arXiv subjects

Sean J. Carey

Publications and source records attributed to Sean J. Carey.

17 recordsLinked to original sources

arXiv2026

A Spitzer Space Telescope Exploration Science Program to Search for Y Dwarf Variability

We present the results of a Spitzer Space Telescope Exploration Science Program to search for and characterize variability in Y dwarfs. We observed 14 Y dwarfs over a 24 hr period at [3.6] and [4.5] and then repeated the observations a few months later. We add two Y dwarfs, WD 0806-661B and WISE J085510.83-071442.5, that were also observed with Spitzer so that our sample includes all Y dwarfs observed for variability with Spitzer. We infer variability fractions of 59%+-15% and 64%+10%-13% for [3.6], and [4.5], in the first epoch, and 35%+17% -11% and 75%+8% -15% in the second epoch. We also find that Y dwarf Spitzer light curves are generally stable over timescales of months, but in some cases can show clear changes in amplitude. Combining our results with a similar Spitzer survey of L and T dwarfs by Metchev et al. (2015), we find the mid-infrared variability fraction of L, T, and Y dwarfs weakly supports the hypothesis that brown dwarf variability is caused by variations in the horizontal and/or vertical structure of condensate clouds.

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arXiv2023

The Near-Earth Object Surveyor Mission

The Near-Earth Object (NEO) Surveyor mission is a NASA observatory designed to discover and characterize near-Earth asteroids and comets. The mission's primary objective is to find the majority of objects large enough to cause severe regional impact damage ($>$140 m in effective spherical diameter) within its five-year baseline survey. Operating at the Sun-Earth L1 Lagrange point, the mission will survey to within 45 degrees of the Sun in an effort to find the objects in the most Earth-like orbits. The survey cadence is optimized to provide observational arcs long enough to reliably distinguish near-Earth objects from more distant small bodies that cannot pose an impact hazard. Over the course of its survey, NEO Surveyor will discover $\sim$200,000 - 300,000 new NEOs down to sizes as small as $\sim$10 m and thousands of comets, significantly improving our understanding of the probability of an Earth impact over the next century.

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arXiv2021

Initial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), using Hubble, Spitzer, ZTF, Keck, APO and GROWTH Visible & Infrared Imaging and Spectroscopy

We present visible and mid-infrared imagery and photometry of temporary Jovian co-orbital comet P/2019 LD$_2$ taken with HST/WFC3, Spitzer/IRAC, the GROWTH telescope network, visible spectroscopy from Keck/LRIS and archival ZTF observations taken between 2019 April and 2020 August. Our observations indicate that the nucleus of LD$_2$ has a radius between 0.2-1.8 km assuming a 0.08 albedo and a coma dominated by $\sim$100$μ$ m-scale dust ejected at $\sim$1 m/s speeds with a $\sim$1'' jet pointing in the SW direction. LD$_2$ experienced a total dust mass loss of $\sim$10$^8$ kg at a loss rate of $\sim$6 kg/s with Af$ρ$/cross-section varying between $\sim$85 cm/125 km$^2$ and $\sim$200 cm/310 km$^2$ from 2019 April 9 to 2019 Nov 8. If the increase in Af$ρ$/cross-section remained constant, it implies LD$_2$'s activity began $\sim$2018 November when within 4.8 au of the Sun, implying the onset of H$_2$O sublimation. We measure CO/CO$_2$ gas production of $\lesssim$10$^{27}$ mol/s /$\lesssim$10$^{26}$ mol/s from our 4.5 $μ$m Spitzer observations, $g$-$r$ = 0.59$\pm$0.03, $r$-$i$ = 0.18$\pm$0.05, $i$-$z$ = 0.01$\pm$0.07 from GROWTH observations, H$_2$O gas production of $\lesssim$80 kg/s scaling from our estimated $C_2$ production of $Q_{C_2}\lesssim$7.5$\times10^{24}$ mol/s from Keck/LRIS spectroscopy. We determine that the long-term orbit of LD$_2$ is similar to Jupiter family comets having close encounters with Jupiter within $\sim$0.5 Hill radius in the last $\sim$3 y, within 0.8 Hill radius in $\sim$9 y. Additionally, 78.8$\%$ of our orbital clones are ejected from the Solar System within $1 \times 10^{6}$ years having a dynamical half-life of 3.4 $\times 10^5$ years.

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arXiv2018

The Science Advantage of a Redder Filter for WFIRST

WFIRST will be capable of providing Hubble-quality imaging performance over several thousand square degrees of the sky. The wide-area, high spatial resolution survey data from WFIRST will be unsurpassed for many decades into the future. With the current baseline design, the WFIRST filter complement will extend from the bluest wavelength allowed by the optical design to a reddest filter (F184W) that has a red cutoff at 2.0 microns. In this white paper, we outline some of the science advantages for adding a Ks filter with a 2.15 micron central wavelength in order to extend the wavelength coverage for WFIRST as far to the red as the possible given the thermal performance of the observatory and the sensitivity of the detectors.

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arXiv2018

Early 2017 observations of TRAPPIST-1 with $\textit{Spitzer}$

The recently detected TRAPPIST-1 planetary system, with its seven planets transiting a nearby ultracool dwarf star, offers the first opportunity to perform comparative exoplanetology of temperate Earth-sized worlds. To further advance our understanding of these planets' compositions, energy budgets, and dynamics, we are carrying out an intensive photometric monitoring campaign of their transits with the $\textit{Spitzer Space Telescope}$. In this context, we present 60 new transits of the TRAPPIST-1 planets observed with $\textit{Spitzer}$/IRAC in February and March 2017. We combine these observations with previously published $\textit{Spitzer}$ transit photometry and perform a global analysis of the resulting extensive dataset. This analysis refines the transit parameters and provides revised values for the planets' physical parameters, notably their radii, using updated properties for the star. As part of our study, we also measure precise transit timings that will be used in a companion paper to refine the planets' masses and compositions using the transit timing variations method. TRAPPIST-1 shows a very low level of low-frequency variability in the IRAC 4.5-$μ$m band, with a photometric RMS of only 0.11$\%$ at a 123-s cadence. We do not detect any evidence of a (quasi-)periodic signal related to stellar rotation. We also analyze the transit light curves individually, to search for possible variations in the transit parameters of each planet due to stellar variability, and find that the $\textit{Spitzer}$ transits of the planets are mostly immune to the effects of stellar variations. These results are encouraging for forthcoming transmission spectroscopy observations of the TRAPPIST-1 planets with the $\textit{James Webb Space Telescope}$.

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arXiv2017

Exploring the Dust Population in Cold Diffuse Clouds

The formation and evolution of cold diffuse clouds (CDCs), the parent objects of dense molecular clouds, affects both the star formation process and that of larger-scale galactic evolution. We have begun a pilot study of one CDC's dust content, with the goal of quantifying the abundances of different types of dust and relating these to the relative abundance of molecular gas, the cloud's physical properties, and its general stage of development. Using photometry from AKARI and other surveys, we have extracted a sample spectral energy distribution (SED) of the CDC dust thermal emission over the near-infrared to submillimeter range. The extracted SED closely resembles others in the literature, confirming our isolation of the cloud emission from other sources along the sight line. We plan to fit this SED with dust models at each position in the cloud, automating our procedure to map out the structure of this CDC and others.

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arXiv2017

Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1

One focus of modern astronomy is to detect temperate terrestrial exoplanets well-suited for atmospheric characterisation. A milestone was recently achieved with the detection of three Earth-sized planets transiting (i.e. passing in front of) a star just 8% the mass of the Sun 12 parsecs away. Indeed, the transiting configuration of these planets with the Jupiter-like size of their host star - named TRAPPIST-1 - makes possible in-depth studies of their atmospheric properties with current and future astronomical facilities. Here we report the results of an intensive photometric monitoring campaign of that star from the ground and with the Spitzer Space Telescope. Our observations reveal that at least seven planets with sizes and masses similar to the Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.21, 12.35 days) are near ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inward. The seven planets have equilibrium temperatures low enough to make possible liquid water on their surfaces.

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arXiv2016

The First Detection of Photometric Variability in a Y Dwarf: WISE J140518.39+553421.3

We present the first detection of photometric variability of a spectroscopically-confirmed Y dwarf. The Infrared Array Camera on board the Spitzer Space Telescope was used to obtain times series photometry at 3.6 and 4.5 microns over a twenty four hour period at two different epochs separated by 149 days. Variability is evident at 4.5 um in the first epoch and at 3.6 and 4.5 um in the second epoch which suggests that the underlying cause or causes of this variability change on the timescales of months. The second-epoch [3.6] and [4.5] light curves are nearly sinusoidal in form, in phase, have periods of roughly 8.5 hours, and have semi-amplitudes of 3.5%. We find that a simple geometric spot model with a single bright spot reproduces these observations well. We also compare our measured semi-amplitudes of the second epoch light curves to predictions of the static, one-dimensional, partly cloudy and hot spot models of Morley and collaborators and find that neither set of models can reproduce the observed [3.6] and[4.5] semi-amplitudes simultaneously. More advanced two- or three-dimensional models that include time-dependent phenomena like vertical mixing, cloud formation, and thermal relaxation are therefore sorely needed in order to properly interpret our observations.

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arXiv2015

Spectroscopic infrared extinction mapping as a probe of grain growth in IRDCs

We present spectroscopic tests of MIR to FIR extinction laws in IRDC G028.36+00.07, a potential site of massive star and star cluster formation. Lim & Tan (2014) developed methods of FIR extinction mapping of this source using ${\it Spitzer}$-MIPS ${\rm 24μm}$ and ${\it Herschel}$-PACS ${\rm 70μm}$ images, and by comparing to MIR ${\it Spitzer}$-IRAC $3$--${\rm 8μm}$ extinction maps, found tentative evidence for grain growth in the highest mass surface density regions. Here we present results of spectroscopic infrared extinction (SIREX) mapping using ${\it Spitzer}$-IRS (14 to ${\rm 38μm}$) data of the same IRDC. These methods allow us to first measure the SED of the diffuse Galactic ISM that is in the foreground of the IRDC. We then carry out our primary investigation of measuring the MIR to FIR opacity law and searching for potential variations as a function of mass surface density within the IRDC. We find relatively flat, featureless MIR-FIR opacity laws that lack the $\sim{\rm 12μm}$ and $\sim{\rm 35μm}$ features associated with the thick water ice mantle models of Ossenkopf & Henning (1994). Their thin ice mantle models and the coagulating aggregate dust models of Ormel et al. (2011) are a generally better match to the observed opacity laws. We also find evidence for generally flatter MIR to FIR extinction laws as mass surface density increases, strengthening the evidence for grain and ice mantle growth in higher density regions.

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arXiv2014

Magnetic Fields in High-Mass Infrared Dark Clouds

High-mass Stars are cosmic engines known to dominate the energetics in the Milky Way and other galaxies. However, their formation is still not well understood. Massive, cold, dense clouds, often appearing as Infrared Dark Clouds (IRDCs), are the nurseries of massive stars. No measurements of magnetic fields in IRDCs in a state prior to the onset of high-mass star formation (HMSF) have previously been available, and prevailing HMSF theories do not consider strong magnetic fields. Here, we report observations of magnetic fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong magnetic field is as important as turbulence and gravity for HMSF. The main dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the lower density filament merging onto the main filament is parallel to the magnetic field. The implied magnetic field is strong enough to suppress fragmentation sufficiently to allow HMSF. Other mechanisms reducing fragmentation, such as the entrapment of heating from young stars via high mass surface densities, are not required to facilitate HMSF.

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arXiv2014

The pros and cons of the inversion method approach to derive 3D dust emission properties of the ISM: the Hi-GAL field centred on (l,b)=(30$^{\circ}$,0$^{\circ}$)

Herschel FIR continuum data obtained as part of the Hi-GAL survey have been used, together with the GLIMPSE 8 $μ$m and MIPSGAL 24 $μ$m data, to attempt the first 3D-decomposition of dust emission associated with atomic, molecular and ionized gas at 15 arcmin angular resolution. Our initial test case is a 2$\times$2 square degrees region centred on (l,b)=(30$^{\circ}$,0$^{\circ}$), a direction that encompasses the origin point of the Scutum-Crux Arm at the tip of the Galactic Bar. Coupling the IR maps with velocity maps specific for different gas phases (HI 21cm, $^{12}$CO and $^{13}$CO, and RRLs), we estimate the properties of dust blended with each of the gas components and at different Galactocentric distances along the LOS. A statistical Pearson's coefficients analysis is used to study the correlation between the column densities and the intensity of the IR emission. This analysis provides evidence that the 2$\times$2 square degree field under consideration is characterized by the presence of a gas component not accounted for by the standard tracers, possibly associated with warm H$_{2}$ and cold HI. We demonstrate that the IR radiation in the range 8 $μ$m $<$ $λ$ $<$ 500 $μ$m is systematically dominated by emission originating within the Scutum-Crux Arm. By applying an inversion method, we recover the dust emissivities associated with atomic, molecular and ionized gas. Using the DustEM model we obtain an indication for PAHs depletion in the diffuse ionized gas. However, the main goal of this work is to discuss the impact of the missing column density associated with the dark gas component on the accurate evaluation of the dust properties, and to shed light on the limitations of the inversion method approach when this is applied to a small section of the Galactic Plane and when the working resolution allows sufficient de-blending of the gas components along the LOS.

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arXiv2012

A Spitzer IRAC Measure of the Zodiacal Light

The dominant non-instrumental background source for space-based infrared observatories is the zo- diacal light. We present Spitzer Infrared Array Camera (IRAC) measurements of the zodiacal light at 3.6, 4.5, 5.8, and 8.0 μm, taken as part of the instrument calibrations. We measure the changing surface brightness levels in approximately weekly IRAC observations near the north ecliptic pole (NEP) over the period of roughly 8.5 years. This long time baseline is crucial for measuring the annual sinusoidal variation in the signal levels due to the tilt of the dust disk with respect to the ecliptic, which is the true signal of the zodiacal light. This is compared to both Cosmic Background Explorer Diffuse Infrared Background Experiment (COBE DIRBE) data and a zodiacal light model based thereon. Our data show a few percent discrepancy from the Kelsall et al. (1998) model including a potential warping of the interplanetary dust disk and a previously detected overdensity in the dust cloud directly behind the Earth in its orbit. Accurate knowledge of the zodiacal light is important for both extragalactic and Galactic astronomy including measurements of the cosmic infrared background, absolute measures of extended sources, and comparison to extrasolar interplanetary dust models. IRAC data can be used to further inform and test future zodiacal light models.

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arXiv2010

Circumstellar Structure around Evolved Stars in the Cygnus-X Star Formation Region

We present observations of newly discovered 24 micron circumstellar structures detected with the Multiband Imaging Photometer for Spitzer (MIPS) around three evolved stars in the Cygnus-X star forming region. One of the objects, BD+43 3710, has a bipolar nebula, possibly due to an outflow or a torus of material. A second, HBHA 4202-22, a Wolf-Rayet candidate, shows a circular shell of 24 micron emission suggestive of either a limb-brightened shell or disk seen face-on. No diffuse emission was detected around either of these two objects in the Spitzer 3.6-8 micron Infrared Array Camera (IRAC) bands. The third object is the luminous blue variable candidate G79.29+0.46. We resolved the previously known inner ring in all four IRAC bands. The 24 micron emission from the inner ring extends ~1.2 arcmin beyond the shorter wavelength emission, well beyond what can be attributed to the difference in resolutions between MIPS and IRAC. Additionally, we have discovered an outer ring of 24 micron emission, possibly due to an earlier episode of mass loss. For the two shell stars, we present the results of radiative transfer models, constraining the stellar and dust shell parameters. The shells are composed of amorphous carbon grains, plus polycyclic aromatic hydrocarbons in the case of G79.29+0.46. Both G79.29+0.46 and HBHA 4202-22 lie behind the main Cygnus-X cloud. Although G79.29+0.46 may simply be on the far side of the cloud, HBHA 4202-22 is unrelated to the Cygnus-X star formation region.

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arXiv2009

Primordial Circumstellar Disks in Binary Systems: Evidence for Reduced Lifetimes

We combine the results from several multiplicity surveys of pre-main-sequence stars located in four nearby star-forming regions with Spitzer data from three different Legacy Projects. This allows us to construct a sample of 349 targets, including 125 binaries, which we use to to investigate the effect of companions on the evolution of circumstellar disks. We find that the distribution of projected separations of systems with Spitzer excesses is significantly different (P ~2.4e-5, according to the KS test for binaries with separations < 400 AU) from that of systems lacking evidence for a disk. As expected, systems with projected separations < 40 AU are half as likely to retain at least one disk than are systems with projected separations in the 40-400 AU range. These results represent the first statistically significant evidence for a correlation between binary separation and the presence of an inner disk (r ~ 1 AU). Several factors (e.g., the incompleteness of the census of close binaries, the use of unresolved disk indicators, and projection effects) have previously masked this correlation in smaller samples. We discuss the implications of our findings for circumstellar disk lifetimes and the formation of planets in multiple systems.

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arXiv2006

Ammonia in Infrared Dark Clouds

Infrared Dark Clouds appear to be the long sought population of cold and dense aggregations with the potential of harbouring the earliest stages of massive star formation. Up to now there has been no systematic study on the temperature distribution, velocity fields, chemical and physical state toward this new cloud population. Knowing these properties is crucial for understanding the presence, absence and the very potential of star formation. The present paper aims at addressing these questions. We analyse temperature structures and velocity fields and gain information on their chemical evolution. The gas emission is remarkably coextensive with the extinction seen at infrared wavelengths and with the submillimeter dust emission. Our results show that IRDCs are on average cold (T < 20 K) and have variations among the different cores. IRDC cores are in virial equilibrium, are massive (M > 100 M_sun), highly turbulent (1 -- 3 km/s) and exhibit significant velocity structure (variations around 1 -- 2 km/s over the cloud). We find an increasing trend in temperature from IRDCs with high ammonia column density to high mass protostellar objects and hot core/Ultracompact Hii regions stages of early warm high-mass star formation while linewidths of IRDCs are smaller. On the basis of this sample, we infer that while active star formation is not yet pervasive in most IRDCs, local condensations might collapse in the future or have already begun forming stars.

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arXiv2004

Structure and Colors of Diffuse Emission in the Spitzer Galactic First Look Survey

We investigate the density structure of the interstellar medium using new high-resolution maps of the 8 micron, 24 micron, and 70 micron surface brightness towards a molecular cloud in the Gum Nebula, made as part of the Spitzer Space Telescope Galactic First Look Survey. The maps are correlated with 100 micron images measured with IRAS. At 24 and 70 micron, the spatial power spectrum of surface brightness follows a power law with spectral index -3.5. At 24 micron, the power law behavior is remarkably consistent from the 0.2 degree size of our maps down to the 5 arcsecond spatial resolution. Thus, the structure of the 24 micron emission is self-similar even at milliparsec scales. The combined power spectrum produced from Spitzer 24 micron and IRAS 25 micron images is consistent with a change in the power law exponent from -2.6 to -3.5. The decrease may be due to the transition from a two-dimensional to three-dimensional structure. Under this hypothesis, we estimate the thickness of the emitting medium to be 0.3 pc.

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arXiv2003

The G11.11-0.12 Infrared-Dark Cloud: Anomalous Dust and a Non-Magnetic Isothermal Model

The G11.11-0.12 Infrared-Dark Cloud has a filamentary appearance, both in absorption against the diffuse 8micron Galactic background, and in emission from cold dust at 850micron. Detailed comparison of the dust properties at these two wavelengths reveals that standard models for the diffuse interstellar dust in the Galaxy are not consistent with the observations. The ratio of absorption coefficients within the cloud is kappa_8/kappa_850 <= 1010, which is well below that expected for the diffuse ISM where kappa_8/kappa_850 ~ 1700. This may be due to the formation of ice mantles on the dust and grain coagulation, both of which are expected within dense regions of molecular clouds. The 850micron emission probes the underlying radial structure of the filament. The profile is well represented by a marginally resolved central region and a steeply falling envelope, with Sigma(r) proportional to r^(-a), where a <= 3, indicating that G11.11-0.12 is the first observed filament with a profile similar to that of a non-magnetic isothermal cylinder.

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