curve shape as a function of wavelength. These data will allow us to: (a) provide otherwise unobtainable constraints on the structure of the inner disks in Class I and II YSOs - and hence, perhaps, provide clues to the formation and migration of planets at young ages; (b) measure the short and long-term stability of hot spots on the surfaces of YSO's of all evolutionary stages; and (c) determine rotational periods for the largest sample to date of Class I YSO's and hence obtain the best measure of the initial angular momentum distribution of young stars.
We propose to re-observe and enhance the original GLIMPSE survey region by following the Galactic... more We propose to re-observe and enhance the original GLIMPSE survey region by following the Galactic warp at a Galactocentric distance of 13 kpc to survey the far outer Galaxy. The new survey will cover longitudes of 15 < l < 65, -15 > l > -95, with a latitude width of 2.5 degrees; 55% of the survey will overlap with GLIMPSE, and 45% will cover new territory. The survey strategy of 3 HDR visits on each sky position will increase the dynamic range over GLIMPSE by a factor of 13 on the faint end and 3 on the bright end. The survey will be enhanced by several complementary surveys at near-IR, mid-IR, far-IR, submm, and radio wavelengths. Deep GLIMPSE will allow us to map stellar Galactic structure out to the edge of the stellar disk; map star formation and H II regions in the far outer Galaxy; improve our estimate of the star formation rate (SFR) of the Galaxy and study the SFR as a function of Galactocentric distance; study low-mass as well as high-mass star formation in the Galactic plane and determine stellar mass functions and evolutionary timescales of YSOs as a function of stellar mass and environment; continue to catalog PAH bubbles, IRDCs, YSO outflows, stellar clusters, external galaxies, brown dwarfs, and more; and search for dwarf galaxies hidden by the Galactic midplane.
We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a sample of 41 young lo... more We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a sample of 41 young low-mass stars and brown dwarfs, to study the characteristics of their disks and to search for evidence of ongoing planet formation. We also propose to use IRAC to observe an additional 43 newly-identified brown dwarfs in Taurus and Chamaeleon, for future IRS spectroscopic followup. This guaranteed-time observing program has a total of 50.1 hours, with 14 hours provided by IRAC PI Giovanni Fazio, and 36.1 hours by IRS PI Jim Houck.
We present multi-band photometry of 185 type-Ia supernovae (SN Ia), with over 11500 observations.... more We present multi-band photometry of 185 type-Ia supernovae (SN Ia), with over 11500 observations. These were acquired between 2001 and 2008 at the F. L. Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics (CfA). This sample contains the largest number of homogeneously-observed and reduced nearby SN Ia (z < 0.08) published to date. It more than doubles the nearby sample, bringing SN Ia cosmology to the point where systematic uncertainties dominate. Our natural system photometry has a precision of 0.02 mag or better in BVRIr'i' and roughly 0.04 mag in U for points brighter than 17.5 mag. We also estimate a systematic uncertainty of 0.03 mag in our SN Ia standard system BVRIr'i' photometry and 0.07 mag for U. Comparisons of our standard system photometry with published SN Ia light curves and comparison stars, where available for the same SN, reveal agreement at the level of a few hundredths mag in most cases. We find that 1991bg-like SN Ia are sufficiently distinct from other SN Ia in their color and light-curve-shape/luminosity relation that they should be treated separately in light-curve/distance fitter training samples. The CfA3 sample will contribute to the development of better light-curve/distance fitters, particularly in the few dozen cases where near-infrared photometry has been obtained and, together, can help disentangle host-galaxy reddening from intrinsic supernova color, reducing the systematic uncertainty in SN Ia distances due to dust.
We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in... more We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 micron data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (Ic) and/or near-infrared (JKs) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence (PMS) eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 & 4.5 micron variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009, and we highlight our light curves for AA-Tau analogs - YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.
We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a large sample of young... more We propose to obtain IRS complete (5-40 micron) low-resolution spectra of a large sample of young stellar objects with protoplanetary disks in the Orion A and B molecular cloud complexes. The aim is to study the influence of the wide range of star-formation conditions in the Orion clouds on the structure, composition and evolution of protoplanetary disks, and on the conditions and occurrence of planet formation in these disks. This guaranteed-time observing program has a total of 100.9 hours, with 20 hours provided by IRAC PI Giovanni Fazio, 20 hours by MIPS PI George Rieke, and 60.9 hours by IRS PI Jim Houck.
We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the youn... more We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (~ 3 Myr) sigma Orionis cluster. 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. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% in the T Tauri mass range (1-0.1 msun). We find that the disk fraction does not decrease significantly toward the brown dwarf candidates (<0.1 msun). The IRAC infrared excesses found in stellar clusters and associations with and without central high mass stars are similar, suggesting that external photoevaporation is not very important in many clusters. Finally, we find no correlation between the X-ray luminosity and the disk infrared excess, suggesting that the X-rays are not strongly affected by disk accretion.
Spitzer and Caltech Submillimeter Observatory (CSO) images and spectrophotometry of epsilon Erida... more Spitzer and Caltech Submillimeter Observatory (CSO) images and spectrophotometry of epsilon Eridani at wavelengths from 3.5 to 350 um reveal new details of its bright debris disk. The 350 um map confirms the presence of a ring at r = 11-28 arcsec (35-90 AU) observed previously at longer sub-mm wavelengths. The Spitzer mid- and far-IR images do not show the ring, but rather a featureless disk extending from within a few arcsec of the star across the ring to r ~ 34 arcsec (110 AU). The spectral energy distribution (SED) of the debris system implies a complex structure. A model constrained by the surface brightness profiles and the SED indicates that the sub-mm ring emission is primarily from large (a ~ 135 um) grains, with smaller (a ~ 15 um) grains also present in and beyond the ring. The Spitzer IRS and MIPS SED-mode spectrophotometry data clearly show the presence of spatially compact excess emission at lambda > 15 um that requires the presence of two additional narrow belts of dust within the sub-mm ring's central void. The innermost belt at r ~ 3 AU is composed of silicate dust. A simple dynamical model suggests that dust produced collisionally by a population of about 11 M_Earth of planetesimals in the sub-mm ring could be the source of the emission from both in and beyond the sub-mm ring. Maintaining the inner belts and the inner edge to the sub-mm ring may require the presence of three planets in this system including the candidate radial velocity object.
Publications of The Astronomical Society of The Pacific, 2006
This paper summarizes contributions and suggestions as presented at the Chandra Workshop Star For... more This paper summarizes contributions and suggestions as presented at the Chandra Workshop Star Formation in the Era of Three Great Observatories conducted in July 2005. One of the declared goals of the workshop was to raise recognition within the star formation research community about the sensible future utilization of the space observatories Spitzer, Hubble, and Chandra in their remaining years of operation to tackle imminent questions of our understanding of stellar formation and the early evolution of stars. A white paper was generated to support the continuous and simultaneous usage of observatory time for star formation research. The contents of this paper have been presented and discussed at several other meetings during the course of 2005 and January 2006.
We propose a Spitzer Legacy Program to perform a uniform, unbiased survey of the Cygnus-X complex... more We propose a Spitzer Legacy Program to perform a uniform, unbiased survey of the Cygnus-X complex (~6x6 deg area) with IRAC and MIPS. This region contains the richest known concentration of massive protostars and the largest OB associations in the nearest 2 kpc. With this survey we can 1) analyze the evolution of high mass protostars with a large and statistically robust sample at a single distance, 2) study the role of clustering in high mass star formation, 3) study low mass star formation in a massive molecular cloud complex dominated by the energetics of ~100 O-stars, 4) assess what fraction of all young low mass stars in the nearest 2 kpc are forming in this one massive complex, and 5) provide an unbiased survey of the region and produce a legacy data set which can be used in conjunction with future studies of this region (e.g., with Herschel and JWST). The Cygnus-X survey will be an important step in constructing one of Spitzer's greatest legacies: surveying with high sensitivity and spatial resolution a representative sample of galactic star forming regions, from Bok globules to complexes containing millions of solar masses of gas and hundreds of O-stars.
The origin and evolution of the X-rays in very young stellar objects (YSOs) are not yet well unde... more The origin and evolution of the X-rays in very young stellar objects (YSOs) are not yet well understood since it is very hard to observe YSOs in the protostellar phase. We study the X-ray properties of Class 0-I objects in the Orion Nebula Cluster (ONC) and compare them with those of the more evolved Class II and III members. Using Chandra Orion Ultradeep Project (COUP) data, we study the X-ray properties of stars in different evolutionary classes: luminosities, NH, temperatures and time variability are compared in order to understand if the interaction between the circumstellar material and the central object can influence the X-ray emission. We have assembled the deepest and most complete photometric catalog of objects in the ONC region from the UV to 8 microns using data from HST, WFI@2.2m ESO and ISPI@4m CTIO telescopes, and Spitzer IRAC. We select high probability candidate Class 0-I protostars, distinguishing between those having a spectral energy distribution which rises from K up to 8 microns (Class 0-Ia) from those where the SED rises from K up to 4.5 microns and decreasing afterwards (Class 0-Ib). We select a sample of bona fide Class II stars and a set of Class III stars with IR emission consistent with normal photospheres. Our principal result is that Class 0-Ia objects are significantly less luminous in X-rays, both in the total and hard bands, than the more evolved Class II stars with mass larger than 0.5 Msun; these latter show X-ray luminosities similar to those of Class 0-Ib stars. This result supports the hypothesis that the onset of X-ray emission occurs at a very early stage of star formation. Temporal variability and spectral properties of Class 0-I stars are similar to those of the more evolved Class II and III objects, except for a larger absorption likely due to gas in the circumstellar material.
We present multi-wavelength optical and infrared photometry of 170 previously known low mass star... more We present multi-wavelength optical and infrared photometry of 170 previously known low mass stars and brown dwarfs of the 5 Myr Collinder 69 cluster (Lambda Orionis). The new photometry supports cluster membership for most of them, with less than 15% of the previous candidates identified as probable non-members. The near infrared photometry allows us to identify stars with IR excesses, and we find that the Class II population is very large, around 25% for stars (in the spectral range M0 - M6.5) and 40% for brown dwarfs, down to 0.04 Msun, despite the fact that the H(alpha) equivalent width is low for a significant fraction of them. In addition, there are a number of substellar objects, classified as Class III, that have optically thin disks. The Class II members are distributed in an inhomogeneous way, lying preferentially in a filament running toward the south-east. The IR excesses for the Collinder 69 members range from pure Class II (flat or nearly flat spectra longward of 1 micron), to transition disks with no near-IR excess but excesses beginning within the IRAC wavelength range, to two stars with excess only detected at 24 micron. Collinder 69 thus appears to be at an age where it provides a natural laboratory for the study of primordial disks and their dissipation.
We will conduct a survey of the Orion A (L1641) and Orion B (l1630) Molecular Clouds in all four ... more We will conduct a survey of the Orion A (L1641) and Orion B (l1630) Molecular Clouds in all four IRAC bands. From this survey we can achieve the following: 1.) probe the spatial distribution of young stars in the Orion clouds, comparing the number of clustered and isolated stars, 2.) study the evolution of circumstellar disks and envelopes during the first few millions years of pre--main sequence evolution, and 3.) measure the Orion cloud IMF down to 10-20 Mjupiter.
The Workshop “12 Questions on Star and Massive Star Cluster Formation” was held in Garching from ... more The Workshop “12 Questions on Star and Massive Star Cluster Formation” was held in Garching from 3 to 6 July 2007. The programme was set up to allow long (and fruitful) discussions around several questions connecting the formation of stars and star clusters. Here we summarise some of the discussions, and encourage interest- ed readers to download the contributions from http://www.eso.org/star07.
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Papers by Tom Megeath