Showing papers by "Joseph L. Hora published in 2009"

Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud

Abstract: We present a catalog of 1750 massive stars in the Large Magellanic Cloud (LMC), with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 1268 of these stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE survey database, for which we present uniform photometry from 0.3 to 24 μm in the UBVIJHKs +IRAC+MIPS24 bands. The resulting infrared color-magnitude diagrams illustrate that the supergiant B[e], red supergiant, and luminous blue variable (LBV) stars are among the brightest infrared point sources in the LMC, due to their intrinsic brightness, and at longer wavelengths, due to dust. We detect infrared excesses due to free-free emission among ~900 OB stars, which correlate with luminosity class. We confirm the presence of dust around 10 supergiant B[e] stars, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs. The similar luminosities of B[e] supergiants (log L/L ☉ ≥ 4) and the rare, dusty progenitors of the new class of optical transients (e.g., SN 2008S and NGC 300 OT), plus the fact that dust is present in both types of objects, suggests a common origin for them. We find the infrared colors for Wolf-Rayet stars to be independent of spectral type and their SEDs to be flatter than what models predict. The results of this study provide the first comprehensive roadmap for interpreting luminous, massive, resolved stellar populations in nearby galaxies at infrared wavelengths.

THE MASS LOSS RETURN FROM EVOLVED STARS TO THE LARGE MAGELLANIC CLOUD: EMPIRICAL RELATIONS FOR EXCESS EMISSION AT 8 AND 24 μm

Abstract: We present empirical relations describing excess emission from evolved stars in the Large Magellanic Cloud (LMC) using data from the Spitzer Space Telescope Surveying the Agents of a Galaxy's Evolution (SAGE) survey which includes the Infrared Array Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 {mu}m and Multiband Imaging Photometer (MIPS) 24, 70, and 160 {mu}m bands. We combine the SAGE data with the Two Micron All Sky Survey (2MASS; J, H, and K {sub s}) and the optical Magellanic Cloud Photometric Survey (MCPS; U, B, V, and I) point source catalogs in order to create complete spectral energy distributions (SEDs) of the asymptotic giant branch (AGB) star candidates in the LMC. AGB star outflows are among the main producers of dust in a galaxy, and this mass loss results in an excess in the fluxes observed in the 8 and 24 {mu}m bands. The aim of this work is to investigate the mass loss return by AGB stars to the interstellar medium of the LMC by studying the dependence of the infrared excess flux on the total luminosity. We identify oxygen-rich, carbon-rich, and extreme AGB star populations in our sample based on their 2MASS and IRAC colors. The SEDs ofmore » oxygen- and carbon-rich AGB stars are compared with appropriate stellar photosphere models to obtain the excess flux in all the IRAC bands and the MIPS 24 {mu}m band. Extreme AGB stars are dominated by circumstellar emission at 8 and 24 {mu}m; thus we approximate their excesses with the flux observed in these bands. We find about 16,000 O-rich, 6300 C-rich, and 1000 extreme sources with reliable 8 {mu}m excesses, and about 4500 O-rich, 5300 C-rich, and 960 extreme sources with reliable 24 {mu}m excesses. The excesses are in the range 0.1 mJy to 5 Jy. The 8 and 24 {mu}m excesses for all three types of AGB candidates show a general increasing trend with luminosity. The color temperature of the circumstellar dust derived from the ratio of the 8 and 24 {mu}m excesses decreases with an increase in excess, while the 24 {mu}m optical depth increases with excess. The extreme AGB candidates are the major contributors to the mass loss, and we estimate the total AGB mass loss return to the LMC to be (5.9-13) x 10{sup -3} M {sub sun} yr{sup -1}.« less

The Mass-Loss Return From Evolved Stars to the LMC: Empirical Relations for Excess Emission at 8 and 24 \mu m

Abstract: We present empirical relations describing excess emission from evolved stars in the Large Magellanic Cloud (LMC) using data from the SAGE (Surveying the Agents of a Galaxy's Evolution) survey which includes the IRAC 3.6, 4.5, 5.8 and 8.0 \mu m and MIPS 24, 70 and 160 \mu m bands. We combine the SAGE data with the Two Micron All Sky Survey (2MASS; J, H and Ks) and the optical Magellanic Cloud Photometric Survey (MCPS; U, B, V and I) point source catalogs to create complete spectral energy distributions (SEDs) of the asymptotic giant branch (AGB) star candidates in the LMC. AGB star outflows are among the main producers of dust in a galaxy, and this mass loss results in an excess in the fluxes observed in the 8 and 24 \mic m bands. We identify oxygen-rich, carbon-rich and extreme AGB star populations in our sample based on their 2MASS and IRAC colors. We calculate excesses fluxes in the mid-IR bands by comparison of the SEDs with model photospheres. We find about 16,000 O-rich, 6300 C-rich and 1000 extreme sources with reliable 8 \mu m excesses, and about 4500 O-rich, 5300 C-rich and 960 extreme sources with reliable 24 \mic m excesses. The excesses are in the range 0.1 mJy--5 Jy. The 8 and 24 \mic m excesses for all three types of AGB candidates show a general increasing trend with luminosity. The dust color temperature derived from the ratio of the 8 and 24 \mic m excesses decreases with an increase in excess, while the 24 \mic m optical depth increases with excess. The extreme AGB candidates are the major contributors to the mass loss, and we estimate the total AGB mass-loss return to the LMC to be (5.9--13)\times 10^{-3} Msun yr^{-1}.

DETECTION OF THERMAL EMISSION OF XO-2b: EVIDENCE FOR A WEAK TEMPERATURE INVERSION

Abstract: We estimate flux ratios of the extrasolar planet XO-2b to its host star XO-2 at 3.6, 4.5, 5.8, and 8.0 μm with Infrared Array Camera on the Spitzer Space Telescope to be 0.00081 ± 0.00017, 0.00098 ± 0.00020, 0.00167 ± 0.00036, and 0.00133 ± 0.00049, respectively. The fluxes provide tentative evidence for a weak temperature inversion in the upper atmosphere, the precise nature of which would need to be confirmed by longer wavelength observations. XO-2b substellar flux of 0.76 × 109 erg cm–2 s–1 lies in the predicted transition region between atmospheres with and without upper atmospheric temperature inversion.

A spectroscopic study of young stellar objects in the serpens cloud core and ngc 1333

Abstract: We present spectral observations of 130 young stellar objects (YSOs) in the Serpens Cloud Core and NGC 1333 embedded clusters. The observations consist of near-IR spectra in the H and K bands from SpeX on the IRTF and far-red spectra (6000-9000 A) from Hectospec on the Multi-Mirror Telescope. These YSOs were identified in previous Spitzer and Chandra observations, and the evolutionary classes of the YSOs were determined from the Spitzer mid-IR photometry. With these spectra we search for corroborating evidence for the pre-main-sequence nature of the objects, study the properties of the detected emission lines as a function of evolutionary class, and obtain spectral types for the observed YSOs. The temperatures implied by the spectral types are combined with luminosities determined from the near-IR photometry to construct Hertzsprung-Russell (H-R) diagrams for the clusters. By comparing the positions of the YSOs in the H-R diagrams with the pre-main-sequence tracks of Baraffe (1998), we determine the ages of the embedded sources and study the relative ages of the YSOs with and without optically thick circumstellar disks. The apparent isochronal ages of the YSOs in both clusters range from less than 1 Myr to 10 Myr, with most objects below 3 Myr. Themore » observed distributions of ages for the Class II and Class III objects are statistically indistinguishable. We examine the spatial distribution and extinction of the YSOs as a function of their isochronal ages. We find the sources 3 Myr show all the characteristics of YSOs in their spectra, their IR spectral energy distributions, and their X-ray emission; we find no evidence that they are contaminating background giants or foreground dwarfs. However, we find no corresponding decrease in the fraction of sources with infrared excess with isochronal age; this suggests that the older isochronal ages may not measure the true age of the >3 Myr YSOs. Thus, the nature of the apparently older sources and their implications for cluster formation remain unresolved.« less

Variable evolved stars and young stellar objects discovered in the large magellanic cloud using the sage survey

Abstract: We present initial results and source lists of variable sources in the Large Magellanic Cloud (LMC) for which we detect thermal infrared variability from the Surveying the Agents of a Galaxy's Evolution (SAGE) survey, which had two epochs of photometry separated by 3 months. The SAGE survey mapped a 7° × 7° region of the LMC using the Infrared Array Camera (IRAC) and the MIPS instruments on board Spitzer. Variable sources are identified using a combination of the IRAC 3.6, 4.5, 5.8, 8.0 μ bands and the MIPS 24 μ bands. An error-weighted flux difference between the two epochs is used to assess the variability. Of the ~3 million sources detected at both epochs, we find ~2000 variable sources for which we provide electronic catalogs. Most of the variable sources can be classified as asymptotic giant branch (AGB) stars. A large fraction (>66%) of the extreme AGB stars are variable and only smaller fractions of carbon-rich (6.1%) and oxygen-rich (2.0%) stars are detected as variable sources. We also detect a population of variable young stellar object candidates.

Spitzer/IRAC Limits to Planetary Companions of Fomalhaut and epsilon Eridani

Abstract: Fomalhaut and epsilon Eridani are two young, nearby stars that possess extended debris disks whose structures suggest the presence of perturbing planetary objects. With its high sensitivity and stable point spread function, Spitzer/IRAC is uniquely capable of detecting cool, Jupiter-like planetary companions whose peak emission is predicted to occur near 4.5 um. We report on deep IRAC imaging of these two stars, taken at 3.6 and 4.5 um using subarray mode and in all four channels in wider-field full array mode. Observations acquired at two different telescope roll angles allowed faint surrounding objects to be separated from the stellar diffraction pattern. No companion candidates were detected at the reported position of Fomalhaut b with 3 sigma model-dependent mass upper limits of 3 MJ (for an age of 200 Myr). Around epsilon Eridani we instead set a limit of 4 and <1 MJ (1 Gyr model age) at the inner and outer edge of the sub-millimeter debris ring, respectively. These results are consistent with non-detections in recent near-infrared imaging searches, and set the strongest limits to date on the presence of planets outside epsilon Eridani sub-millimeter ring.

SPITZER/INFRARED ARRAY CAMERA LIMITS TO PLANETARY COMPANIONS OF FOMALHAUT AND ϵ ERIDANI

Abstract: Fomalhaut and Eridani are two young, nearby stars that possess extended debris disks whose structures suggest the presence of perturbing planetary objects. With its high sensitivity and stable point-spread function, Spitzer/Infrared Array Camera (IRAC) is uniquely capable of detecting cool, Jupiter-like planetary companions whose peak emission is predicted to occur near 4.5 ?m. We report on deep IRAC imaging of these two stars, taken at 3.6 and 4.5 ?m using subarray mode and in all four channels in wider-field full array mode. Observations acquired at two different telescope roll angles allowed faint surrounding objects to be separated from the stellar diffraction pattern. No companion candidates were detected at the reported position of Fomalhaut b with 3? model-dependent mass upper limits of 3M J (for an age of 200 Myr). Around Eridani, we instead set a limit of 4 and 1M J (1 Gyr model age) at the inner and outer edge of the submillimeter debris ring, respectively. These results are consistent with non-detections in recent near-infrared imaging searches, and set the strongest limits to date on the presence of planets outside Eridani submillimeter ring.

Spitzer detection of polycyclic aromatic hydrocarbons and silicate features in post-agb stars and young planetary nebulae

Abstract: We have observed a small sample of hot post-asymptotic giant branch (AGB) stars with the Infrared Array Camera (IRAC) and the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The stars were selected from the literature on the basis of their far-infrared (IR) excess (i.e., post-AGB candidates) and B spectral type (i.e., close to the ionization of the envelope). The combination of our IRAC observations with Two Micron All Sky Survey and IRAS catalog data, along with previous radio observations in the cm range (where available) allowed us to model the spectral energy distributions of our targets and find that in almost all of them at least two shells of dust at different temperatures must be present, the hot dust component ranging up to 103 K. In several targets, grains larger than 1 μm are needed to match the far-IR data points. In particular, in IRAS 17423-1755 grains up to 100 μm must be introduced to match the emission in the millimeter range. We obtained IRS spectra to identify the chemistry of the envelopes and found that more than one-third of the sources in our sample have mixed chemistry, showing both mid-IR bands attributed to polycyclic aromatic hydrocarbons (PAHs) and silicate features. The analysis of the PAH features indicates that these molecules are located in the outflows, far away from the central stars. We consider the larger than expected percentage of mixed-chemistry targets as a selection bias toward stars with a disk or torus around them. Our results strengthen the current picture of mixed chemistry being due to the spatial segregation of different dust populations in the envelopes.

A spatially resolved study of photoelectric heating and (C II) cooling in the LMC Comparison with dust emission as seen by SAGE

Abstract: Context. Photoelectric heating is a dominant heating mechanism for many phases of the interstellar medium. We study this mechanism throughout the Large Magellanic Cloud (LMC). Aims. We aim to quantify the importance of the [C II] cooling line and the photoelectric heating process of various environments in the LMC and to investigate which parameters control the extent of photoelectric heating. Methods. We use the BICE [C II] map and the Spitzer/SAGE infrared maps. We examine the spatial variations in the efficiency of photoelectric heating: photoelectric heating rate over power absorbed by grains, i.e. the observed [C II] line strength over the integrated infrared emission. We correlate the photoelectric heating efficiency and the emission from various dust constituents and study the variations as a function of Ha emission, dust temperatures, and the total infrared luminosity. The observed variations are interpreted in a theoretical framework. From this we estimate radiation field, gas temperature, and electron density. Results. We find systematic variations in photoelectric efficiency. The highest efficiencies are found in the diffuse medium, while the lowest coincide with bright star-forming regions (similar to 1.4 times lower). The [C II] line emission constitutes 1.32% of the far infrared luminosity across the whole of the LMC. We find correlations between the [C II] emission and ratios of the mid infrared and far infrared bands, which comprise various dust constituents. The correlations are interpreted in light of the spatial variations of the dust abundance and by the local environmental conditions that affect the dust emission properties. As a function of the total infrared surface brightness, S-TIR, the [C II] surface brightness can be described as: S [C II] = 1.25 S-TIR(0.69) [10(-3) erg s(-1) cm(-2) sr(-1)], for S-TIR greater than or similar to 3.2 x 10(-4) erg s(-1) cm(-2) sr(-1). We provide a simple model of the photoelectric efficiency as a function of the total infrared luminosity. We find a power-law relation between radiation field and electron density, consistent with other studies. The [ C II] emission is well-correlated with the 8 mu m emission, suggesting that the polycyclic aromatic hydrocarbons play a dominant role in the photoelectric heating process.

Spitzer 24 um Images of Planetary Nebulae

Abstract: Spitzer MIPS 24 um images were obtained for 36 Galactic planetary nebulae (PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective temperatures of ~100,000 K or higher. Diffuse 24 um emission is detected in 28 of these PNe. The eight non-detections are angularly large PNe with very low H-alpha surface brightnesses. We find three types of correspondence between the 24 um emission and H-alpha line emission of these PNe: six show 24 um emission more extended than H-alpha emission, nine have a similar extent at 24 um and H-alpha, and 13 show diffuse 24 um emission near the center of the H-alpha shell. The sizes and surface brightnesses of these three groups of PNe and the non-detections suggest an evolutionary sequence, with the youngest ones being brightest and the most evolved ones undetected. The 24 um band emission from these PNe is attributed to [O IV] 25.9 um and [Ne V] 24.3 um line emission and dust continuum emission, but the relative contributions of these three components depend on the temperature of the central star and the distribution of gas and dust in the nebula.

Spitzer 24 μm images of planetary nebulae

Abstract: Spitzer MIPS 24 μm images were obtained for 36 Galactic planetary nebulae (PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective temperatures of ~100,000 K or higher. Diffuse 24 μm emission is detected in 28 of these PNe. The eight nondetections are angularly large PNe with very low Hα surface brightnesses. We find three types of correspondence between the 24 μm emission and Hα line emission of these PNe: six show 24 μm emission more extended than Hα emission, nine have a similar extent at 24 μm and Hα, and 13 show diffuse 24 μm emission near the center of the Hα shell. The sizes and surface brightnesses of these three groups of PNe and the nondetections suggest an evolutionary sequence, with the youngest ones being brightest and the most evolved ones undetected. The 24 μm band emission from these PNe is attributed to [O IV] 25.9 μm and [Ne V] 24.3 μm line emission and dust continuum emission, but the relative contributions of these three components depend on the temperature of the central star and the distribution of gas and dust in the nebula.

Spatial Variations of Dust Abundances Across the Large Magellanic Cloud

Abstract: Using the data obtained with the Spitzer Space Telescope as part of the Surveying the Agents of a Galaxy's Evolution (SAGE) legacy survey, we have studied the variations of the dust composition and abundance across the Large Magellanic Cloud (LMC). Such variations are expected, as the explosive events which have lead to the formation of the many H I shells observed should have affected the dust properties. Using a model and comparing with a reference spectral energy distribution from our Galaxy, we deduce the relative abundance variations of small dust grains across the LMC. We examined the infrared color ratios as well as the relative abundances of very small grains (VSGs) and polycyclic aromatic hydrocarbons (PAHs) relative to the big grain abundance. Results show that each dust component could have different origins or evolution in the interstellar medium (ISM). The VSG abundance traces the star formation activity and could result from shattering of larger grains, whereas the PAH abundance increases around molecular clouds as well as in the stellar bar, where they could have been injected into the ISM during mass loss from old stars.

Spitzer detection of pah and silicate features in post-agb stars and young planetary nebulae

Abstract: We have observed a small sample of hot post-AGB stars with the InfraRed Array Camera (IRAC) and the InfraRed Spectrograph (IRS) on-board the Spitzer Space Telescope The stars were selected from the literature on the basis of their far-Infrared excess (ie, post-AGB candidates) and B spectral type (ie, close to the ionization of the envelope) The combination of our IRAC observations with 2MASS and IRAS catalog data, along with previous radio observations in the cm range (where available) allowed us to model the SEDs of our targets and find that in almost all of them at least two shells of dust at different temperatures must be present, the hot dust component ranging up to 10 3 K In several targets grains larger than 1 µm are needed to match the far-IR data points In particular, in IRAS 17423-1755 grains up to 100 µm must be introduced to match the emission in the mm range We obtained IRS spectra to identify the chemistry of the envelopes and found that more than 1 /3 of the sources in our sample have mixed chemistry, showing both mid-IR bands attributed to Polycyclic Aromatic Hydrocarbons (PAH) and silicate features The analysis of the PAH features indicates that these molecules are located in the outflows, far away from the central stars We consider the larger than expected percentage of mixed-chemistry targets as a selection bias towards stars with a disk or torus around them Our results strengthen the current picture of mixed chemistry being due to the spatial segregation of different dust populations in the envelopes Subject headings: Infrared: general, Stars: AGB and post-AGB, Planetary Nebulae: general

Physical Conditions in the Ionized Gas of 30 Doradus

Abstract: We present a mid-infrared spectroscopic data cube of the central part of 30 Doradus, observed with Spitzer’s IRS and MIPS/SED mode. Aromatic dust emission features and emission lines from molecular and atomic hydrogen are detected but not particularly strong. The dominant spectral features are emission lines from moderately ionized species of argon, neon, and sulphur, which are used to determine the physical conditions in the ionized gas. The ionized gas excitation shows strong variations on parsec scales, some of which can plausibly be associated with individual hot stars. We fit the ionic line strengths with photoionization and shock models, and find that photoionization dominates in the region. The ionization parameter U traces the rim of the central bubble, as well as highlighting isolated sources of ionization, and at least one quiescent clump. The hardness of the ionizing radiation field Trad reveals several “hot spots” that are either the result of individual very hot stars or trace the propagation of the diffuse ionizing field through the surrounding neutral cloud. Consistent with other measurements of giant H II regions, log(U) ranges between -3 and -0.75, and Trad between 30000 and 85000K.

The Dust-to-Gas Ratio in the Small Magellanic Cloud Tail

Abstract: The Tail region of the Small Magellanic Cloud (SMC) was imaged using the MIPS instrument on the Spitzer Space Telescope as part of the SAGE-SMC Spitzer Legacy. Diffuse infrared emission from dust was detected in all the MIPS bands. The Tail gas-to-dust ratio was measured to be 1200 ± 350 using the MIPS observations combined with existing IRAS and H I observations. This gas-to-dust ratio is higher than the expected 500-800 from the known Tail metallicity indicating possible destruction of dust grains. Two cluster regions in the Tail were resolved into multiple sources in the MIPS observations and local gas-to-dust ratios were measured to be ~ 440 and ~ 250 suggest dust formation and/or significant amounts of ionized gas in these regions. These results support the interpretation that the SMC Tail is a tidal Tail recently stripped from the SMC that includes gas, dust, and young stars.

A spatially resolved study of photoelectric heating and [C II] cooling in the LMC

Abstract: Context. Photoelectric heating is a dominant heating mechanism for many phases of the interstellar medium. We study this mechanism throughout the Large Magellanic Cloud (LMC). Aims. We aim to quantify the importance of the [C II] cooling line and the photoelectric heating process of various environments in the LMC and to investigate which parameters control the extent of photoelectric heating. Methods. We use the BICE [C II] map and the Spitzer/SAGE infrared maps. We examine the spatial variations in the efficiency of photoelectric heating: photoelectric heating rate over power absorbed by grains, i.e. the observed [C II] line strength over the integrated infrared emission. We correlate the photoelectric heating efficiency and the emission from various dust constituents and study the variations as a function of H emission, dust temperatures, and the total infrared luminosity. The observed variations are interpreted in a theoretical framework. From this we estimate radiation field, gas temperature, and electron density. Results. We find systematic variations in photoelectric efficiency. The highest efficiencies are found in the diffuse medium, while the lowest coincide with bright star-forming regions (~1.4 times lower). The [C II] line emission constitutes 1.32% of the far infrared luminosity across the whole of the LMC. We find correlations between the [C II] emission and ratios of the mid infrared and far infrared bands, which comprise various dust constituents. The correlations are interpreted in light of the spatial variations of the dust abundance and by the local environmental conditions that affect the dust emission properties. As a function of the total infrared surface brightness, S_(TIR), the [C II] surface brightness can be described as: S_([C II]) = 1.25 S^(0.69)_(TIR)[10^(-3) erg s^(-1) cm^(-2) sr^(-1)], for S_(TIR) ≳ 3.2 x 10^(-4) erg s^(-1) cm^(-2) sr^(-1). We provide a simple model of the photoelectric efficiency as a function of the total infrared luminosity. We find a power-law relation between radiation field and electron density, consistent with other studies. The [C II] emission is well-correlated with the 8 µm emission, suggesting that the polycyclic aromatic hydrocarbons play a dominant role in the photoelectric heating process.

A Spectroscopic Study of Young Stellar Objects in the Serpens Cloud Core and NGC 1333

Abstract: We present spectral observations of 130 young stellar objects (YSOs) in the Serpens Cloud Core and NGC 1333 embedded clusters. The observations consist of near-IR spectra in the H and K-bands, from SpeX on the IRTF and far-red spectra (6000 - 9000 A) from Hectospec on the MMT. These YSOs were identified in previous Spitzer and Chandra observations, and the evolutionary classes of the YSOs were determined from the Spitzer mid-IR photometry. With these spectra, we search for corroborating evidence for the pre-main sequence nature of the objects, study the properties of the detected emission lines as a function of evolutionary class, and obtain spectral types for the observed YSOs. By comparing the positions of the YSOs in the HR diagrams with the pre-main sequence tracks of Baraffe (1998), we determine ages of the embedded sources and study the relative ages of the YSOs with and without optically thick circumstellar disks. The apparent isochronal ages of the YSOs in both clusters range from less than 1 Myr to 10 Myr, with most objects below 3 Myr. The observed distribution of ages for the Class II and Class III objects are statistically indistinguishable. We examine the spatial distribution and extinction of the YSOs as a function of their isochronal ages. We find the sources 3 Myr show all the characteristics of young stellar objects in their spectra, their IR spectral energy distributions, and their X-ray emission.

Dust production and mass loss in the galactic globular cluster ngc 362

Abstract: We investigate dust production and stellar mass loss in the Galactic globular cluster NGC 362. Due to its close proximity to the Small Magellanic Cloud (SMC), NGC 362 was imaged with the Infrared Array Camera and Multiband Imaging Photometer cameras onboard the Spitzer Space Telescope as part of the Surveying the Agents of Galaxy Evolution (SAGE-SMC) Spitzer Legacy program. We detect several cluster members near the tip of the red giant branch (RGB) that exhibit infrared excesses indicative of circumstellar dust and find that dust is not present in measurable quantities in stars below the tip of the RGB. We modeled the spectral energy distribution (SED) of the stars with the strongest IR excess and find a total cluster dust mass-loss rate of 3.0{sup +2.0}{sub -1.2} x 10{sup -9} M{sub sun} yr{sup -1}, corresponding to a gas mass-loss rate of 8.6{sup +5.6}{sub -3.4} x 10{sup -6} M{sub sun} yr{sup -1}, assuming [Fe/H] =-1.16. This mass loss is in addition to any dustless mass loss that is certainly occurring within the cluster. The two most extreme stars, variables V2 and V16, contribute up to 45% of the total cluster dust-traced mass loss. The SEDs of the more moderate stars indicate themore » presence of silicate dust, as expected for low-mass, low-metallicity stars. Surprisingly, the SED shapes of the stars with the strongest mass-loss rates appear to require the presence of amorphous carbon dust, possibly in combination with silicate dust, despite their oxygen-rich nature. These results corroborate our previous findings in omega Centauri.« less

The dusty nebula surrounding hr car: a spitzer view

Abstract: We present mid-IR observations of the Galactic Luminous Blue Variable (LBV) HR Car and its associated nebula carried out with the Spitzer Space Telescope using both Infrared Array Camera and Infrared Spectrograph, as part of a GTO program aimed to study stellar ejecta from evolved stars. Our observations reveal a rich mid-IR spectrum of the inner nebula showing both solid state and atomic gas signatures. Strong low-excitation atomic fine structure lines such as 26.0 μm [Fe II] and 34.8 μm [Si II], indicate, for the first time, the presence of a PDR in this object class. While the physics and chemistry of the low-excitation gas appears to be dominated by photodissociation, a possible contribution due to shocks can be inferred from the evidence of gas phase Fe abundance enhancement. The presence of amorphous silicates, inferred from the observed characteristic broad feature at 10 μm located in the inner nebula, suggests that dust has formed during the LBV outburst. This is in contrast with the detection of crystalline dust in other probably more evolved Galactic LBVs, which is similar to the crystalline dust observed in red supergiants. This has been considered to be evidence of dust production during evolutionary phases prior to the outburst.

Physical Conditions in the Ionized Gas of 30 Doradus

Abstract: We present a mid-infrared spectroscopic data cube of the central part of 30 Doradus, observed with Spitzer's IRS and MIPS/SED mode. Aromatic dust emission features and emission lines from molecular and atomic hydrogen are detected but not particularly strong. The dominant spectral features are emission lines from moderately ionized species of argon, neon, and sulphur, which are used to determine the physical conditions in the ionized gas. The ionized gas excitation shows strong variations on parsec scales, some of which can plausibly be associated with individual hot stars. We fit the ionic line strengths with photoionization and shock models, and find that photoionization dominates in the region. The ionization parameter U traces the rim of the central bubble, as well as highlighting isolated sources of ionization, and at least one quiescent clump. The hardness of the ionizing radiation field T_rad reveals several "hot spots" that are either the result of individual very hot stars or trace the propagation of the diffuse ionizing field through the surrounding neutral cloud. Consistent with other measurements of giant molecular hydrogen regions, log(U) ranges between -3 and -0.75, and T_rad between 30000 and 85000K.

Imaging the cool hypergiant nml cygni's dusty circumstellar envelope with adaptive optics

Abstract: We present subarcsecond angular resolution, high-Strehl ratio mid-IR adaptive optics images of the powerful OH/IR source and cool hypergiant NML Cyg at 8.8, 9.8, and 11.7 ?m. These images reveal once more the complexity in the dusty envelope surrounding this star. We spatially resolve the physical structures (radius ~ 014, ~240 AU adopting a distance of 1.74 kpc) responsible for NML Cyg's deep 10 ?m silicate dust absorption feature. We also detect an asymmetric excess, at separations of ~03-05 (~520-870 AU), northwest from the star. The colors of this excess are consistent with thermal emission of hot, optically thin dust. This excess is oriented in the direction of the Cyg OB2 stellar association, and is likely due to the disruption of NML Cyg's dusty wind with the near-UV radiation flux from the massive hot stars within Cyg OB2. This interaction was predicted in our previous paper to explain the geometry of an inverted photodissociation region observed at optical wavelengths.

Galaxy Clusters in the IRAC Dark Field. II. Mid-Infrared Sources

Abstract: We present infrared (IR) luminosities, star formation rates (SFR), colors, morphologies, locations, and active galactic nuclei (AGNs) properties of 24 μm detected sources in photometrically detected high-redshift clusters in order to understand the impact of environment on star formation (SF) and AGN evolution in cluster galaxies. We use three newly identified z = 1 clusters selected from the IRAC dark field; the deepest ever mid-IR survey with accompanying, 14 band multiwavelength data including deep Hubble Space Telescope imaging and deep wide-area Spitzer MIPS 24 μm imaging. We find 90 cluster members with MIPS detections within two virial radii of the cluster centers, of which 17 appear to have spectral energy distributions dominated by AGNs and the rest dominated by SF. We find that 43% of the star-forming sample have IR luminosities L_(IR) > 10^(11) L_☉(luminous IR galaxies). The majority of sources (81%) are spirals or irregulars. A large fraction (at least 25%) show obvious signs of interactions. The MIPS-detected member galaxies have varied spatial distributions as compared to the MIPS-undetected members with one of the three clusters showing SF galaxies being preferentially located on the cluster outskirts, while the other two clusters show no such trend. Both the AGN fraction and the summed SFR of cluster galaxies increase from redshift zero to one, at a rate that is a few times faster in clusters than over the same redshift range in the field. Cluster environment does have an effect on the evolution of both AGN fraction and SFR from redshift one to the present, but does not affect the IR luminosities or morphologies of the MIPS sample. SF happens in the same way regardless of environment making MIPS sources look the same in the cluster and field, however the cluster environment does encourage a more rapid evolution with time as compared to the field.

The infrared array camera dark field: far-infrared to x-ray data

Abstract: We present 20 band photometry from the far-IR to X-ray in the Spitzer Infrared Array Camera (IRAC) dark field. The bias for the near-IR camera on Spitzer is calibrated by observing a ~20' diameter "dark" field near the north ecliptic pole roughly every two-to-three weeks throughout the mission duration of Spitzer. The field is unique for its extreme depth, low background, high quality imaging, time-series information, and accompanying photometry including data taken with Akari, Palomar, MMT, KPNO, Hubble, and Chandra. This serendipitous survey contains the deepest mid-IR data taken to date. This data set is well suited for studies of intermediate-redshift galaxy clusters, high-redshift galaxies, the first generation of stars, and the lowest mass brown dwarfs, among others. This paper provides a summary of the data characteristics and catalog generation from all bands collected to date as well as a discussion of photometric redshifts and initial and expected science results and goals. To illustrate the scientific potential of this unique data set, we also present here IRAC color-color diagrams.

Diverse protostellar evolutionary states in the young cluster afgl961

Abstract: We present arcsecond resolution mid-infrared and millimeter observations of the center of the young stellar cluster AFGL961 in the Rosette molecular cloud. Within 0.2 pc of each other, we find an early B star embedded in a dense core, a neighboring star of similar luminosity with no millimeter counterpart, a protostar that has cleared out a cavity in the circumcluster envelope, and two massive, dense cores with no infrared counterparts. An outflow emanates from one of these cores, indicating a deeply embedded protostar, but the other is starless, bound, and appears to be collapsing. The diversity of states implies either that protostellar evolution is faster in clusters than in isolation or that clusters form via quasi-static rather than dynamic collapse. The existence of a pre-stellar core at the cluster center shows that some star formation continues after and in close proximity to massive, ionizing stars.

Diverse protostellar evolutionary states in the young cluster AFGL961

Abstract: We present arcsecond resolution mid-infrared and millimeter observations of the center of the young stellar cluster AFGL961 in the Rosette molecular cloud. Within 0.2 pc of each other, we find an early B star embedded in a dense core, a neighboring star of similar luminosity with no millimeter counterpart, a protostar that has cleared out a cavity in the circumcluster envelope, and two massive, dense cores with no infrared counterparts. An outflow emanates from one of these cores, indicating a deeply embedded protostar, but the other is starless, bound, and appears to be collapsing. The diversity of states implies either that protostellar evolution is faster in clusters than in isolation or that clusters form via quasi-static rather than dynamic collapse. The existence of a pre-stellar core at the cluster center shows that that some star formation continues after and in close proximity to massive, ionizing stars.

The IRAC Dark Field; Far- Infrared to X-ray Data

Abstract: We present 20 band photometry from the far-IR to X-ray in the Spitzer IRAC dark field. The bias for the near-IR camera on Spitzer is calibrated by observing a ~20 arcminute diameter "dark" field near the north ecliptic pole roughly every two-to-three weeks throughout the mission duration of Spitzer. The field is unique for its extreme depth, low background, high quality imaging, time-series information, and accompanying photometry including data taken with Akari, Palomar, MMT, KPNO, Hubble, and Chandra. This serendipitous survey contains the deepest mid-IR data taken to date. This dataset is well suited for studies of intermediate redshift galaxy clusters, high redshift galaxies, the first generation of stars, and the lowest mass brown dwarfs, among others. This paper provides a summary of the data characteristics and catalog generation from all bands collected to date as well as a discussion of photometric redshifts and initial and expected science results and goals. To illustrate the scientific potential of this unique dataset, we also present here IRAC color color diagrams.

Galaxy Clusters in the IRAC Dark Field II: Mid-IR Sources

Abstract: We present infrared luminosities, star formation rates, colors, morphologies, locations, and AGN properties of 24 micron-detected sources in photometrically detected high-redshift clusters in order to understand the impact of environment on star formation and AGN evolution in cluster galaxies. We use three newly-identified z=1 clusters selected from the IRAC dark field; the deepest ever mid-IR survey with accompanying, 14 band multiwavelength data including deep HST imaging and deep wide-area Spitzer MIPS 24 micron imaging. We find 90 cluster members with MIPS detections within two virial radii of the cluster centers, of which 17 appear to have spectral energy distributions dominated by AGN and the rest dominated by star formation. We find that 43 of the star forming are luminous infrared galaxies (LIRGs). The majority of sources (81%) are spirals or irregulars. A large fraction (at least 25%) show obvious signs of interactions. The MIPS -detected member galaxies have varied spatial distributions as compared to the MIPS-undetected members with one of the three clusters showing SF galaxies being preferentially located on the cluster outskirts, while the other 2 clusters show no such trend. Both the AGN fraction and the summed SFR of cluster galaxies increases from z=0 to 1, at a rate that is a few times faster in clusters than over the same redshift range in the field. Cluster environment does have an effect on the evolution of both AGN fraction and SFR from redshift one to the present, but does not effect the infrared luminosities or morphologies of the MIPS sample. Star formation happens in the same way regardless of environment making MIPS sources look the same in the cluster and field, however the cluster environment does encourage a more rapid evolution with time as compared to the field.

Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud

Abstract: We present a catalog of 1750 massive stars in the Large Magellanic Cloud, with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 1268 of these stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE survey database, for which we present uniform photometry from 0.3-24 microns in the UBVIJHKs+IRAC+MIPS24 bands. The resulting infrared color-magnitude diagrams illustrate that the supergiant B[e], red supergiant and luminous blue variable (LBV) stars are among the brightest infrared point sources in the Large Magellanic Cloud, due to their intrinsic brightness, and at longer wavelengths, due to dust. We detect infrared excesses due to free-free emission among ~900 OB stars, which correlate with luminosity class. We confirm the presence of dust around 10 supergiant B[e] stars, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs. The similar luminosities of B[e] supergiants (log L/Lo>=4) and the rare, dusty progenitors of the new class of optical transients (e.g. SN 2008S and NGC 300 OT), plus the fact that dust is present in both types of objects, suggests a common origin for them. We find the infrared colors for Wolf-Rayet stars to be independent of spectral type and their SEDs to be flatter than what models predict. The results of this study provide the first comprehensive roadmap for interpreting luminous, massive, resolved stellar populations in nearby galaxies at infrared wavelengths.