Showing papers by "Massimo Marengo published in 2006"

UBVRI Light Curves of 44 Type Ia Supernovae

Abstract: We present UBVRI photometry of 44 Type Ia supernovae (SNe Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to the B band.

Spitzer IRAC Photometry of M, L, and T Dwarfs

Abstract: We present the results of a program to acquire photometry for 86 late M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M_(5.8) versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one-parameter family in T_(eff). Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of 5 (~70M_J to ~15M_J), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 μm fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 μm. This suggests that equilibrium CH_4/CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of K_s, IRAC 3.6 μm, and 4.5 μm bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M_k versus K_s-[4.5] relation, which shows a smooth progression over spectral type, and splits the M, L, and T types cleanly.

Identifying Primordial Substructure in NGC 2264

Abstract: We present new Spitzer Space Telescope observations of the young cluster NGC 2264. Observations at 24 μm with the Multiband Imaging Photometer have enabled us to identify the most highly embedded and youngest objects in NGC 2264. This Letter reports on one particular region of NGC 2264 where bright 24 μm sources are spatially configured in curious linear structures with quasi-uniform separations. The majority of these sources (~60%) are found to be protostellar in nature, with Class I spectral energy distributions. Comparison of their spatial distribution with submillimeter data from Wolf-Chase et al. and millimeter data from Peretto et al. shows a close correlation between the dust filaments and the linear spatial configurations of the protostars, indicating that star formation is occurring primarily within dense, dusty filaments. Finally, the quasi-uniform separations of the protostars are found to be comparable in magnitude to the expected Jeans length, suggesting thermal fragmentation of the dense filamentary material.

Outflows from Massive YSOs as Seen with the Infrared Array Camera

Abstract: The bipolar outflow from the massive star forming cluster in DR21 is one of the most powerful known, and in IRAC images the outflow stands out by virtue of its brightness at 4.5 um (Band 2). Indeed, IRAC images of many galactic and extragalactic star formation regions feature prominent Band 2 morphologies. We have analyzed archival ISOSWS spectra of the DR21 outflow, and compare them to updated H2 shocked and UV-excitation models. We find that H2 line emission contributes about 50% of the flux of the IRAC bands at 3.6 um, 4.5 um , and 5.8 um, and is a significant contributor to the 8.0 um band as well, and confirm that the outflow contains multiple excitation mechanisms. Other potentially strong features, in particular Br alpha and CO emission, have been suggested as contributing to IRAC fluxes in outflows, but they are weak or absent in DR21; surprisingly, there also is no evidence for strong PAH emission. The results imply that IRAC images can be a powerful detector of, and diagnostic for, outflows caused by massive star formation activity in our galaxy, and in other galaxies as well. They also suggest that IRAC color-color diagnostic diagrams may need to take into account the possible influence of these strong emission lines. IRAC images of the general ISM in the region, away from the outflow, are in approximate but not precise agreement with theoretical models.

Outflows from Massive Young Stellar Objects as Seen with the Infrared Array Camera

Abstract: The bipolar outflow from the massive star-forming cluster in DR 21 is one of the most powerful known, and in IRAC images the outflow stands out by virtue of its brightness at 4.5 ?m (band 2). Indeed, IRAC images of many Galactic and extragalactic star formation regions feature prominent band 2 morphologies. We have analyzed archival ISO SWS spectra of the DR 21 outflow and compare them to updated H2 shocked and UV excitation models. We find that H2 line emission contributes about 50% of the flux of the IRAC bands at 3.6, 4.5, and 5.8 ?m, and is a significant contributor to the 8.0 ?m band as well, and confirm that the outflow contains multiple excitation mechanisms. Other potentially strong features, in particular Br? and CO emission, have been suggested as contributing to IRAC fluxes in outflows, but they are weak or absent in DR 21; surprisingly, there also is no evidence for strong PAH emission. The results imply that IRAC images can be a powerful detector of, and diagnostic for, outflows caused by massive star formation activity in our Galaxy, and in other galaxies as well. They also suggest that IRAC color-color diagnostic diagrams may need to take into account the possible influence of these strong emission lines. IRAC images of the general ISM in the region, away from the outflow, are in approximate, but not precise, agreement with theoretical models.

The circumstellar environments of NML Cygni and the cool hypergiants

Abstract: We present high-resolution HST WFPC2 images of compact nebulosity surrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. The powerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers. We show that NML Cyg's circumstellar envelope is likely shaped by photodissociation from the powerful, nearby association Cyg OB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Per have marginally resolved envelopes. S Per's circumstellar nebula appears elongated in a northeast/southwest orientation similar to that for its OH and H2O masers, while VX Sgr is embedded in a spheroidal envelope. We find no evidence for circumstellar nebulosity around the intermediate-type hypergiants ρ Cas, HR 8752, and HR 5171a, nor around the normal M-type supergiant μ Cep. We conclude that there is no evidence for high mass loss events prior to 500–1000 years ago for these four stars.

IRACproc: a software suite for processing and analyzing Spitzer/IRAC data

Abstract: We have developed a post-Basic Calibrated Data pipeline processing software suite called "IRACproc". This package facilitates the co-addition of dithered or mapped Spitzer /IRAC data to make them ready for further analysis with application to a wide variety of IRAC observing programs. In acting as a wrapper for the Spitzer Science Center's MOPEX software, IRACproc improves the rejection of cosmic rays and other transients in the co-added data. In addition, IRACproc performs (optional) Point Spread Function (PSF) fitting, subtraction, and masking of saturated stars. The under/critically sampled IRAC PSFs are subject to large variations in shape between successive frames as a result of sub-pixel shifts from dithering or telescope jitter. IRACproc improves cosmic ray and other transient rejection by using spatial derivative images to map the locations and structure of astronomical sources. By protecting sources with a metric that accounts for these large variations in the PSFs, our technique maintains the structure and photometric reliability of the PSF, while at the same time removing transients at the lowest level. High Dynamic Range PSFs for each IRAC band were obtained by combining an unsaturated core, derived from stars in the IRAC PSF calibration project, with the wings of a number of bright stars. These PSFs have dynamic ranges of ~10 7 and cover the entire IRAC field of view. PSF subtraction can drastically reduce the light from a bright star outside the saturated region. For a bright star near the array center it is possible to detect faint sources as close as ~15-20" that would otherwise be lost in the glare. In addition, PSF fitting has been shown to provide photometry accurate to 1-2% for over-exposed stars.

Infrared observations of the helix planetary nebula

Abstract: We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRAC instrument on the Spitzer Space Telescope. The Helix is one of the closest bright PNs and therefore provides an opportunity to resolve the small-scale structure in the nebula. The emission from this PN in the 5.8 and 8 μm IRAC bands is dominated by the pure rotational lines of molecular hydrogen, with a smaller contribution from forbidden line emission such as [Ar III] in the ionized region. The IRAC images resolve the "cometary knots," which have been previously studied in this PN. The "tails" of the knots and the radial rays extending into the outer regions of the PN are seen in emission in the IRAC bands. IRS spectra on the main ring and the emission in the IRAC bands are consistent with shock-excited H_2 models, with a small (~10%) component from photodissociation regions. In the northeast arc, the H_2 emission is located in a shell outside the Hα emission.

A Spitzer IRAC Search for Substellar Companions of the Debris Disk Star ∊ Eridani

Abstract: We have used the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope to search for low-mass companions of the nearby debris disk star Eri. The star was observed in two epochs 39 days apart, with different focal plane rotation to allow the subtraction of the instrumental point-spread function, achieving a maximum sensitivity of 0.01 MJy sr-1 at 3.6 and 4.5 ?m, and 0.05 MJy sr-1 at 5.8 and 8.0 ?m. This sensitivity is not sufficient to directly detect scattered or thermal radiation from the Eri debris disk. It is, however, sufficient to allow the detection of Jovian planets with mass as low as 1MJ in the IRAC 4.5 ?m band. In this band, we detected over 460 sources within the 570 field of view of our images. To test if any of these sources could be a low-mass companion to Eri, we have compared their colors and magnitudes with models and photometry of low-mass objects. Of the sources detected in at least two IRAC bands, none fall into the range of mid-IR color and luminosity expected for cool, 1 Gyr substellar and planetary mass companions of Eri, as determined by both models and observations of field M, L, and T dwarfs. We identify three new sources that have detections at 4.5 ?m only, the lower limit placed on their [3.6]-[4.5] color consistent with models of planetary mass objects. Their nature cannot be established with the currently available data, and a new observation at a later epoch will be needed to measure their proper motion in order to determine if they are physically associated to Eri.

A Spitzer/IRAC Search for Substellar Companions of the Debris Disk Star epsilon Eridani

Abstract: We have used the InfraRed Array Camera (IRAC) onboard the Spitzer Space telescope to search for low mass companions of the nearby debris disk star epsilon Eridani. The star was observed in two epochs 39 days apart, with different focal plane rotation to allow the subtraction of the instrumental Point Spread Function, achieving a maximum sensitivity of 0.01 MJy/sr at 3.6 and 4.5 um, and 0.05 MJy/sr at 5.8 and 8.0 um. This sensitivity is not sufficient to directly detect scattered or thermal radiation from the epsilon Eridani debris disk. It is however sufficient to allow the detection of Jovian planets with mass as low as 1 MJ in the IRAC 4.5 um band. In this band, we detected over 460 sources within the 5.70 arcmin field of view of our images. To test if any of these sources could be a low mass companion to epsilon Eridani, we have compared their colors and magnitudes with models and photometry of low mass objects. Of the sources detected in at least two IRAC bands, none fall into the range of mid-IR color and luminosity expected for cool, 1 Gyr substellar and planetary mass companions of epsilon Eridani, as determined by both models and observations of field M, L and T dwarf. We identify three new sources which have detections at 4.5 um only, the lower limit placed on their [3.6]-[4.5] color consistent with models of planetary mass objects. Their nature cannot be established with the currently available data and a new observation at a later epoch will be needed to measure their proper motion, in order to determine if they are physically associated to epsilon Eridani.

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

Abstract: 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, using 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 times 104 yr. 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.

Spitzer and Magellan Observations of NGC 2264: A Remarkable Star Forming Core Near IRS-2

Abstract: We analyze {\it 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.

Resolving the Dusty Circumstellar Structure of the Enigmatic Symbiotic Star CH Cygni with the MMT Adaptive Optics System

Abstract: We imaged the symbiotic star CH Cyg and two PSF calibration stars using the unique 6.5 m MMT deformable secondary adaptive optics system. Our high-resolution (FWHM = 03), very high Strehl (98% ± 2%), mid-infrared (9.8 and 11.7 μm) images of CH Cyg allow us to probe finer length scales than ever before for this object. CH Cyg is significantly extended compared to our unresolved PSF calibration stars (μ UMa and α Her) at 9.8 and 11.7 μm. We estimated the size of the extension by convolving a number of simple Gaussian models with the μ UMa PSF and determining which model provided the best fit to the data. Adopting the Hipparcos distance for this object of 270 pc, we found a nearly Gaussian extension with a FWHM at 9.8 μm of ~40.5 ± 2.7 AU (015 ± 001) and a FWHM at 11.7 μm of 45.9 ± 2.7 AU (017 ± 001). After subtracting out the Gaussian component of the emission (convolved with our PSF), we found a faint ~ 07 asymmetric extension, which peaks in flux ~05 north of the stars. This extension is roughly coincident with the northern knotlike feature seen in HST WFPC2 images obtained in 1999.

Spitzer/IRAC Photometry of M, L, and T Dwarfs

Abstract: We present the results of a program to acquire photometry for eighty-six late-M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M_5.8 versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one parameter family in Teff. Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of five (~70 M_Jup to ~15 M_Jup), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 micron fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 microns. This suggests that equilibrium CH_4/CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of Ks, IRAC 3.6 micron, and 4.5 micron bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M_Ks versus Ks-[4.5] relation, which shows a smooth progression over spectral type and splits the M, L, and T types cleanly.

Discovery of Two T Dwarf Companions with the Spitzer Space Telescope

Abstract: We report the discovery of T dwarf companions to the nearby stars HN Peg (G0V, 18.4 pc, ~0.3 Gyr) and HD 3651 (K0V, 11.1 pc, ~7 Gyr). During an ongoing survey of 5'x5' fields surrounding stars in the solar neighborhood with IRAC aboard the Spitzer Space Telescope, we identified these companions as candidate T dwarfs based on their mid-IR colors. Using near-IR spectra obtained with SpeX at the NASA IRTF, we confirm the presence of methane absorption that characterizes T dwarfs and measure spectral types of T2.5+/-0.5 and T7.5+/-0.5 for HN Peg B and HD 3651 B, respectively. By comparing our Spitzer data to images from 2MASS obtained several years earlier, we find that the proper motions of HN Peg B and HD 3651 B are consistent with those of the primaries, confirming their companionship. HN Peg B and HD 3651 B have angular separations of 43.2" and 42.9" from their primaries, which correspond to projected physical separations of 795 and 476 AU, respectively. A comparison of their luminosities to the values predicted by theoretical evolutionary models implies masses of 0.021+/-0.009 and 0.051+/-0.014 Msun for HN Peg B and HD 3651 B. In addition, the models imply an effective temperature for HN Peg B that is significantly lower than the values derived for other T dwarfs at similar spectral types, which is the same behavior reported by Metchev & Hillenbrand for the young late-L dwarf HD 203030 B. Thus, the temperature of the L/T transition appears to depend on surface gravity. Meanwhile, HD 3651 B is the first substellar companion directly imaged around a star that is known to harbor a close-in planet from RV surveys. The discovery of this companion supports the notion that the high eccentricities of close-in planets like the one near HD 3651 may be the result of perturbations by low-mass companions at wide separations.

Identification of the Microlens in Event MACHO-LMC-20

Abstract: We report on the identification of the lens responsible for microlensing event MACHO-LMC-20. As part of a Spitzer IRAC program conducting mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields, we discovered a significant flux excess at the position of the source star for this event. These data, in combination with high-resolution near-infrared Magellan PANIC data, have allowed us to classify the lens as an early M dwarf in the thick disk of the Milky Way, at a distance of ~2 kpc. This is only the second microlens to have been identified, the first also being a M dwarf star in the disk. Together, these two events are still consistent with the expected frequency of nearby stars in the Milky Way thin and thick disks acting as lenses.

The Identification of the Microlens in Event MACHO-LMC-20

Abstract: We report on the identification of the lens responsible for microlensing event MACHO-LMC-20. As part of a Spitzer IRAC program conducting mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields, we discovered a significant flux excess at the position of the source star for this event. These data, in combination with high-resolution near-infrared Magellan PANIC data, have allowed us to classify the lens as an early M dwarf in the thick disk of the Milky Way, at a distance of ~2 kpc. This is only the second microlens to have been identified, the first also being a M dwarf star in the disk. Together, these two events are still consistent with the expected frequency of nearby stars in the Milky Way thin and thick disks acting as lenses.

Spitzer/IRAC Characterization of Galactic AGB Stars

Abstract: The Spitzer Space Telescope and in particular its InfraRed Array Camera (IRAC) is an ideal facility to study the distribution of AGB stars in our own and other galaxies because of its efficiency in surveying vast areas of the sky and its ability to detect sources with infrared excess. The IRAC colors of AGB stars, however, are not well known because cool stars have numerous molecular absorption features in the spectral region covered by the IRAC photometric system. The presence and strength of these features depends on the chemistry of the stellar atmosphere and the mass loss rate and can change with time due to the star's variability. To characterize the IRAC colors of AGB stars, we are carrying out a Spitzer Guaranteed Time Observation program to observe a sample of AGB stars with IRAC. The results will be made available to the community in the form of template magnitudes and colors for each target with the goal of aiding the identification of AGB stars in already available and future IRAC surveys. We present here the first results of this project.

Resolving the Dusty Circumstellar Structure of the Enigmatic Symbiotic Star CH Cygni with the MMT Adaptive Optics System

Abstract: We imaged the symbiotic star CH Cyg and two PSF calibration stars using the unique 6.5m MMT deformable secondary adaptive optics system. Our high-resolution (FWHM=0.3"), very high Strehl (98%+-2%) mid-infrared (9.8 and 11.7 um) images of CH Cyg allow us to probe finer length scales than ever before for this object. CH Cyg is significantly extended compared to our unresolved PSF calibration stars (Mu UMa and Alpha Her) at 9.8 and 11.7 um. We estimated the size of the extension by convolving a number of simple Gaussian models with the Mu UMa PSF and determining which model provided the best fit to the data. Adopting the Hipparcos distance for this object of 270 pc, we found a nearly Gaussian extension with a FWHM at 9.8 um of ~40.5+-2.7 AU (0.15+-0.01") and a FWHM at 11.7 um of 45.9+-2.7 AU (0.17+-0.01"). After subtracting out the Gaussian component of the emission (convolved with our PSF), we found a faint \~0.7" asymmetric extension which peaks in flux ~0.5" north of the stars. This extension is roughly coincident with the northern knotlike feature seen in HST WFPC2 images obtained in 1999.

Identification of the Microlens in Event MACHO-LMC-20

Abstract: We report on the identification of the lens responsible for microlensing event MACHO-LMC-20. As part of a \textit{Spitzer}/IRAC program conducting mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields, we discovered a significant flux excess at the position of the source star for this event. These data, in combination with high resolution near-infrared \textit{Magellan}/PANIC data has allowed us to classify the lens as an early M dwarf in the thick disk of the Milky Way, at a distance of $\sim 2$ kpc. This is only the second microlens to have been identified, the first also being a M dwarf star in the disk. Together, these two events are still consistent with the expected frequency of nearby stars in the Milky Way thin and thick disks acting as lenses.