arXiv subjects

Thayne Currie

Publications and source records attributed to Thayne Currie.

At least 181 records · Page 10Linked to original sources

arXiv2012

CfA4: Light Curves for 94 Type Ia Supernovae

We present multi-band optical photometry of 94 spectroscopically-confirmed Type Ia supernovae (SN Ia) in the redshift range 0.0055 to 0.073, obtained between 2006 and 2011. There are a total of 5522 light curve points. We show that our natural system SN photometry has a precision of roughly 0.03 mag or better in BVr'i', 0.06 mag in u', and 0.07 mag in U for points brighter than 17.5 mag and estimate that it has a systematic uncertainty of 0.014, 0.010, 0.012, 0.014, 0.046, and 0.073 mag in BVr'i'u'U, respectively. Comparisons of our standard system photometry with published SN Ia light curves and comparison stars reveal mean agreement across samples in the range of ~0.00-0.03 mag. We discuss the recent measurements of our telescope-plus-detector throughput by direct monochromatic illumination by Cramer et al (in prep.). This technique measures the whole optical path through the telescope, auxiliary optics, filters, and detector under the same conditions used to make SN measurements. Extremely well-characterized natural-system passbands (both in wavelength and over time) are crucial for the next generation of SN Ia photometry to reach the 0.01 mag accuracy level. The current sample of low-z SN Ia is now sufficiently large to remove most of the statistical sampling error from the dark energy error budget. But pursuing the dark-energy systematic errors by determining highly-accurate detector passbands, combining optical and near-infrared (NIR) photometry and spectra, using the nearby sample to illuminate the population properties of SN Ia, and measuring the local departures from the Hubble flow will benefit from larger, carefully measured nearby samples.

astro-ph.CO↗

arXiv2012

Dust Attenuation and H-alpha Star Formation Rates of z~0.5 Galaxies

Using deep narrow-band and broad-band imaging, we identify 401 z~0.40 and 249 z~0.49 H-alpha line-emitting galaxies in the Subaru Deep Field. Compared to other H-alpha surveys at similar redshifts, our samples are unique since they probe lower H-alpha luminosities, are augmented with multi-wavelength (rest-frame 1000AA--1.5 microns) coverage, and a large fraction (20%) of our samples has already been spectroscopically confirmed. Our spectra allow us to measure the Balmer decrement for nearly 60 galaxies with H-beta detected above 5-sigma. The Balmer decrements indicate an average extinction of A(H-alpha)=0.7^{+1.4}_{-0.7} mag. We find that the Balmer decrement systematically increases with higher H-alpha luminosities and with larger stellar masses, in agreement with previous studies with sparser samples. We find that the SFRs estimated from modeling the spectral energy distribution (SED) is reliable---we derived an "intrinsic" H-alpha luminosity which is then reddened assuming the color excess from SED modeling. The SED-predicted H-alpha luminosity agrees with H-alpha narrow-band measurements over 3 dex (rms of 0.25 dex). We then use the SED SFRs to test different statistically-based dust corrections for H-alpha and find that adopting one magnitude of extinction is inappropriate: galaxies with lower luminosities are less reddened. We find that the luminosity-dependent dust correction of Hopkins et al. yields consistent results over 3 dex (rms of 0.3 dex). Our comparisons are only possible by assuming that stellar reddening is roughly half of nebular reddening. The strong correspondence argue that with SED modeling, we can derive reliable intrinsic SFRs even in the absence of H-alpha measurements at z~0.5.

astro-ph.CO↗

arXiv2011

A 5 Micron Image of beta Pictoris b at a Sub-Jupiter Projected Separation: Evidence for a Misalignment Between the Planet and the Inner, Warped Disk

We present and analyze a new M' detection of the young exoplanet beta Pictoris b from 2008 VLT/NaCo data at a separation of ~ 4 AU and a high signal-to-noise rereduction of L' data taken in December 2009. Based on our orbital analysis, the planet's orbit is viewed almost perfectly edge-on (i ~ 89 degrees) and has a Saturn-like semimajor axis of 9.50 (+3.93, -1.7) AU. Intriguingly, the planet's orbit is aligned with the major axis of the outer disk (Omega ~ 31 degrees) but probably misaligned with the warp/inclined disk at 80 AU often cited as a signpost for the planet's existence. Our results motivate new studies to clarify how $β$ Pic b sculpts debris disk structures and whether a second planet is required to explain the warp/inclined disk.

astro-ph.EP↗

arXiv2011

Model atmospheres for massive gas giants with thick clouds: Application to the HR 8799 planets and predictions for future detections

We have generated an extensive new suite of massive giant planet atmosphere models and used it to obtain fits to photometric data for the planets HR 8799b, c, and d. We consider a wide range of cloudy and cloud-free models. The cloudy models incorporate different geometrical and optical thicknesses, modal particle sizes, and metallicities. For each planet and set of cloud parameters, we explore grids in gravity and effective temperature, with which we determine constraints on the planet's mass and age. Our new models yield statistically significant fits to the data, and conclusively confirm that the HR 8799 planets have much thicker clouds than those required to explain data for typical L and T dwarfs. Both models with 1) physically thick forsterite clouds and a 60-micron modal particle size and 2) clouds made of 1 micron-sized pure iron droplets and 1% supersaturation fit the data. Current data are insufficient to accurately constrain the microscopic cloud properties, such as composition and particle size. The range of best-estimated masses for HR 8799b, HR 8799c, and HR 8799d conservatively span 2-12 M_J, 6-13 M_J, and 3-11 M_J, respectively and imply coeval ages between ~10 and ~150 Myr, consistent with previously reported stellar age. The best-fit temperatures and gravities are slightly lower than values obtained by Currie et al. (2011) using even thicker cloud models. Finally, we use these models to predict the near-to-mid IR colors of soon-to-be imaged planets. Our models predict that planet-mass objects follow a locus in some near-to-mid IR color-magnitude diagrams that is clearly separable from the standard L/T dwarf locus for field brown dwarfs.

astro-ph.EP↗

arXiv2011

Spitzer IRS Spectroscopy of the 10 Myr-old EF Cha Debris Disk: Evidence for Phyllosilicate-Rich Dust in the Terrestrial Zone

We describe Spitzer IRS spectroscopic observations of the 10 Myr-old star, EF Cha. Compositional modeling of the spectra from 5 μm to 35 μm confirms that it is surrounded by a luminous debris disk with LD/L\star ~ 10-3, containing dust with temperatures between 225 K and 430 K characteristic of the terrestrial zone. The EF Cha spectrum shows evidence for many solid-state features, unlike most cold, low-luminosity debris disks but like some other 10-20 Myr-old luminous, warm debris disks (e.g. HD 113766A). The EF Cha debris disk is unusually rich in a species or combination of species whose emissivities resemble that of finely powdered, laboratory-measured phyllosilicate species (talc, saponite, and smectite), which are likely produced by aqueous alteration of primordial anhydrous rocky materials. The dust and, by inference, the parent bodies of the debris also contain abundant amorphous silicates and metal sulfides, and possibly water ice. The dust's total olivine to pyroxene ratio of ~ 2 also provides evidence of aqueous alteration. The large mass volume of grains with sizes comparable to or below the radiation blow-out limit implies that planetesimals may be colliding at a rate high enough to yield the emitting dust but not so high as to devolatize the planetesimals via impact processing. Because phyllosilicates are produced by the interactions between anhydrous rock and warm, reactive water, EF Cha's disk is a likely signpost for water delivery to the terrestrial zone of a young planetary system.

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arXiv2011

The Transitional Protoplanetary Disk Frequency as a Function of Age: Disk Evolution in the Coronet Cluster, Taurus, and Other 1--8 Myr-old Regions

We present Spitzer 3.6--24 micron photometry and spectroscopy for stars in the 1--3 Myr-old Coronet Cluster, expanding upon the survey of Sicilia-Aguilar et al. (2008). Using sophisticated radiative transfer models, we analyze these new data and those from Sicilia-Aguilar et al. (2008) to identify disks with evidence for substantial dust evolution consistent with disk clearing: transitional disks. We then analyze data in Taurus and others young clusters -- IC 348, NGC 2362, and eta Cha -- to constrain the transitional disk frequency as a function of time. Our analysis confirms previous results finding evidence for two types of transitional disks -- those with inner holes and those that are homologously depleted. The percentage of disks in the transitional phase increases from ~ 15--20% at 1--2 Myr to > 50% at 5--8 Myr; the mean transitional disk lifetime is closer to ~ 1 Myr than 0.1--0.5 Myr, consistent with previous studies by Currie et al. (2009) and Sicilia-Aguilar et al. (2009). In the Coronet Cluster and IC 348, transitional disks are more numerous for very low-mass M3--M6 stars than for more massive K5--M2 stars, while Taurus lacks a strong spectral type-dependent frequency. Assuming standard values for the gas-to-dust ratio and other disk properties, the lower limit for the masses of optically-thick primordial disks is Mdisk ~ 0.001--0.003 M*. We find that single color-color diagrams do not by themselves uniquely identify transitional disks or primordial disks. Full SED modeling is required to accurately assess disk evolution for individual sources and inform statistical estimates of the transitional disk population in large samples using mid-IR colors.

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arXiv2011

A Combined Subaru/VLT/MMT 1--5 Micron Study of Planets Orbiting HR 8799: Implications for Atmospheric Properties, Masses, and Formation

We present new 1--1.25 micron (z and J band) Subaru/IRCS and 2 micron (K band) VLT/NaCo data for HR 8799 and a rereduction of the 3--5 micron MMT/Clio data first presented by Hinz et al. (2010). Our VLT/NaCo data yields a detection of a fourth planet at a projected separation of ~ 15 AU -- "HR 8799e". We also report new, albeit weak detections of HR 8799b at 1.03 microns and 3.3 microns. Empirical comparisons to field brown dwarfs show that at least HR 8799b and HR8799c, and possibly HR 8799d, have near-to-mid IR colors/magnitudes significantly discrepant from the L/T dwarf sequence. Standard cloud deck atmosphere models appropriate for brown dwarfs provide only (marginally) statistically meaningful fits to HR 8799b and c for unphysically small radii. Models with thicker cloud layers not present in brown dwarfs reproduce the planets' SEDs far more accurately and without the need for rescaling the planets' radii. Our preliminary modeling suggests that HR 8799b has log(g) = 4--4.5, Teff = 900K, while HR 8799c, d, and (by inference) e have log(g) = 4--4.5, Teff = 1000--1200K. Combining results from planet evolution models and new dynamical stability limits implies that the masses of HR 8799b, c, d, and e are 6--7 Mj, 7--10 Mj, 7--10 Mj and 7--10 Mj. 'Patchy" cloud prescriptions may provide even better fits to the data and may lower the estimated surface gravities and masses. Finally, contrary to some recent claims, forming the HR 8799 planets by core accretion is still plausible, although such systems are likely rare.

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arXiv2010

High-Contrast 3.8 Micron Imaging Of The Brown Dwarf/Planet-Mass Companion to GJ 758

We present L' band (3.8 $μm$) MMT/Clio high-contrast imaging data for the nearby star GJ 758, which was recently reported by Thalmann et al. (2009) to have one -- possibly two-- faint comoving companions (GJ 758B and ``C", respectively). GJ 758B is detected in two distinct datasets. Additionally, we report a \textit{possible} detection of the object identified by Thalmann et al as ``GJ 758C" in our more sensitive dataset, though it is likely a residual speckle. However, if it is the same object as that reported by Thalmann et al. it cannot be a companion in a bound orbit. GJ 758B has a H-L' color redder than nearly all known L--T8 dwarfs. Based on comparisons with the COND evolutionary models, GJ 758B has T$_{e}$ $\sim$ 560 K$^{^{+150 K}_{-90K}}$ and a mass ranging from $\sim$ 10--20 M$_{J}$ if it is $\sim$ 1 Gyr old to $\sim$ 25--40 M$_{J}$ if it is 8.7 Gyr old. GJ 758B is likely in a highly eccentric orbit, e $\sim$ 0.73$^{^{+0.12}_{-0.21}}$, with a semimajor axis of $\sim$ 44 AU$^{^{+32 AU}_{-14 AU}}$. Though GJ 758B is sometimes discussed within the context of exoplanet direct imaging, its mass is likely greater than the deuterium-burning limit and its formation may resemble that of binary stars rather than that of jovian-mass planets.

astro-ph.SR↗

arXiv2010

The Morphologies and Lifetimes of Transitional Protoplanetary Disks

I describe new constraints on the lifetimes and morphologies of transitional protoplanetary disks from observations of 1--10 Myr old stars with the \textit{Spitzer Space Telescope}. New Spitzer results clearly show evidence for two kinds of transitional disks and thus two main disk evolutionary pathways: disks which form an inner hole/gap and clear from the inside out and disks that deplete more homologously. Analyzing the disk populations of 1--10 Myr old clusters such as Taurus, IC 348, NGC 2362, and $η$ Cha show that the mean transitional disk lifetime must be an appreciable fraction of the mean protoplanetary disk lifetime: $\approx$ 1 Myr out of 3--5 Myr. The varieties of transitional disk SEDs and correlations with other disk diagnostics are consistent with multiple mechanisms responsible for clearing disks.

astro-ph.EP↗

arXiv2009

The Stellar Population of h and chi Persei: Cluster Properties, Membership, and the Intrinsic Colors and Temperatures of Stars

(Abridged) From photometric observations of $\sim$ 47,000 stars and spectroscopy of $\sim$ 11,000 stars, we describe the first extensive study of the stellar population of the famous Double Cluster, h and $χ$ Persei, down to subsolar masses. Both clusters have E(B-V) $\sim$ 0.52--0.55 and dM = 11.8--11.85; the halo population, while more poorly constrained, likely has identical properties. As determined from the main sequence turnoff, the luminosity of M supergiants, and pre-main sequence isochrones, ages for h Persei, $χ$ Persei and the halo population all converge on $\approx$ 14 Myr. From these data, we establish the first spectroscopic and photometric membership lists of cluster stars down to early/mid M dwarfs. At minimum, there are $\sim$ 5,000 members within 10' of the cluster centers, while the entire h and $χ$ Persei region has at least $\sim$ 13,000 and as many as 20,000 members. The Double Cluster contains $\approx$ 8,400 M$_{\odot}$ of stars within 10' of the cluster centers. We estimate a total mass of at least 20,000 M$_{\odot}$. We conclude our study by outlining outstanding questions regarding the properties of h and $χ$ Persei. From comparing recent work, we compile a list of intrinsic colors and derive a new effective temperature scale for O--M dwarfs, giants, and supergiants.

astro-ph.SR↗

arXiv2009

Analysis of B and Be Star Populations of the Double Cluster h and chi Persei

We present blue optical spectra of 92 members of h and chi Per obtained with the WIYN telescope at Kitt Peak National Observatory. From these spectra, several stellar parameters were measured for the B-type stars, including V sin i, T_eff, log g_polar, M_star, and R_star. Stromgren photometry was used to measure T_eff and log g_polar for the Be stars. We also analyze photometric data of cluster members and discuss the near-to-mid IR excesses of Be stars.

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arXiv2009

The Pulsar Planets: A Test Case of Terrestrial Planet Assembly

We model the assembly of planets from planetary embryos under the conditions suggested by various scenarios for the formation of the planetary system around the millisecond pulsar B1257+12. We find that the most likely models fall at the low angular momentum end of the proposed range. Models that invoke supernova fallback produce such disks, although we find that a solar composition disk produces a more likely evolution than one composed primarily of heavy elements. Furthermore, we find that dust sedimentation must occur rapidly as the disk cools, in order that the solid material be confined to a sufficiently narrow range of radii. A quantitative comparison between the observations and the best-fit models shows that the simulations can reproduce the observed eccentricities and masses, but have difficulty reproducing the compactness of the pulsar planet system. Finally, we examine the results of similar studies of solar system terrestrial planet accumulation and discuss what can be learned from the comparison.

astro-ph.EP↗

arXiv2009

Deep MIPS Observations of the IC 348 Nebula: Constraints on the Evolutionary State of Anemic Circumstellar Disks and the Primordial-to-Debris Disk Transition

We describe new, deep MIPS photometry and new high signal-to-noise optical spectroscopy of the 2.5 Myr-old IC 348 Nebula. To probe the properties of the IC 348 disk population, we combine these data with previous optical/infrared photometry and spectroscopy to identify stars with gas accretion, to examine their mid-IR colors,and to model their spectral energy distributions. IC 348 contains many sources in different evolutionary states, including protostars and stars surrounded by primordial disks, two kinds of transitional disks, and debris disks. Most disks surrounding eary/intermediate spectral-type stars (> 1.4 Msun at 2.5 Myr) are debris disks; most disks surrounding solar and subsolar-mass stars are primordial disks. At the 1--2 sigma level, more massive stars also have a smaller frequency of gas accretion and smaller mid-IR luminosities than lower-mass stars. These trends are suggestive of a stellar mass-dependent evolution of disks, where most disks around high/intermediate-mass stars shed their primordial disks on rapid, 2.5 Myr timescales. The frequency of MIPS-detected transitional disks is ~ 15--35% for stars plausibly more massive than 0.5 Msun. The relative frequency of transitional disks in IC 348 compared to that for 1 Myr-old Taurus and 5 Myr-old NGC 2362 is consistent with a transition timescale that is a significant fraction of the total primordial disk lifetime.

astro-ph↗

arXiv2009

The Last Gasp of Gas Giant Planet Formation: A Spitzer Study of the 5 Myr-old Cluster NGC 2362

(Abridged) We describe Spitzer IRAC and MIPS observations of the populous, 5 Myr-old open cluster NGC 2362. Early/intermediate-type confirmed/candidate cluster members either have photospheric mid-IR emission or weak, optically-thin infrared excess emission at < 24 microns consistent with debris disks. Few late-type, solar/subsolar-mass stars have primordial disks. The disk population around late-type stars is dominated by disks with inner holes (canonical 'transition disks') and 'homologously depleted' disks. Both types of disks represent an intermediate stage between primordial disks and debris disks. Thus, we find that multiple paths for the primordial-to-debris disk transition exist. Our results undermine standard arguments in favor of a ~ 0.01 Myr year timescale for the transition based on data from Taurus-Auriga and rule out standard UV photoevaporation scenarios as the primary mechanism to explain the transition. Combining our data with other Spitzer surveys, we investigate the evolution of debris disks around high/intermediate-mass stars and investigate timescales for giant planet formation. If the gas and dust in disks evolve on similar timescales, the formation timescale for gas giant planets surrounding early-type, high/intermediate-mass stars is likely 1--5 Myr. Most solar/subsolar-mass stars detected by Spitzer have SEDs that indicate their disks may be actively leaving the primordial disk phase. Thus, gas giant planet formation may also occur by 5 Myr around solar/subsolar-mass stars as well.

astro-ph.EP↗

arXiv2009

On the Semimajor Axis Distribution of Extrasolar Gas Giant Planets: Why Hot Jupiters Are Rare Around High-Mass Stars

Based on a suite of Monte Carlo simulations, I show that a stellar-mass dependent lifetime of the gas disks from which planets form can explain the lack of hot Jupiters/close-in giant planets around high-mass stars and other key features of the observed semimajor axis distribution of radial velocity-detected giant planets. Using reasonable parameters for the Type II migration rate, regions of planet formation, and timescales for gas giant core formation, I construct synthetic distributions of jovian planets. A planet formation/migration model assuming a stellar mass-dependent gas disk lifetime reproduces key features in the observed distribution by preferentially stranding planets around high-mass stars at large semimajor axes.

astro-ph.EP↗

arXiv2009

Lyman Break Galaxies at z~1.8-2.8: GALEX/NUV Imaging of the Subaru Deep Field

Abridged: A photometric sample of ~7100 V<25.3 Lyman break galaxies (LBGs) has been selected by combining Subaru/Suprime-Cam BVRci'z' data with deep GALEX/NUV imaging of the Subaru Deep Field. Follow-up spectroscopy confirmed 24 LBGs at 1.5<z<2.7. Among the optical spectra, 12 have Ly-alpha emission with rest-frame equivalent widths of ~5-60AA. The success rate for identifying LBGs as NUV-dropouts at 1.5<z<2.7 is 86%. The rest-frame UV (1700AA) luminosity function (LF) is constructed from the photometric sample with corrections for stellar contamination and z<1.5 interlopers. The LF is 1.7+/-0.1 times higher than those of z~2 BXs and z~3 LBGs. Three explanations were considered, and it is argued that significantly underestimating low-z contamination or effective comoving volume is unlikely: the former would be inconsistent with the spectroscopic sample at 93% confidence, and the second explanation would not resolve the discrepancy. The third scenario is that different photometric selection of the samples yields non-identical galaxy populations, such that some BX galaxies are LBGs and vice versa. This argument is supported by a higher surface density of LBGs at all magnitudes while the redshift distribution of the two populations is nearly identical. This study, when combined with other star-formation rate (SFR) density UV measurements from LBG surveys, indicates that there is a rise in the SFR density: a factor of 3-6 (3-10) increase from z~5 (z~6) to z~2, followed by a decrease to z~0. This result, along with past sub-mm studies that find a peak at z~2 in their redshift distribution, suggest that z~2 is the epoch of peak star-formation. Additional spectroscopy is required to characterize the complete shape of the z~2 LBG UV LF via measurements of contamination and accurate distances.

astro-ph.CO↗

arXiv2008

The X-Ray Environment During the Epoch of Terrestrial Planet Formation: Chandra Observations of h Persei

We describe Chandra/ACIS-I observations of the massive ~ 13--14 Myr-old cluster, h Persei, part of the famous Double Cluster (h and chi Persei) in Perseus. Combining the list of Chandra-detected sources with new optical/IR photometry and optical spectroscopy reveals ~ 165 X-ray bright stars with V < 23. Roughly 142 have optical magnitudes and colors consistent with cluster membership. The observed distribution of Lx peaks at Lx ~ 10^30.3 ergs/s and likely traces the bright edge of a far larger population of ~ 0.4--2 Msun X-ray active stars. From a short list of X-ray active stars with IRAC 8 micron excess from warm, terrestrial-zone dust, we derive a maximum X-ray flux incident on forming terrestrial planets. Although there is no correlation between X-ray activity and IRAC excess, the fractional X-ray luminosity correlates with optical colors and spectral type. By comparing the distribution of Lx/L* vs. spectral type and V-I in h Per with results for other 1--100 Myr-old clusters, we show that stars slightly more massive than the Sun (> 1.5 Msun) fall out of X-ray saturation by ~ 10--15 Myr. Changes in stellar structure for > 1.5 Msun stars likely play an important role in this decline of X-ray emission.

astro-ph↗

arXiv2008

A Spitzer Study of Debris Disks In The Young Nearby Cluster NGC 2232: Icy Planets Are Common Around ~ 1.5--3 Solar-Mass Stars

We describe Spitzer IRAC and MIPS observations of the nearby 25 Myr-old open cluster NGC 2232. Combining these data with ROSAT All-Sky Survey observations, proper motions, and optical photometry/spectroscopy, we construct a list of highly probable cluster members. We identify 1 A-type star, HD 45435, with definite excess emission at 4.5--24 micron indicative of debris from terrestrial planet formation. We also identify 2--4 late-type stars with possible 8 micron excesses, and 8 early-type stars with definite 24 micron excesses. Constraints on the dust luminosity and temperature suggest that the detected excesses are produced by debris disks. From our sample of B and A stars, stellar rotation appears correlated with 24 micron excess, a result expected if massive primordial disks evolve into massive debris disks. To explore the evolution of the frequency and magnitude of debris around A-type stars, we combine our results with data for other young clusters. The frequency of debris disks around A-type stars appears to increase from ~ 25% at 5 Myr to ~ 50--60% at 20--25 Myr. Older A-type stars have smaller debris disk frequencies: ~ 20% at 50--100 Myr. For these ages, the typical level of debris emission rises from 5--20 Myr and then declines. Because 24 micron dust emission probes icy planet formation around A-type stars, our results suggest that the frequency of icy planet formation is eta(i) > 0.5--0.6. Thus, most A-type stars (approx. 1.5--3 Msun) produce icy planets.

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