arXiv subjects

Andrew W. Stephens

Publications and source records attributed to Andrew W. Stephens.

At least 19 recordsLinked to original sources

arXiv2026

Paving the Road to the Habitable Worlds Observatory with High-Resolution Imaging I: New and Archival Speckle Observations of Potential HWO Target Stars

One of the key goals of the Habitable Worlds Observatory (HWO) is to directly image about 25 potentially habitable exoplanets and determine their properties. This challenge will require a large survey of nearby, bright stars -- ~100 according to the Astro2020 Decadel Survey. To ensure the success of the mission and to help guide design decisions, the stellar multiplicity of the target stars must be well-understood. To this end, we present optical speckle imaging of stars in the NASA Exoplanet Exploration Program (ExEP) provisional HWO star list, which is currently the Tier 1 target list for the HWO Target Stars and Systems Sub-Working Group. We obtained new observations using `Alopeke and Zorro at Gemini Observatory and queried the Exoplanet Follow-up Observing Program Archive for archival observations, resulting in speckle imaging data for 80 of the 164 stars. We confirmed one candidate companion detected previously by Gaia (HD 90089) and obtained an ambiguous detection of a known companion (HD 212330). To examine our sensitivity to companions, we simulated stellar companions down to ~0.1 $M_{\odot}$ for each target and found that 75%-85% would be detected in our speckle images; the remaining simulated companions are either too faint or too close-in, and will require follow-up using other methods such as long-term spectroscopic measurements and space-based techniques. This work represents a first step towards surveying potential HWO targets for close-in stellar companions and helping to inform the target selection process for the HWO direct-imaging survey, bringing us closer towards the discovery of potential habitable worlds.

astro-ph.SR↗

arXiv2025

Keck and Gemini characterization of $Hayabusa2\#$ rendezvous target 1998 KY$_{26}$

Near-earth object (NEO) 1998 KY$_{26}$ is a target of the $Hayabusa2\#$ spacecraft, which it will rendezvous with in July 2031. The asteroid is a rapid rotator and has a large out-of-plane nongravitational acceleration. We present deep $g$ and $R$ band imaging obtained with the Keck I/Low Resolution Imaging Spectrometer and visible spectroscopy from Gemini North/Gemini Multi-Object Spectrograph taken of 1998 KY$_{26}$ on 2024 June 8-9 when the asteroid was $\sim$0.037 au from the Earth. The asteroid lacks evidence of a dust coma in the deep images and its spectrum most closely resembles Xe-type asteroids, possessing a spectral slope of 6.71$\pm$0.43 $\%$ 100 nm$^{-1}$, and colors $g$-$r$ = 0.63$\pm$0.03, $r$-$i$ = 0.15$\pm$0.03, $i$-$z$ = 0.05$\pm$0.04, and implies a diameter of $\sim$10 m. From our images, we compute a 3$\sigma$ upper limit on the dust production of 1998 KY$_{26}$ of $<$10$^{-5}$ kg s$^{-1}$, $<$10$^{-2}$ kg s$^{-1}$, and $<$10$^{-1}$ kg s$^{-1}$ assuming $\mathrm{\mu}$m, mm, and cm size dust particles. Additionally, we compare the orbit of 1998 KY$_{26}$ and large nongravitational parameters asteroids to NEO population models and find that the majority, including 1998 KY$_{26}$, likely originated from the inner Main Belt, while the second most numerous group originates from the outer Main Belt, followed by a third group originating from the Jupiter Family Comet population. Given its inner Main Belt origin, its Xe-type spectrum, and rapid rotation, we hypothesize that the nongravitational acceleration of 1998 KY$_{26}$ may be caused by the shedding of large dust grains from its surface due to its rotation rather than H$_2$O vapor outgassing.

astro-ph.EP↗

arXiv2024

Hydroxyl Lines and Moonlight: a High Spectral Resolution Investigation of NIR skylines from Maunakea to guide NIR spectroscopic surveys

Subtracting the changing sky contribution from the near-infrared (NIR) spectra of faint astronomical objects is challenging and crucial to a wide range of science cases such as estimating the velocity dispersions of dwarf galaxies, studying the gas dynamics in faint galaxies, measuring accurate redshifts, and any spectroscopic studies of faint targets. Since the sky background varies with time and location, NIR spectral observations, especially those employing fiber spectrometers and targeting extended sources, require frequent sky-only observations for calibration. However, sky subtraction can be optimized with sufficient a priori knowledge of the sky's variability. In this work, we explore how to optimize sky subtraction by analyzing 1075 high-resolution NIR spectra from the CFHT's SPIRou on Maunakea, and we estimate the variability of 481 hydroxyl (OH) lines. These spectra were collected during two sets of three nights dedicated to obtaining sky observations every five and a half minutes. During the first set, we observed how the Moon affects the NIR, which has not been accurately measured at these wavelengths. We suggest accounting for the Moon contribution at separation distances less than 10 degrees when 1) reconstructing the sky using principal component analysis 2) observing targets at Y JHK mags fainter than ~15 and 3) attempting a sky subtraction better than 1%. We also identified 126 spectral doublets, or OH lines that split into at least two components, at SPIRou's resolution. In addition, we used Lomb-Scargle Periodograms and Gaussian process regression to estimate that most OH lines vary on similar timescales, which provides a valuable input for IR spectroscopic survey strategies. The data and code developed for this study are publicly available.

astro-ph.IM↗

arXiv2024

The SN 2023ixf Progenitor in M101: II. Properties

We follow our first paper with an analysis of the ensemble of the extensive pre-explosion ground- and space-based infrared observations of the red supergiant (RSG) progenitor candidate for the nearby core-collapse supernova SN 2023ixf in Messier 101, together with optical data prior to explosion obtained with the Hubble Space Telescope (HST). We have confirmed the association of the progenitor candidate with the SN, as well as constrained the metallicity at the SN site, based on SN observations with instruments at Gemini-North. The internal host extinction to the SN has also been confirmed from a high-resolution Keck spectrum. We fit the observed spectral energy distribution (SED) for the star, accounting for its intrinsic variability, with dust radiative-transfer modeling, which assume a silicate-rich dust shell ahead of the underlying stellar photosphere. The star is heavily dust-obscured, likely the dustiest progenitor candidate yet encountered. We found median estimates of the star's effective temperature and luminosity of 2770 K and 9.0e4 L_Sun, with 68% credible intervals of 2340--3150 K and (7.5--10.9)e4 L_sun. The candidate may have a Galactic RSG analog, IRC -10414, with a strikingly similar SED and luminosity. Via comparison with single-star evolutionary models we have constrained the initial mass of the progenitor candidate from 12 M_sun to as high as 14 M_sun. We have had available to us an extraordinary view of the SN 2023ixf progenitor candidate, which should be further followed up in future years with HST and the James Webb Space Telescope.

astro-ph.SR↗

arXiv2024

Planet Hunters NGTS: New Planet Candidates from a Citizen Science Search of the Next Generation Transit Survey Public Data

We present the results from the first two years of the Planet Hunters NGTS citizen science project, which searches for transiting planet candidates in data from the Next Generation Transit Survey (NGTS) by enlisting the help of members of the general public. Over 8,000 registered volunteers reviewed 138,198 light curves from the NGTS Public Data Releases 1 and 2. We utilize a user weighting scheme to combine the classifications of multiple users to identify the most promising planet candidates not initially discovered by the NGTS team. We highlight the five most interesting planet candidates detected through this search, which are all candidate short-period giant planets. This includes the TIC-165227846 system that, if confirmed, would be the lowest-mass star to host a close-in giant planet. We assess the detection efficiency of the project by determining the number of confirmed planets from the NASA Exoplanet Archive and TESS Objects of Interest (TOIs) successfully recovered by this search and find that 74% of confirmed planets and 63% of TOIs detected by NGTS are recovered by the Planet Hunters NGTS project. The identification of new planet candidates shows that the citizen science approach can provide a complementary method to the detection of exoplanets with ground-based surveys such as NGTS.

astro-ph.EP↗

arXiv2022

End-to-end science operations in the era of extremely large telescopes

Observatory end-to-end science operations is the overall process starting with a scientific question, represented by a proposal requesting observing time, and ending with the analysis of observation data addressing that question, and including all the intermediate steps needed to plan, schedule, obtain, and process these observations. Increasingly complex observing facilities demand a highly efficient science operations approach and at the same time be user friendly to the astronomical user community and enable the highest possible scientific return. Therefore, this process is supported by a collection of tools. In this paper, we describe the overall end-to-end process and its implementation for the three upcoming extremely large telescopes (ELTs), ESO's ELT, the Thirty Meter Telescope (TMT), and the Giant Magellan Telescope (GMT).

astro-ph.IM↗

arXiv2021

Gemini/GMOS Transmission Spectroscopy of the Grazing Planet Candidate WD 1856+534 b

WD 1856+534 b is a Jupiter-sized, cool giant planet candidate transiting the white dwarf WD 1856+534. Here, we report an optical transmission spectrum of WD 1856+534 b obtained from ten transits using the Gemini Multi-Object Spectrograph. This system is challenging to observe due to the faintness of the host star and the short transit duration. Nevertheless, our phase-folded white light curve reached a precision of 0.12 %. WD 1856+534 b provides a unique transit configuration compared to other known exoplanets: the planet is $8\times$ larger than its star and occults over half of the stellar disc during mid-transit. Consequently, many standard modeling assumptions do not hold. We introduce the concept of a `limb darkening corrected, time-averaged transmission spectrum' and propose that this is more suitable than $(R_{\mathrm{p}, λ} / R_{\mathrm{s}})^2$ for comparisons to atmospheric models for planets with grazing transits. We also present a modified radiative transfer prescription. Though the transmission spectrum shows no prominent absorption features, it is sufficiently precise to constrain the mass of WD 1856+534 b to be > 0.84 M$_\mathrm{J}$ (to $2 \, σ$ confidence), assuming a clear atmosphere and a Jovian composition. High-altitude cloud decks can allow lower masses. WD 1856+534 b could have formed either as a result of common envelope evolution or migration under the Kozai-Lidov mechanism. Further studies of WD 1856+534 b, alongside new dedicated searches for substellar objects around white dwarfs, will shed further light on the mysteries of post-main sequence planetary systems.

astro-ph.EP↗

arXiv2021

A local baseline of the black hole mass scaling relations for active galaxies. IV. Correlations between $M_{\rm BH}$ and host galaxy $σ$, stellar mass, and luminosity

The tight correlations between the mass of supermassive black holes ($M_{\rm BH}$) and their host-galaxy properties have been of great interest to the astrophysical community, but a clear understanding of their origin and fundamental drivers still eludes us. The local relations for active galaxies are interesting in their own right and form the foundation for any evolutionary study over cosmic time. We present Hubble Space Telescope optical imaging of a sample of 66 local active galactic nuclei (AGNs); for 14 objects, we also obtained Gemini near-infrared images. We use state of the art methods to perform surface photometry of the AGN host galaxies, decomposing them in spheroid, disk and bar (when present) and inferring the luminosity and stellar mass of the components. We combine this information with spatially-resolved kinematics obtained at the Keck Telescopes to study the correlations between $M_{\rm BH}$ (determined from single-epoch virial estimators) and host galaxy properties. The correlations are uniformly tight for our AGN sample, with intrinsic scatter 0.2-0.4 dex, smaller than or equal to that of quiescent galaxies. We find no difference between pseudo and classical bulges or barred and non-barred galaxies. We show that all the tight correlations can be simultaneously satisfied by AGN hosts in the 10$^7$-10$^9$ $M_{\odot}$ regime, with data of sufficient quality. The MBH-$σ$ relation is also in agreement with that of AGNs with $M_{\rm BH}$ obtained from reverberation mapping, providing an indirect validation of single-epoch virial estimators of $M_{\rm BH}$.

astro-ph.GA↗

arXiv2016

Kinematic Downsizing at z~2

We present results from a survey of the internal kinematics of 49 star-forming galaxies at z$\,\sim\,$2 in the CANDELS fields with the Keck/MOSFIRE spectrograph (SIGMA, Survey in the near-Infrared of Galaxies with Multiple position Angles). Kinematics (rotation velocity $V_{rot}$ and integrated gas velocity dispersion $σ_g$) are measured from nebular emission lines which trace the hot ionized gas surrounding star-forming regions. We find that by z$\,\sim\,$2, massive star-forming galaxies ($\log\,M_*/M_{\odot}\gtrsim10.2$) have assembled primitive disks: their kinematics are dominated by rotation, they are consistent with a marginally stable disk model, and they form a Tully-Fisher relation. These massive galaxies have values of $V_{rot}/σ_g$ which are factors of 2-5 lower than local well-ordered galaxies at similar masses. Such results are consistent with findings by other studies. We find that low mass galaxies ($\log\,M_*/M_{\odot}\lesssim10.2$) at this epoch are still in the early stages of disk assembly: their kinematics are often supported by gas velocity dispersion and they fall from the Tully-Fisher relation to significantly low values of $V_{rot}$. This "kinematic downsizing" implies that the process(es) responsible for disrupting disks at z$\,\sim\,$2 have a stronger effect and/or are more active in low mass systems. In conclusion, we find that the period of rapid stellar mass growth at z$\,\sim\,$2 is coincident with the nascent assembly of low mass disks and the assembly and settling of high mass disks.

astro-ph.GA↗

arXiv2014

VISTA Variables in the Vía Láctea (VVV): Halfway Status and Results

The VISTA Variables in the Vía Láctea (VVV) survey is one of six public ESO surveys, and is now in its 4th year of observing. Although far from being complete, the VVV survey has already delivered many results, some directly connected to the intended science goals (detection of variables stars, microlensing events, new star clusters), others concerning more exotic objects, e.g. novae. Now, at the end of the fourth observing period, and comprising roughly 50% of the proposed observations, the actual status of the survey, as well some of the results based on the VVV data, are presented.

astro-ph.IM↗

arXiv2013

Limits on Quaoar's Atmosphere

Here we present high cadence photometry taken by the Acquisition Camera on Gemini South, of a close passage by the $\sim540$ km radius Kuiper Belt Object, (50000) Quaoar, of a r'=20.2 background star. Observations before and after the event show that the apparent impact parameter of the event was $0.019\pm0.004$", corresponding to a close approach of $580\pm120$ km to the centre of Quaoar. No signatures of occultation by either Quaoar's limb or its potential atmosphere are detectable in the relative photometry of Quaoar and the target star, which were unresolved during closest approach. From this photometry we are able to put constraints on any potential atmosphere Quaoar might have. Using a Markov chain Monte Carlo and likelihood approach, we place pressure upper limits on sublimation supported, isothermal atmospheres of pure N$_2$, CO, and CH$_4$. For N$_2$ and CO, the upper limit surface pressures are 1 and 0.7 $μ{bar}$ respectively. The surface temperature required for such low sublimation pressures is $\sim33$ K, much lower than Quaoar's mean temperature of $\sim44$ K measured by others. We conclude that Quaoar cannot have an isothermal N$_2$ or CO atmosphere. We cannot eliminate the possibility of a CH$_4$ atmosphere, but place upper surface pressure and mean temperature limits of $\sim138$ nbar and $\sim44$ K respectively.

astro-ph.EP↗

arXiv2013

Kuiper Belt Occultation Predictions

Here we present observations of 7 large Kuiper Belt Objects. From these observations, we extract a point source catalog with $\sim0.01"$ precision, and astrometry of our target Kuiper Belt Objects with $0.04-0.08"$ precision within that catalog. We have developed a new technique to predict the future occurrence of stellar occultations by Kuiper Belt Objects. The technique makes use of a maximum likelihood approach which determines the best-fit adjustment to cataloged orbital elements of an object. Using simulations of a theoretical object, we discuss the merits and weaknesses of this technique compared to the commonly adopted ephemeris offset approach. We demonstrate that both methods suffer from separate weaknesses, and thus, together provide a fair assessment of the true uncertainty in a particular prediction. We present occultation predictions made by both methods for the 7 tracked objects, with dates as late as 2015. Finally, we discuss observations of three separate close passages of Quaoar to field stars, which reveal the accuracy of the element adjustment approach, and which also demonstrate the necessity of considering the uncertainty in stellar position when assessing potential occultations.

astro-ph.EP↗

arXiv2007

The birth place of the type Ic Supernova 2007gr

We report our attempts to locate the progenitor of the peculiar type Ic SN 2007gr in HST pre-explosion images of the host galaxy, NGC 1058. Aligning adaptive optics Altair/NIRI imaging of SN 2007gr from the Gemini (North) Telescope with the pre-explosion HST WFPC2 images, we identify the SN position on the HST frames with an accuracy of 20 mas. Although nothing is detected at the SN position we show that it lies on the edge of a bright source, 134+/-23 mas (6.9 pc) from its nominal centre. Based on its luminosity we suggest that this object is possibly an unresolved, compact and coeval cluster and that the SN progenitor was a cluster member, although we note that model profile fitting favours a single bright star. We find two solutions for the age of this assumed cluster; 7-/+0.5 Myrs and 20-30 Myrs, with turn-off masses of 28+/-4 Msun and 12-9 Msun respectively. Pre-explosion ground-based K-band images marginally favour the younger cluster age/higher turn-off mass. Assuming the SN progenitor was a cluster member, the turn-off mass provides the best estimate for its initial mass. More detailed observations, after the SN has faded, should determine if the progenitor was indeed part of a cluster, and if so allow an age estimate to within ~2 Myrs thereby favouring either a high mass single star or lower mass interacting binary progenitor.

astro-ph↗

arXiv2006

The Star Formation Histories of the Bulge and Disk of M31 from Gemini North+NIRI/Altair and HST/NICMOS

We discuss H and K observations of three fields in the bulge and disk of M31 obtained with the Altair adaptive optics system and NIRI instrument on Gemini North. These are the highest resolution and deepest near-infrared observations obtained to date of the inner regions of M31, and demonstrate the promise of ground-based adaptive optics for studying the crowded regions of nearby galaxies. We have combined our observations with previously published HST/NICMOS observations of nine M31 fields and have derived the coarse star formation histories of M31's bulge and inner disk. From fits to the M_K luminosity functions, we find the stellar population mix to be dominated by old, nearly solar-metallicity stars. The old populations, which we define as having age >~6 Gyr, indeed dominate the star formation histories at all radii independent of the relative contributions of bulge and disk stars. Although all of our fields contain some bulge contribution, our results suggest that there is no age difference between the bulge and disk to the limit of our precision.

astro-ph↗

arXiv2005

WLM-1: A Non-Rotating, Gravitationally Unperturbed, Highly Elliptical Extragalactic Globular Cluster

Globular clusters have long been known for presenting (at times) significant deviations from spherical symmetry. While rotation has been the main proposed explanation, other complicating factors such as their constant interaction with the strong gravitational potential of their host galaxy have made it difficult for a consensus to be reached. To address this question we have obtained high-resolution spectra of WLM-1, the lone globular cluster associated with the isolated, low-mass dwarf irregular galaxy WLM. Using archival HST WFPC2 data, we measure the radial ellipticity profile of WLM-1, finding it to be highly elliptical, with a mean value of 0.17 in the region 0.5-5" -- which is comparable to what is found in our Galaxy for the most elliptical globular clusters. There is no evidence of isophote twisting, except for the innermost regions of the cluster (r < 0.5"). To investigate whether the observed flattening can be ascribed to rotation, we have obtained long-slit high-resolution VLT/UVES spectra of this cluster along and perpendicular to the axis of flattening. Using cross-correlation we find that the velocity profile of the cluster is consistent with zero rotation along either axis. Thus neither cluster rotation nor galactic tides can be responsible for the flattened morphology of WLM-1. We argue that the required velocity dispersion anisotropy between the semi-major and semi-minor axes that would be required to account for the observed flattening is relatively small, of order 1 km/s. Even though our errors preclude us from conclusively establishing that such a difference indeed exists, velocity anisotropy remains at present the most plausible explanation for the shape of this cluster.

astro-ph↗

arXiv2003

Infrared Spectroscopic Study of 8 Galactic Globular Clusters

We have obtained medium-resolution infrared K band spectra of 44 giants in seven heavily reddened clusters in the Galactic bulge, as well as 12 giants in omega Centauri. We measure the equivalent widths of the Na doublet, the Ca triplet, and the CO band head, and then apply the new technique of Frogel et al to determine the metallicity of each star. Averaging these values, we estimate the metallicity for each cluster, and compare our new [Fe/H] values with previous determinations from the literature. Our estimates for each cluster are: NGC 6256 (-1.35), NGC 6539 (-0.79), HP 1 (-1.30), Liller 1 (-0.36), Palomar 6 (-0.52), Terzan 2 (-0.87), and Terzan 4 (-1.62). We briefly discuss differences between the various [Fe/H] scales on which it was possible to base our calibration, which is found to be the largest uncertainty in using this technique to determine metallicities.

astro-ph↗

arXiv2003

The Stellar Content of M31's Bulge

In this paper we analyze the stellar populations present in M31 using nine sets of adjacent HST-NICMOS Camera 1 and 2 fields with galactocentric distances ranging from 2' to 20'. These infrared observations provide some of the highest spatial resolution measurements of M31 to date; our data place tight constraints on the maximum luminosities of stars in the bulge of M31. The tip of the red giant branch is clearly visible at Mbol ~ -3.8, and the tip of the asymptotic giant branch (AGB) extends to Mbol ~ -5. This AGB peak luminosity is significantly fainter than previously claimed; through direct comparisons and simulations we show that previous measurements were affected by image blending. We do observe field-to-field variations in the luminosity functions, but simulations show that these differences can be produced by blending in the higher surface brightness fields. We conclude that the red giant branch of the bulge of M31 is not measurably different from that of the Milky Way's bulge. We also find an unusually high number of bright blueish stars (7.3/arcmin^2) which appear to be Galactic foreground stars.

astro-ph↗

arXiv2002

The Structure and Stellar Content of the Central Region of M33

Using Gemini QuickStart infrared observations of the central 22" of M33, we analyze the stellar populations in this controversial region. Based on the slope of the giant branch we estimate the mean metallicity to be -0.26+/-0.27, and from the luminosities of the most luminous stars, we estimate that there were two bursts of star formation ~2 and ~0.5 Gyr ago. We show that the stellar luminosity function not only has a different bright end cutoff, but also a significantly different slope than that of the Galactic bulge, and suggest that this difference is due to the young stellar component in M33. We combine our infrared Gemini data with optical HST-WFPC2 measurements revealing a CMD populated with young, intermediate, and old age stellar populations. Using surface brightness profiles from 0.1" to 18', we perform simple decompositions and show that the data are best fit by a three-component, core + bulge + disk model. Finally, we find no evidence for radial variations of the stellar populations in the inner 3"-10" of M33 based on a spatial analysis of the color-magnitude diagrams and luminosity functions.

astro-ph↗