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Multi-Star Wavefront Control at the Occulting Mask Coronagraph Testbed: Monochromatic Laboratory Demonstration for the Roman Coronagraph Instrument

The Astro2020 decadal survey recommended a direct imaging flagship mission with a goal for mission yield of 25 or more potentially habitable exoplanets. A majority of Sun-like stars have a stellar companion that can introduce additional noise into the field of view of any high-contrast imaging instrument and enabling exoplanet discovery around binary stars represents a path to increased coronagraphic instrument efficiency. This includes both of the Alpha Centauri A and B stars which would represent the top science target for direct imaging if companion leakage can be suppressed. Multi-Star Wavefront Control (MSWC) is a technique that removes stellar leakage from both stellar components, enabling direct imaging of exoplanets in many binary star systems. We present the latest testbed results obtained with MSWC as part of the technology development effort focusing on demonstrations conducted on the Occulting Mask Coronagraph (OMC) testbed at JPL. OMC has a layout similar to the Roman Space Telescope coronagraph instrument (CGI) and is outfitted with a MSWC mask with the same design as the contributed mask for the Roman CGI. Our testbed results represent the first demonstrations of this technique using the recently installed full binary source for a geometry matching potential Alpha Centauri observations. Technical Review Abstract: The Astro2020 decadal survey recommended a direct imaging flagship mission with a goal for mission yield of 25 or more potentially habitable exoplanets. A majority of Sun-like stars have a stellar companion that can introduce additional noise into the field of view of any high-contrast imaging instrument and enabling exoplanet discovery around binary stars represents a path to increased coronagraphic instrument efficiency by increasing the available science target pool of bright nearby stars. This includes both of the Alpha Centauri A and B stars which would represent the top science target for direct imaging if companion leakage can be suppressed. Multi-Star Wavefront Control (MSWC) is a technique that removes stellar leakage from both stellar components, enabling direct imaging of exoplanets in many binary star systems . We present the latest testbed results obtained with MSWC as part of the technology development effort focusing on demonstrations conducted on the Occulting Mask Coronagraph (OMC) testbed at JPL during the completed first vacuum window and recent results from the ongoing second vacuum window. The MSWC mask consists of a shaped pupil mask similar to the one used for the Wide-Field of View mode but also includes a set of superimposed, regularly-spaced dots that serve as a diffraction grating. OMC has a layout similar to the Roman Space Telescope coronagraph instrument (CGI) and is configured with a binary imaging mode with a MSWC mask using same design as the contributed mask for the Roman CGI. Our testbed results represent the first demonstrations of this technique using the recently installed full binary source. We present results obtained in Super-Nyquist regime demonstrating suppression in the Super-Nyquist regime for the 3rd diffraction order reaching 8.7e-9 contrast with the Roman pupil. In addition, we present results obtained with the binary-star regime demonstrating 9.6e-8 contrast for a geometry matching potential Alpha Centauri observations in a monochromatic wavelength similar to Band 1. Planned demonstrations in the 2nd vacuum window will focus on Band 3d and Band 4 using the full MSWC mode for an Alpha Centauri geometry.

High-contrast imaging↗
DOE OSTI2015

THE DUSTIEST POST-MAIN SEQUENCE STARS IN THE MAGELLANIC CLOUDS

Using observations from the Herschel Inventory of The Agents of Galaxy Evolution (HERITAGE) survey of the Magellanic Clouds (MC), we have found 35 evolved stars and stellar end products that are bright in the far-infrared. These 28 (LMC) and 7 (SMC) sources were selected from the 529 evolved star candidates in the HERITAGE far-infrared point source catalogs. Our source identification method is based on spectral confirmation, spectral energy distribution characteristics, careful examination of the multiwavelength images and includes constraints on the luminosity, resulting in a thoroughly vetted list of evolved stars. These sources span a wide range in luminosity and hence initial mass. We found 13 low- to intermediate-mass evolved stars, including asymptotic giant branch (AGB) stars, post-AGB stars, planetary nebulae, and a symbiotic star. We also identify 10 high mass stars, including 4 of the 15 known B[e] stars in the MC, 3 extreme red supergiants that are highly enshrouded by dust, a Luminous Blue Variable, a Wolf–Rayet star, and two supernova remnants. Further, we report the detection of 9 probable evolved objects which were previously undescribed in the literature. These sources are likely to be among the dustiest evolved objects in the MC. The Herschel emission may either be due to dust produced by the evolved star or it may arise from swept-up interstellar medium material.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Different to the core: The pre-supernova structures of massive single and binary-stripped stars

The majority of massive stars live in binary or multiple systems and will interact with a companion during their lifetimes, which helps to explain the observed diversity of core-collapse supernovae. Donor stars in binary systems can lose most of their hydrogen-rich envelopes through mass transfer. As a result, not only are the surface properties affected, but so is the core structure. However, most calculations of the core-collapse properties of massive stars rely on single-star models. We present a systematic study of the difference between the pre-supernova structures of single stars and stars of the same initial mass (11–21 M ⊙ ) that have been stripped due to stable post-main-sequence mass transfer at solar metallicity. We present the pre-supernova core composition with novel diagrams that give an intuitive representation of the isotope distribution. As shown in previous studies, at the edge of the carbon-oxygen core, the binary-stripped star models contain an extended gradient of carbon, oxygen, and neon. This layer remains until core collapse and is more extended in mass for higher initial stellar masses. It originates from the receding of the convective helium core during core helium burning in binary-stripped stars, which does not occur in single-star models. We find that this same evolutionary phase leads to systematic differences in the final density and nuclear energy generation profiles. Binary-stripped star models have systematically higher total masses of carbon at the moment of core collapse compared to single-star models, which likely results in systematically different supernova yields. In about half of our models, the silicon-burning and oxygen-rich layers merge after core silicon burning. We discuss the implications of our findings for the “explodability”, supernova observations, and nucleosynthesis of these stars. Our models are publicly available and can be readily used as input for detailed supernova simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Results of a search for γ Dor and δ SCT stars with the Kepler spacecraft

The light curves of 2768 stars with effective temperatures and surface gravities placing them near the gamma Doradus (γ Dor)/delta Scuti (δ Sct) instability region were observed as part of the Kepler Guest Observer program from Cycles 1 through 5. The light curves were analyzed in a uniform manner to search for γ Dor, δ Sct, and hybrid star pulsations. The γ Dor, δ Sct, and hybrid star pulsations extend asteroseismology to stars slightly more massive (1.4–2.5 M{sub ⊙}) than our Sun. We find 207 γ Dor, 84 δ Sct, and 32 hybrid candidate stars. Many of these stars are cooler than the red edge of the γ Dor instability strip as determined from ground-based observations made before Kepler. A few of our γ Dor candidate stars lie on the hot side of the ground-based γ Dor instability strip. The hybrid candidate stars cover the entire region between 6200 K and the blue edge of the ground-based δ Sct instability strip. None of our candidate stars are hotter than the hot edge of the ground-based δ Sct instability strip. Our discoveries, coupled with the work of others, show that Kepler has discovered over 2000 γ Dor, δ Sct, and hybrid star candidates in the 116 square degree Kepler field of view. We found relatively few variable stars fainter than magnitude 15, which may be because they are far enough away to lie between spiral arms in our Galaxy, where there would be fewer stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DETAILED ABUNDANCES OF STARS WITH SMALL PLANETS DISCOVERED BY KEPLER. I. THE FIRST SAMPLE

We present newly derived stellar parameters and the detailed abundances of 19 elements of seven stars with small planets discovered by NASA's Kepler Mission. Each star, save one, has at least one planet with a radius ≤1.6 R{sub ⊕}, suggesting a primarily rocky composition. The stellar parameters and abundances are derived from high signal-to-noise ratio, high-resolution echelle spectroscopy obtained with the 10 m Keck I telescope and High Resolution Echelle Spectrometer using standard spectroscopic techniques. The metallicities of the seven stars range from −0.32 to +0.13 dex, with an average metallicity that is subsolar, supporting previous suggestions that, unlike Jupiter-type giant planets, small planets do not form preferentially around metal-rich stars. The abundances of elements other than iron are in line with a population of Galactic disk stars, and despite our modest sample size, we find hints that the compositions of stars with small planets are similar to stars without known planets and with Neptune-size planets, but not to those of stars with giant planets. This suggests that the formation of small planets does not require exceptional host-star compositions and that small planets may be ubiquitous in the Galaxy. We compare our derived abundances (which have typical uncertainties of ≲0.04 dex) to the condensation temperature of the elements; a correlation between the two has been suggested as a possible signature of rocky planet formation. None of the stars demonstrate the putative rocky planet signature, despite at least three of the stars having rocky planets estimated to contain enough refractory material to produce the signature, if real. More detailed abundance analyses of stars known to host small planets are needed to verify our results and place ever more stringent constraints on planet formation models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The X-ray luminosity function of M37 and the evolution of coronal activity in low-mass stars

We use a 440.5 ks Chandra observation of the ≈500 Myr old open cluster M37 to derive the X-ray luminosity functions of its ≤1.2 M{sub ⊙} stars. Combining detections of 162 M37 members with upper limits for 160 non-detections, we find that its G, K, and M stars have a similar median (0.5–7 keV) X-ray luminosity L{sub X}=10{sup 29.0} erg s{sup −1}, whereas the L{sub X}-to-bolometric-luminosity ratio (L{sub X}/L{sub bol}) indicates that M stars are more active than G and K stars by ≈1 order of magnitude at 500 Myr. To characterize the evolution of magnetic activity in low-mass stars over their first ≈600 Myr, we consolidate X-ray and optical data from the literature for stars in six other open clusters: from youngest to oldest they are, the Orion Nebula Cluster (ONC), NGC 2547, NGC 2516, the Pleiades, NGC 6475, and the Hyades. For these, we homogenize the conversion of instrumental count rates to L{sub X} by applying the same one-temperature emission model as for M37, and obtain masses using the same empirical mass-absolute magnitude relation (except for the ONC). We find that for G and K stars X-ray activity decreases ≈2 orders of magnitude over their first 600 Myr, and for M stars, ≈1.5. The decay rate of the median L{sub X} follows the relation L{sub X}∝ t{sup b}, where b=−0.61±0.12 for G stars, −0.82 ± 0.16 for K stars, and −0.40 ± 0.17 for M stars. In L{sub X}/L{sub bol} space, the slopes are −0.68 ± 0.12, −0.81 ± 0.19, and −0.61 ± 0.12, respectively. These results suggest that for low-mass stars the age-activity relation steepens after ≈625 Myr, consistent with the faster decay in activity observed in solar analogs at t>1 Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Validating the C i 5052.17 Å/Mg ii 4481 Å Equivalent Width Ratio as a Diagnostic for F-type Lambda Boo Stars

The Lambda Boo-type stars are chemically peculiar stars with deficiencies of iron-peak elements but near-solar C, N, O, and S abundances. Since the prototype Lambda Boötis was first reported as peculiar, this group has been expanded from a small group of early A-type stars to a larger group of late B to early F-type dwarfs. Although a detailed abundance analysis that supports the Lambda Boo-like abundance pattern is the definitive confirmation of this Lambda Boo characteristic, the rapid rotation of many Lambda Boo stars generally limits how much detail can be derived from an abundance analysis. Traditionally, Lambda Boo candidates have been classified by visually examining the difference between their spectra and spectra of standard stars. Therefore, some ambiguity remains especially for mild or borderline Lambda Boo stars. This is the third paper in a series that establishes a straightforward yet reliable way to identify Lambda Boo-type stars. In previous papers, we identified line equivalent width (EW) ratios in the ultraviolet and visible regions that can distinguish Lambda Boo stars from other metal-weak stars. In this paper, we apply the visible line EW ratio diagnostic to 25 Lambda Boo candidates and carry out a detailed abundance analysis of HD 81290, an F2 star with a C i/Mg ii EW ratio in the range expected for Lambda Boo-type stars. Our elemental abundance analysis results confirm HD 81290's Lambda Boo membership and demonstrate the utility of our EW ratio as a diagnostic for cooler F-type Lambda Boo stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Hot Stars with Kepler Planets Have High Obliquities

It has been known for a decade that hot stars with hot Jupiters tend to have high obliquities. Less is known about the degree of spin–orbit alignment for hot stars with other kinds of planets. Here, we reassess the obliquities of hot Kepler stars with transiting planets smaller than Neptune, based on spectroscopic measurements of their projected rotation velocities (vsin i). The basis of the method is that a lower obliquity—all other things being equal—causes sin i to be closer to unity and increases the value of vsin i. We sought evidence for this effect using a sample of 150 Kepler stars with effective temperatures between 5950 and 6550 K and a control sample of 101 stars with matching spectroscopic properties and random orientations. The planet hosts have systematically higher values of vsin i than the control stars, but not by enough to be compatible with perfect spin–orbit alignment. The mean value of sin i is 0.856 ± 0.036, which is 4σ away from unity (perfect alignment), and 2σ away from π/4 (random orientations). There is also evidence that the hottest stars have a broader obliquity distribution: when modeled separately, the stars cooler than 6250 K have 〈sin i〉=0.928±0.042 while the hotter stars are consistent with random orientations. This is similar to the pattern previously noted for stars with hot Jupiters. Based on these results, obliquity excitation for early-G and late-F stars appears to be a general outcome of star and planet formation, rather than being exclusively linked to hot Jupiter formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Rotational Velocities of Am and Non-chemical-peculiar Stars Based on Kepler and LAMOST DR5

Rotation is a critical physical process operating in the formation of Am stars. There is a strong correlation between low-velocity rotation and chemical peculiarity. However, the existence of many non-chemical-peculiar slow rotators challenges the understanding of Am stars. The purpose of our work is to search for low-velocity rotating non-chemical-peculiar A-type stars and Am stars and to make a comparative analysis. In this paper, we pick out a sample from the LAMOST–Kepler project, including 21 Am stars, 125 non-chemical-peculiar slow rotators, and 53 non-chemical-peculiar fast rotators. We calculate the rotational frequencies through a periodic change of light curves caused by inhomogeneous stellar surfaces and then obtain the rotational velocities. For slow rotators, the age of Am stars is statistically younger than that of non-chemical-peculiar stars in the same temperature bin. In the comparison of the period, the average amplitude, and stellar mass of Am and non-chemical-peculiar stars, we discover that there is no difference in the photometric variability between Am and non-chemical-peculiar stars, which implies similar inhomogeneities on the surfaces. The average amplitude of non-chemical-peculiar stars has a downward trend with the increase of effective temperature and stellar mass, which is consistent with the theoretical prediction caused by weak dynamo-generated magnetic fields in A-type stars. In addition, we confirm four non-chemical-peculiar stars that have flares by checking field-of-view images, pixel images, and pixel-level light curves.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

The Influence of Disk Composition on the Evolution of Stars in the Disks of Active Galactic Nuclei

Abstract Disks of gas accreting onto supermassive black holes, powering active galactic nuclei (AGN), can capture stars from nuclear star clusters or form stars in situ via gravitational instability. The density and thermal conditions of these disks can result in rapid accretion onto embedded stars, dramatically altering their evolution in comparison to stars in the interstellar medium. Theoretical models predict that, when subjected to sufficiently rapid accretion, fresh gas replenishes hydrogen in the cores of these stars as quickly as it is burned into helium, reaching a quasi-steady state. Such massive, long-lived (“immortal”) stars may be capable of dramatically enriching AGN disks with helium, and would increase the helium abundance in AGN broad-line regions relative to that in the corresponding narrow-line regions and hosts. We investigate how the helium abundance of AGN disks alters the evolution of stars embedded therein. We find, in agreement with analytical arguments, that stars at a given mass are more luminous at higher helium mass fractions, and so undergo more radiation-driven mass loss. We further find that embedded stars tend to be less massive in disks with higher helium mass fractions, and that immortal stars are less common in such disks. Thus, disk composition can alter the rates of electromagnetic and gravitational wave transients as well as further chemical enrichment by embedded stars.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2023

HAZMAT. IX. An Analysis of the UV and X-Ray Evolution of Low-mass Stars in the Era of Gaia

Low-mass stars (≤1 M⊙) are some of the best candidates for hosting planets with detectable life because of these stars' long lifetimes and relative ratios of planet to star mass and radius. An important aspect of these stars to consider is the amount of ultraviolet (UV) and X-ray radiation incident on planets in the habitable zones due to the ability of UV and X-ray radiation to alter the chemistry and evolution of planetary atmospheres. In this work, we build on the results of the HAZMAT I and HAZMAT III M-star studies to determine the intrinsic UV and X-ray flux evolution with age for M stars using Gaia parallactic distances. We then compare these results to the intrinsic fluxes of K stars adapted from HAZMAT V. We find that although the intrinsic M-star UV flux is 10–100 times lower than that of K stars, the UV fluxes in their respective habitable zone are similar. However, the habitable zone X-ray flux evolutions are slightly more distinguishable with a factor of 3–15 times larger X-ray flux for late M stars than for K stars. These results suggest that there may not be a K-dwarf advantage compared to M stars in the UV, but one may still exist in the X-ray.

stars↗
DOE OSTI2014

Extended main sequence turnoffs in intermediate-age star clusters: a correlation between turnoff width and early escape velocity

We present a color-magnitude diagram analysis of deep Hubble Space Telescope imaging of a mass-limited sample of 18 intermediate-age (1-2 Gyr old) star clusters in the Magellanic Clouds, including eight clusters for which new data were obtained. We find that all star clusters in our sample feature extended main-sequence turnoff (eMSTO) regions that are wider than can be accounted for by a simple stellar population (including unresolved binary stars). FWHM widths of the MSTOs indicate age spreads of 200-550 Myr. We evaluate the dynamical evolution of clusters with and without initial mass segregation. Our main results are (1) the fraction of red clump (RC) stars in secondary RCs in eMSTO clusters scales with the fraction of MSTO stars having pseudo-ages of ≲1.35 Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity v {sub esc}, both currently and at an age of 10 Myr. We find that these two results are unlikely to be reproduced by the effects of interactive binary stars or a range of stellar rotation velocities. We therefore argue that the eMSTO phenomenon is mainly caused by extended star formation within the clusters; and (3) we find that v {sub esc} ≥ 15 km s{sup –1} out to ages of at least 100 Myr for all clusters featuring eMSTOs, and v {sub esc} ≤ 12 km s{sup –1} at all ages for two lower-mass clusters in the same age range that do not show eMSTOs. We argue that eMSTOs only occur for clusters whose early escape velocities are higher than the wind velocities of stars that provide material from which second-generation stars can form. The threshold of 12-15 km s{sup –1} is consistent with wind velocities of intermediate-mass asymptotic giant branch stars and massive binary stars in the literature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

LoCuSS: THE SLOW QUENCHING OF STAR FORMATION IN CLUSTER GALAXIES AND THE NEED FOR PRE-PROCESSING

We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 massive clusters at 0.15 < z < 0.30, combining wide-field Spitzer 24 μm and GALEX near-ultraviolet imaging with highly complete spectroscopy of cluster members. The fraction (f{sub SF}) of star-forming cluster galaxies rises steadily with cluster-centric radius, increasing fivefold by 2r{sub 200}, but remains well below field values even at 3r{sub 200}. This suppression of star formation at large radii cannot be reproduced by models in which star formation is quenched in infalling field galaxies only once they pass within r{sub 200} of the cluster, but is consistent with some of them being first pre-processed within galaxy groups. Despite the increasing f{sub SF}-radius trend, the surface density of star-forming galaxies actually declines steadily with radius, falling ∼15× from the core to 2r{sub 200}. This requires star formation to survive within recently accreted spirals for 2–3 Gyr to build up the apparent over-density of star-forming galaxies within clusters. The velocity dispersion profile of the star-forming galaxy population shows a sharp peak of 1.44 σ{sub ν} at 0.3r{sub 500}, and is 10%–35% higher than that of the inactive cluster members at all cluster-centric radii, while their velocity distribution shows a flat, top-hat profile within r{sub 500}. All of these results are consistent with star-forming cluster galaxies being an infalling population, but one that must also survive ∼0.5–2 Gyr beyond passing within r{sub 200}. By comparing the observed distribution of star-forming galaxies in the stacked caustic diagram with predictions from the Millennium simulation, we obtain a best-fit model in which star formation rates decline exponentially on quenching timescales of 1.73 ± 0.25 Gyr upon accretion into the cluster.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Carbon-enhanced metal-poor star candidates from BP/RP spectra in Gaia DR3

ABSTRACT Carbon-enhanced metal-poor (CEMP) stars comprise almost a third of stars with [Fe/H] < −2, although their origins are still poorly understood. It is highly likely that one sub-class (CEMP-s stars) is tied to mass-transfer events in binary stars, while another sub-class (CEMP-no stars) are enriched by the nucleosynthetic yields of the first generations of stars. Previous studies of CEMP stars have primarily concentrated on the Galactic halo, but more recently they have also been detected in the thick disc and bulge components of the Milky Way. Gaia DR3 has provided an unprecedented sample of over 200 million low-resolution (R ≈ 50) spectra from the BP and RP photometers. Training on the CEMP catalogue from the SDSS/SEGUE database, we use XGBoost to identify the largest all-sky sample of CEMP candidate stars to date. In total, we find 58 872 CEMP star candidates, with an estimated contamination rate of 12 per cent. When comparing to literature high-resolution catalogues, we positively identify 60–68 per cent of the CEMP stars in the data, validating our results and indicating a high completeness rate. Our final catalogue of CEMP candidates spans from the inner to outer Milky Way, with distances as close as r ∼ 0.8 kpc from the Galactic centre, and as far as r > 30 kpc. Future higher resolution spectroscopic follow-up of these candidates will provide validations of their classification and enable investigations of the frequency of CEMP-s and CEMP-no stars throughout the Galaxy, to further constrain the nature of their progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A CATALOG OF NEW SPECTROSCOPICALLY CONFIRMED MASSIVE OB STARS IN CARINA

The Carina star-forming region is one of the largest in the Galaxy, and its massive star population is still being unveiled. The large number of stars combined with high, and highly variable, interstellar extinction makes it inherently difficult to find OB stars in this type of young region. We present the results of a spectroscopic campaign to study the massive star population of the Carina Nebula, with the primary goal to confirm or reject previously identified Carina OB star candidates. A total of 141 known O- and B-type stars and 94 candidates were observed, of which 73 candidates had high enough signal-to-noise ratio to classify. We find 23 new OB stars within the Carina Nebula, a 32% confirmation rate. One of the new OB stars has blended spectra and is suspected to be a double-lined spectroscopic binary (SB2). We also reclassify the spectral types of the known OB stars and discover nine new SB2s among this population. Finally, we discuss the spatial distribution of these new OB stars relative to known structures in the Carina Nebula.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A cautionary note about composite galactic star formation relations

We explore the pitfalls that affect the comparison of the star formation relation for nearby molecular clouds with that for distant compact molecular clumps. We show that both relations behave differently in the (Σ{sub gas}, Σ{sub SFR}) space, where Σ{sub gas} and Σ{sub SFR} are, respectively, the gas and star formation rate surface densities, even when the physics of star formation is the same. This is because the star formation relation of nearby clouds relates the gas and star surface densities measured locally, that is, within a given interval of gas surface density, or at a given protostar location. We refer to such measurements as local measurements, and the corresponding star formation relation as the local relation. In contrast, the stellar content of a distant molecular clump remains unresolved. Only the mean star formation rate can be obtained, e.g., from the clump infrared luminosity. One clump therefore provides one single point to the (Σ{sub gas}, Σ{sub SFR}) space, that is, its mean gas surface density and star formation rate surface density. We refer to this star formation relation as a global relation since it builds on the global properties of molecular clumps. Its definition therefore requires an ensemble of cluster-forming clumps. We show that although the local and global relations have different slopes, this cannot per se be taken as evidence for a change in the physics of star formation with gas surface density. It therefore appears that great caution should be taken when physically interpreting a composite star formation relation, that is, a relation combining local and global relations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Variability in the Massive Open Cluster NGC 1817 from K2: A Rich Population of Asteroseismic Red Clump, Eclipsing Binary, and Main-sequence Pulsating Stars

We present a survey of variable stars detected in K2 Campaign 13 within the massive intermediate-age (∼1 Gyr) open cluster NGC 1817. We identify a complete sample of 44 red clump stars in the cluster, and have measured asteroseismic quantities (ν {sub max} and/or Δν) for 29 of them. Five stars showed suppressed dipole modes, and the occurrence rates indicate that mode suppression is unaffected by evolution through core helium burning. A subset of the giants in NGC 1817 (and in the similarly aged cluster NGC 6811) have ν {sub max} and Δν values at or near the maximum observed for core helium–burning stars, indicating they have core masses near the minimum for fully nondegenerate helium ignition. Further asteroseismic study of these stars can constrain the minimum helium core mass in red clump stars and the physics that determines this limit. Two giant stars show photometric variations on timescales similar to previously measured spectroscopic orbits. Thirteen systems in the field show eclipses, but only five are probable cluster members. We identify 32 δ Sct pulsators, 27 γ Dor candidates, and 7 hybrids that are probable cluster members, with most being new detections. We used the ensemble properties of the δ Sct stars to identify stars with possible radial pulsation modes. Among the oddities we have uncovered are: an eccentric orbit for a short-period binary containing a δ Sct pulsating star; a rare subgiant within the Hertzsprung gap showing δ Sct pulsations; and two hot γ Dor pulsating star candidates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Searching for Be stars in the open cluster NGC 663

We present Be star candidates in the open cluster NGC 663, identified by Hα imaging photometry with the Palomar Transient Factory Survey, as a pilot program to investigate how the Be star phenomena, the emission spectra, extended circumstellar envelopes, and fast rotation, correlate with massive stellar evolution. Stellar membership of the candidates was verified by 2MASS magnitudes and colors and by proper motions (PMs). We discover four new Be stars and exclude one known Be star from being a member due to its inconsistent PMs. The fraction of Be stars to member stars [N(Be)/N(members)] in NGC 663 is 3.5%. The spectral type of the 34 Be stars in NGC 663 shows bimodal peaks at B0–B2 and B5–B7, which is consistent with the statistics in most star clusters. Additionally, we also discover 23 emission-line stars of different types, including non-member Be stars, dwarfs, and giants.

79 ASTRONOMY AND ASTROPHYSICS↗