arXiv subjects

S. D. Chojnowski

Publications and source records attributed to S. D. Chojnowski.

At least 19 recordsLinked to original sources

arXiv2026

Results from the first spectropolarimetric survey of $γ$ Dor pulsators

Context. Magnetic fields can have an important influence on stellar structure and evolution. In intermediate-mass (A- and F-type) stars, there are many known stars with directly measured strong, globally organized magnetic fields, as well as indications of weak and/or small-scale variable fields. However, among the intermediate-mass $γ$ Dor pulsators, there are no known stars with strong surface magnetic fields. Aims. The broad goal of this work is to search for evidence of strong, globally organized fields at the surface of $γ$ Dor pulsators. Methods. We identified objects consistent with being $γ$ Dor pulsators based on an analysis of space photometry from the Transiting Exoplanet Survey Satellite (TESS) mission. A spectropolarimetric survey was then conducted on a subset of 47 of these objects, with a precision sufficient to detect dipolar surface magnetic fields down to a threshold of about 10 -- 100 G. Results. We detected strong magnetism in three targets. However, upon closer inspection, none of these appear to be genuine $γ$ Dor pulsators. We found no evidence of surface magnetism in any of the remaining 44 objects. Conclusions. We conclude that either strong, globally organized magnetic fields and $γ$ Dor pulsation are mutually exclusive, or that such stars are exceedingly rare. A possible explanation is that strong global fields inhibit the excitation mechanism, which prevents $γ$ Dor pulsations from being driven in strongly magnetic intermediate-mass stars. The dipolar surface magnetic field strength upper limits we derive for this sample ($\lesssim$ 50 -- 100 G) provide valuable constraints for surface boundary conditions for asteroseismic models that include magnetism for $γ$ Dor stars.

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arXiv2025

Newborn Be star systems observed shortly after mass transfer

Many classical Be stars acquire their very rapid rotation by mass and angular-momentum transfer in massive binaries. Short-lived intermediate-phase objects have only been discovered recently. Data archives and the literature have been searched for additional candidates exhibiting this patterns. Thirteen candidates were identified at various confidence levels. Adding to the two known systems identified as classical Be star+pre-subdwarf binaries (LB-1 and HR6819), two more (V742Cas, HD44637) could be confirmed with interferometry, with V742Cas setting a new record for the smallest visually observed angular semi-major axis, at a=0.663mas. Two further ones (V447Sct, V1362Cyg) are not resolved interferometrically, but other evidence puts them at the same confidence level as LB-1. V2174Cyg is a candidate with very high confidence, but was not observed interferometrically. The remaining ones are either candidates with varying levels of confidence. Of a mostly magnitude complete sample of 328 Be stars, 0.5-1% are found to have recently completed the mass overflow that led to their formation. Another 5% are systems with compact subdwarf companions, i.e., further evolved after a previous overflow, and possibly two more percent harbor white dwarfs. All these systems are of early B-subtypes, however, and if the original sample is restricted to early subtypes (136 objects), these percentages increase by a factor of about 2.5, while dropping to zero for the mid and late subtypes (together 204 objects). This strongly suggests that early- vs. mid- and late-type Be stars have differently weighted channels to acquire their rapid rotation, namely binary interaction vs. evolutionary spin-up.

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arXiv2024

Extreme Magnetic Field Modulus Variability of the Bp star HD 57372

Context. In chemically peculiar Ap/Bp stars with large-scale organised magnetic fields with a simple centred dipole configuration, the ratio between the maximum and the minimum of the mean magnetic field modulus is of the order of 1.25. Values of 2 or more are observed only for very few Ap/Bp stars and are indicative of a very unusual magnetic field geometry. Aims. Determining the magnetic field structure of Ap/Bp stars is bound to provide a different insight into the physics and the origin of the magnetic fields in early-type stars. In this respect, the Bp star HD 57372 is of particular interest because strongly variable magnetically split lines are observed in HARPS and APOGEE spectra. Methods. We obtained and analysed measurements of the mean magnetic field modulus and of the mean longitudinal magnetic field using near-infrared spectra and optical polarimetric spectra distributed over the stellar rotation period. Results. The mean magnetic field modulus of HD 57372, as estimated from absorption lines that are split via the Zeeman effect and resolved in both optical and near-infrared spectra, is found to vary by an extraordinary amount of about 10 kG. The exceptional value of 3 for the ratio between the maximum and the minimum of the field modulus is indicative of a very unusual geometry of HD 57372's magnetic field. All observable quantities are found to vary in phase with the photometric period of 7.889 days. This includes the longitudinal magnetic field , which varies from -6 kG up to 1.7 kG in FORS2 spectra as well as the metal line strengths, whose equivalent widths change by up to 50% of their mean values over the course of the rotation period. The B8 temperature class of HD 57372 also places it among the hottest stars known to exhibit resolved, magnetically split lines.

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arXiv2024

Testing pulsation diagnostics in the rapidly oscillating magnetic Ap star $γ$ Equ using near-infrared CRIRES+ observations

Pulsations of rapidly oscillating Ap stars and their interaction with the stellar magnetic field have not been studied in the near-IR region despite the benefits these observations offer compared to visual wavelengths. The main advantage of the near-IR is the quadratic dependence of the Zeeman effect on the wavelength, as opposed to the linear dependence of the Doppler effect. To test pulsation diagnostics of roAp stars in the near-IR, we aim to investigate the pulsation behaviour of one of the brightest magnetic roAp stars, $γ$Equ, which possesses a strong surface magnetic field of the order of several kilogauss and exhibits magnetically split spectral lines in its spectra. Two magnetically split spectral lines belonging to different elements, the triplet Fe I at 1563.63nm and the pseudo-doublet Ce III at 1629.2nm, were recorded with CRIRES+ over about one hour in the H band with the aim of understanding the character of the line profile variability and the pulsation behaviour of the magnetic field modulus. The profile shapes of both studied magnetically split spectral lines vary in a rather complex manner probably due to a significant decrease in the strength of the longitudinal field component and an increase in the strength of the transverse field components over the last decade. A mean magnetic field modulus of 3.9kG was determined for the Fe I, whereas for the Ce III we observe only about 2.9kG. For comparison, a mean field modulus of 3.4kG was determined using the Zeeman doublet Fe II at 6249.25 in optical PEPSI spectra recorded just about two weeks before the CRIRES+ observations. Different effects that may lead to the differences in the field modulus values are discussed. Our measurements of the mean magnetic field modulus using the line profiles recorded in different pulsational phase bins suggest a field modulus variability of 32G for the Fe I and 102G for the Ce III.

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arXiv2022

KIC 7955301: a hierarchical triple system with eclipse timing variations and an oscillating red giant

KIC 7955301 is a hierarchical triple system with eclipse timing and depth variations discovered by the Kepler mission. It is composed of a non-eclipsing primary star at the bottom of the red giant branch on a 209-day orbit with a K/G-type main-sequence inner eclipsing binary, orbiting in 15.3 days. This system was noted for the large amplitude of its eclipse timing variations (4 hours), and the clear solar-like oscillations of the red-giant component, including p-modes of degree up to l=3 and mixed l=1 modes. The system is a single-lined spectroscopic triple. We perform a dynamical model by combining the Kepler photometric data, eclipse timing variations, and radial-velocity data obtained at Apache Point (ARCES) and Haute Provence (SOPHIE) observatories. The dynamical mass of the red-giant is determined with a 2% precision at 1.30 (+0.03,-0.02) solar mass. We perform asteroseismic modeling based on the global seismic parameters and on the individual frequencies. Both methods lead to a mass of the red giant that matches the dynamical mass within the uncertainties. Asteroseismology also reveals the rotation rate of the core (15 days), the envelope (150 days), and the inclination (75 deg) of the red giant. Three different approaches lead to an age between 3.3 and 5.8 Gyr, which highlights the difficulty of determining stellar ages despite the exceptional wealth of available information. On short timescales, the inner binary exhibits eclipses with varying depths during a 7.3-year long interval, and no eclipses during the consecutive 11.9 years. This is why Kepler could detect its eclipses, TESS will not, and the future ESA PLATO mission should. Over the long term, the system owes its evolution to the evolution of its individual components. It could end its current smooth evolution by merging by the end of the red giant or the asymptotic giant branch of the primary star.

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arXiv2022

MWC656: A Be+BH or a Be+sdO?

MWC656 has been reported as classical Be star with a black hole companion. Revisited spectral variability properties render this unlikely, with a hot subdwarf more probable.

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arXiv2022

Signatures of Feedback in the Spectacular Extended Emission Region of NGC 5972

We present Chandra X-ray Observatory observations and Space Telescope Imaging Spectrograph spectra of NGC 5972, one of the 19 "Voorwerpjes" galaxies. This galaxy contains an Extended Emission Line Region (EELR) and an arc-second scale nuclear bubble. NGC 5972 is a faded AGN, with EELR luminosity suggesting a 2.1 dex decrease in L$_{\textrm{bol}}$ in the last $\sim5\times10^{4}$ yr. We investigate the role of AGN feedback in exciting the EELR and bubble given the long-term variability and potential accretion state changes. We detect broadband (0.3-8 keV) nuclear X-ray emission coincident with the [OIII] bubble, as well as diffuse soft X-ray emission coincident with the EELR. The soft nuclear (0.5-1.5 keV) emission is spatially extended and the spectra are consistent with two APEC thermal populations ($\sim$0.80,$\sim$0.10 keV). We find a bubble age >2.2 Myr, suggesting formation before the current variability. We find evidence for efficient feedback with L$_{\textrm{kin}}/L_{\textrm{bol}}\sim0.8\%$, which may be overestimated given the recent L$_{\textrm{bol}}$ variation. Kinematics suggest an out-flowing 300 km s$^{-1}$ high-ionization [OIII]-emitting gas which may be the line of sight component of a $\sim$780 km s$^{-1}$ thermal X-ray outflow capable of driving strong shocks that could photoionize the precursor material. We explore possibilities to explain the overall jet, radio lobe and EELR misalignment including evidence for a double SMBH which could support a complex misaligned system.

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arXiv2022

The Open Cluster Chemical Abundances and Mapping Survey: V. Chemical Abundances of CTIO/Hydra Clusters using The Cannon

Open clusters are key chemical and age tracers of Milky Way evolution. While open clusters provide significant constraints on galaxy evolution, their use has been limited due to discrepancies in measuring abundances from different studies. We analyze medium resolution (R~19,000) CTIO/Hydra spectra of giant stars in 58 open clusters using The Cannon to determine [Fe/H], [Mg/Fe], [Si/Fe], [Al/Fe], and [O/Fe]. This work adds an additional 55 primarily southern hemisphere open clusters calibrated to the SDSS/APOGEE DR16 metallicity system. This uniform analysis is compared to previous studies [Fe/H] measurements for 23 clusters and we present spectroscopic metallicities for the first time for 35 open clusters.

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arXiv2022

First detection of a magnetic field in low-luminosity B[e] stars: New scenarios for the nature and evolutionary stages of FS CMa stars

We report the first detection of the magnetic field in a star of FS CMa type, a subgroup of objects characterized by the B[e] phenomenon. The split of magnetically sensitive lines in IRAS 17449+2320 determines the magnetic field modulus of 6.2+/-0.2 kG. Spectral lines and their variability reveal the presence of a B-type spectrum and a hot continuum source in the visible. The hot source confirms GALEX UV photometry. Because there is a lack of spectral lines for the hot source in the visible, the spectral fitting gives only the lower temperature limit of the hot source, which is 50 000 K, and the upper limit for the B-type star of 11 100 K. The V/R ratio of the H alpha line shows quasiperiodic behavior on timescale of 800 days. We detected a strong red-shifted absorption in the wings of Balmer and OI lines in some of the spectra. The absorption lines of helium and other metals show no, or very small, variations, indicating unusually stable photospheric regions for FS CMa stars. We detected two events of material infall, which were revealed to be discrete absorption components of resonance lines. The discovery of the strong magnetic field together with the Gaia measurements of the proper motion show that the most probable nature of this star is that of a post-merger object created after the leaving the binary of the birth cluster. Another possible scenario is a magnetic Ap star around Terminal-Age Main Sequence (TAMS). On the other hand, the strong magnetic field defies the hypothesis that IRAS 17449+2320 is an extreme classical Be star. Thus, IRAS 17449+2320 provides a pretext for exploring a new explanation of the nature of FS CMa stars or, at least, a group of stars with very similar spectral properties.

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arXiv2021

Final Targeting Strategy for the SDSS-IV APOGEE-2N Survey

APOGEE-2 is a dual-hemisphere, near-infrared (NIR), spectroscopic survey with the goal of producing a chemo-dynamical mapping of the Milky Way Galaxy. The targeting for APOGEE-2 is complex and has evolved with time. In this paper, we present the updates and additions to the initial targeting strategy for APOGEE-2N presented in Zasowski et al. (2017). These modifications come in two implementation modes: (i) "Ancillary Science Programs" competitively awarded to SDSS-IV PIs through proposal calls in 2015 and 2017 for the pursuit of new scientific avenues outside the main survey, and (ii) an effective 1.5-year expansion of the survey, known as the Bright Time Extension, made possible through accrued efficiency gains over the first years of the APOGEE-2N project. For the 23 distinct ancillary programs, we provide descriptions of the scientific aims, target selection, and how to identify these targets within the APOGEE-2 sample. The Bright Time Extension permitted changes to the main survey strategy, the inclusion of new programs in response to scientific discoveries or to exploit major new datasets not available at the outset of the survey design, and expansions of existing programs to enhance their scientific success and reach. After describing the motivations, implementation, and assessment of these programs, we also leave a summary of lessons learned from nearly a decade of APOGEE-1 and APOGEE-2 survey operations. A companion paper, Santana et al. (submitted), provides a complementary presentation of targeting modifications relevant to APOGEE-2 operations in the Southern Hemisphere.

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arXiv2021

Final Targeting Strategy for the SDSS-IV APOGEE-2S Survey

APOGEE is a high-resolution (R sim 22,000), near-infrared, multi-epoch, spectroscopic survey of the Milky Way. The second generation of the APOGEE project, APOGEE-2, includes an expansion of the survey to the Southern Hemisphere called APOGEE-2S. This expansion enabled APOGEE to perform a fully panoramic mapping of all the main regions of the Milky Way; in particular, by operating in the H-band, APOGEE is uniquely able to probe the dust-hidden inner regions of the Milky Way that are best accessed from the Southern Hemisphere. In this paper we present the targeting strategy of APOGEE-2S, with special attention to documenting modifications to the original, previously published plan. The motivation for these changes is explained as well as an assessment of their effectiveness in achieving their intended scientific objective. In anticipation of this being the last paper detailing APOGEE targeting, we present an accounting of all such information complete through the end of the APOGEE-2S project; this includes several main survey programs dedicated to exploration of major stellar populations and regions of the Milky Way, as well as a full list of programs contributing to the APOGEE database through allocations of observing time by the Chilean National Time Allocation Committee (CNTAC) and the Carnegie Institution for Science (CIS). This work was presented along with a companion article, R. Beaton et al. (submitted; AAS29028), presenting the final target selection strategy adopted for APOGEE-2 in the Northern Hemisphere.

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arXiv2021

Magnetic field geometry and magnetospheric environment of the strongly magnetic Of?p star NGC 1624-2

NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field strength $\ge$ 20 kG, which is the strongest magnetic field ever measured in an O-type star. We study the variability of the mean longitudinal magnetic field $\langle B_z \rangle$ and the mean field modulus $\langle B \rangle$ to obtain constraints on its field geometry. Only one magnetic pole is observable over the rotation cycle. The approximately sinusoidal variation of $\langle B_z \rangle$ and the ratio of the values of the extrema of $\langle B \rangle$ indicate that there is an important component of the field that is dipolar. The $\langle B_z \rangle$ values measured over the rotation cycle are in the range from $-0.2$ to 4.5 kG, whereas the values for $\langle B \rangle$ vary between 9 and 12 kG. The $\langle B_z \rangle$ values obtained using the O III $λ$7455 emission line are in the range from 0.4 to 2.3 kG and show a variability pattern similar to that detected for the absorption lines. The fact that the phase of the $\langle B_z \rangle$ minimum coincides with the phase of the $\langle B \rangle$ maximum, indicates that the field structure must significantly depart from a centred dipole. Further, we discuss the nature of the observed variable Stokes $V$ profiles corresponding to a longitudinal field of negative polarity detected in the emission He I lines and present the first MHD numerical simulations of the gas flow in the magnetosphere of this star.

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arXiv2020

HD 63021: Chromospheric Activity and Mass Transfer in a Close Binary

Prompted by X-ray detections from multiple surveys, we investigated the A-type star HD 63021 and found that it is a double-lined spectroscopic binary with highly variable emission associated with the primary star. Analysis of our multi-epoch spectroscopic observations, the majority of which were carried out on small aperture telescopes, indicates a very short orbital period of just $2.9$ days, and a mass ratio M$_2$/M$_1$ of $0.23$. The A1 V star is a slow rotator, with a rotational speed of $\sim34$ km/s. Assuming its mass is $2.3$ M$_{\odot}$, the present-day secondary is an evolved star of $\sim0.5$ M$_{\odot}$ that nearly fills its Roche lobe. This secondary star rotates comparatively rapidly at $\sim44$ km/s, and we see evidence that it is chromospherically active. Analysis of a photometric lightcurve from TESS reveals two strong periods, one at the orbital period for the system and another at half the orbital period. These findings suggest that HD 63021 is a close binary system undergoing mass transfer from the secondary star onto the primary star -- in all ways like an Algol eclipsing binary system, except without the eclipse. We discuss the system's mass transfer, which is not steady but seems to run in fits and bursts, and infer the system's basic physical properties from an orbital parameter study, the Roche lobe geometry, and its extant X-ray emission.

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arXiv2020

Detection of weak magnetic fields in two HgMn stars

The main-sequence mercury-manganese (HgMn) stars are known to exhibit large overabundances of exotic elements and, similar to magnetic Ap/Bp stars, are spectrum variables, implying the presence of an inhomogeneous element distribution over the stellar surface. A number of magnetic field studies have been attempted in the last decades, indicating that magnetic fields in HgMn stars, if they exist, should be rather weak. The presence of tangled magnetic fields was suggested by several authors who detected quadratic magnetic fields using the moment technique. We employ the least-squares deconvolution technique to carry out a sensitive search for weak magnetic fields in spectropolarimetric observations of three HgMn stars, HD221507, HD65949, and HD101189, which have different fundamental parameters and spectral characteristics. A definite weak longitudinal field is discovered in HD221507 and HD65949 on single epochs, while only marginal field detections were achieved for HD101189. The new measurements indicate that the structure of the magnetic fields is probably rather complex: our analysis reveals the presence of reversed Stokes V profiles at the same observational epoch if individual elements are used in the measurements. This is the first observational evidence that individual elements sample distinct local magnetic fields of different polarity across the stellar surface.

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arXiv2019

The Importance of Telescope Training in Data Interpretation

In this State of the Profession Consideration, we will discuss the state of hands-on observing within the profession, including: information about professional observing trends; student telescope training, beginning at the undergraduate and graduate levels, as a key to ensuring a base level of technical understanding among astronomers; the role that amateurs can take moving forward; the impact of telescope training on using survey data effectively; and the need for modest investments in new, standard instrumentation at mid-size aperture telescope facilities to ensure their usefulness for the next decade.

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arXiv2018

APOGEE DR14/DR15 Abundances in the Inner Milky Way

We present an overview of the distributions of 11 elemental abundances in the Milky Way's inner regions, as traced by APOGEE stars released as part of SDSS Data Release 14/15 (DR14/DR15), including O, Mg, Si, Ca, Cr, Mn, Co, Ni, Na, Al, and K. This sample spans ~4000 stars with R_GC<4 kpc, enabling the most comprehensive study to date of these abundances and their variations within the innermost few kiloparsecs of the Milky Way. We describe the observed abundance patterns ([X/Fe]-[Fe/H]), compare to previous literature results and to patterns in stars at the solar Galactic radius, and discuss possible trends with DR14/DR15 effective temperatures. We find that the position of the [Mg/Fe]-[Fe/H] "knee" is nearly constant with R_GC, indicating a well-mixed star-forming medium or high levels of radial migration in the early inner Galaxy. We quantify the linear correlation between pairs of elements in different subsamples of stars and find that these relationships vary; some abundance correlations are very similar between the alpha-rich and alpha-poor stars, but others differ significantly, suggesting variations in the metallicity dependencies of certain supernova yields. These empirical trends will form the basis for more detailed future explorations and for the refinement of model comparison metrics. That the inner Milky Way abundances appear dominated by a single chemical evolutionary track and that they extend to such high metallicities underscore the unique importance of this part of the Galaxy for constraining the ingredients of chemical evolution modeling and for improving our understanding of the evolution of the Galaxy as a whole.

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arXiv2018

Toward Understanding the B[e] Phenomenon. VII. AS 386, a single-lined binary with a candidate black hole component

We report the results of spectroscopic and photometric observations of the emission-line object AS 386. For the first time, we found that it exhibits the B[e] phenomenon and fits the definition of an FS CMa type object. The optical spectrum shows the presence of a B-type star with the following properties: T_ eff = 11000+/-500 K, log L/L_sun = 3.7+/-0.3, a mass of 7+/-1 M_sun, and a distance D = 2.4+/-0.3 kpc from the Sun. We detected regular radial velocity variations of both absorption and emission lines with the following orbital parameters: P_orb = 131.27+/-0.09 days, semi-amplitude K_1 = 51.7+/-3.0 km/s, systemic radial velocity gamma = -31.8+/-2.6 km/s, and a mass function of f(m) = 1.9+/-0.3 M_sun. AS 386 exhibits irregular variations of the optical brightness (V=10.92+/-0.05 mag), while the near-IR brightness varies up to ~0.3 mag following the spectroscopic period. We explain this behavior by a variable illumination of the dusty disk inner rim by the B-type component. Doppler tomography based on the orbital variations of emission-line profiles shows that the material is distributed near the B-type component and in a circumbinary disk. We conclude that the system has undergone a strong mass transfer that created the circumstellar material and increased the B-type component mass. The absence of any traces of a secondary component, whose mass should be >= 7 M_sun, suggests that it is most likely a black hole.

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arXiv2017

Target Selection for the SDSS-IV APOGEE-2 Survey

APOGEE-2 is a high-resolution, near-infrared spectroscopic survey observing roughly 300,000 stars across the entire sky. It is the successor to APOGEE and is part of the Sloan Digital Sky Survey IV (SDSS-IV). APOGEE-2 is expanding upon APOGEE's goals of addressing critical questions of stellar astrophysics, stellar populations, and Galactic chemodynamical evolution using (1) an enhanced set of target types and (2) a second spectrograph at Las Campanas Observatory in Chile. APOGEE-2 is targeting red giant branch (RGB) and red clump (RC) stars, RR Lyrae, low-mass dwarf stars, young stellar objects, and numerous other Milky Way and Local Group sources across the entire sky from both hemispheres. In this paper, we describe the APOGEE-2 observational design, target selection catalogs and algorithms, and the targeting-related documentation included in the SDSS data releases.

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