bluuebunny/crossref_metadata_2025_split huggingface dataset:
A new method for calculation of closed‐form response of linear time‐invariant systems to periodic input signals
Authors: Safaai‐Jazi Ahmad 
Published: January 2023

AbstractA new method for finding closed‐form time‐domain solutions of linear time‐invariant (LTI) systems with arbitrary periodic input signals is presented. These solutions, unlike those obtained based on the conventional Fourier‐phasor method, have a finite number of terms in one period. To implement the proposed method, the following steps are carried out: (1) For a given system, represented by a transfer function, an impulse response, a block diagram etc., the governing differential equation relating the output of the system, , to its input, , is obtained. (2) An auxiliary differential equation is formed by simply replacing with and equating the input side to alone. The auxiliary differential equation is solved for each time segment of the input signal in one period, leaving the constant coefficients associated with the homogeneous solutions as unknowns. For an nth‐order system with an input signal consisting of q segments in one period, there are such unknown coefficients. (3) Continuity of and its derivatives, at the connection points of successive segments and the periodicity conditions for the beginning and end points of the period are implemented. (4) The outcome of step 3 is a system of equations in terms of unknown coefficients described in step 2. Solving this system of equations, the solution for in one period is obtained. (5) Finally, using the linearity and differentiation properties of the system and the coefficients of the input side of the differential equation of the system, the total response, , in one period is constructed in terms of and its derivatives. For stable LTI systems, the proposed method can be used without any limitations.
The globular cluster system of the nearest Seyfert II galaxy Circinus
Authors: Obasi C. , Gómez M. , Minniti D. , Alonso-García J. , Hempel M. , Pullen J. B. , Gregg M. D. , Baravalle L. D. , Alonso M. V. , Okere B. I. 
Published: January 2023

Context. The globular cluster (GC) system of Circinus galaxy has not been probed previously partly because of the location of the galaxy at −3.8° Galactic latitude, which suffers severely from interstellar extinction, stellar crowding, and Galactic foreground contamination. However, the deep near-infrared (NIR) photometry by the VISTA Variables in the Via Láctea Extended Survey (VVVX) in combination with the precise astrometry of Gaia EDR3 allow us to map GCs in this region.
Aims. Our long-term goal is to study and characterise the distributions of GCs and ultra-compact dwarfs in Circinus galaxy, which is the nearest Seyfert II galaxy. Here we conduct the first pilot search for GCs in this galaxy.
Methods. We used NIR VVVX photometry in combination with Gaia EDR3 astrometric features, such as astrometric excess noise and the ratio of the sum of the blue photometer (BP) and red photometer (RP) to the broad G passband (BRexcess), to build the first homogeneous catalogue of GCs in Circinus galaxy. A robust combination of selection criteria allows us to effectively clean interlopers from our sample.
Results We report the detection of ∼70 GC candidates in this galaxy at a 3σ confidence level. They show a bimodal colour distribution with the blue peak at (G − Ks)0 = 0.985±0.127 mag with a dispersion of 0.211±0.091 mag and the red peak at (G − Ks)0 = 1.625±0.177 mag with a dispersion of 0.482±0.114 mag. A GC specific frequency (S​N) of 1.3±0.2 was derived for the galaxy, and we estimated a total population of 120±40 GCs. Based on the projected radial distribution it appears that Circinus has a different distribution of GC candidates than MW and M 31.
Conclusions. We demonstrate that Circinus galaxy hosts a sizeable number of cluster candidates. This result is the first leap towards understanding the evolution of old stellar clusters in this galaxy.
GaiaData Release 3
Authors: Marton Gábor , Ábrahám Péter , Rimoldini Lorenzo , Audard Marc , Kun Mária , Nagy Zsófia , Kóspál Ágnes , Szabados László , Holl Berry , Gavras Panagiotis , Mowlavi Nami , Nienartowicz Krzysztof , de Fombelle Grégory Jevardat , Lecoeur-Taïbi Isabelle , Karbevska Lea , Lario Pedro Garcia , Eyer Laurent 
Published: January 2023

Context.TheGaiathird Data Release (DR3) presents the first catalogue of full-sky variable young stellar object (YSO) candidates observed by theGaiaspace telescope during the initial 34 months of science operations.Aims.Numerous types of variable stars were classified using photometric data collected byGaia. One of the new classes presented in theGaiaDR3 is the class of YSOs showing brightness variability. We analysed 79 375 sources classified as YSO candidates in order to validate their young nature and investigate the completeness and purity of the sample.Methods.We cross-matched theGaiaDR3 YSO sample with numerous catalogues from the literature, including YSO catalogues based on optical and infrared data, as well as catalogues of extragalactic sources and Galactic variable stars. YSO catalogues were used to quantify the completeness of theGaiaDR3 YSO sample, while others were inspected to calculate the contamination.Results.Among the 79 375 potential YSO candidates published in theGaiaDR3 variable star catalogue, the majority of these objects are distributed along the line of sight of well-known star forming regions and the Galactic midplane. We find that the upper limit of contamination is 26.7%, depending on the external catalogue used for the estimation, but find an average of ∼10% in general, while the completeness is at the percent level, taking into account that theGaiaDR3 YSO sample is based on sources that showed significant variability during the data-collection period. The number of sources in our sample that had not previously been catalogued as YSO candidates is ∼40 000 objects.
Importance of source structure on complex organics emission
Authors: Nazari P. , Tabone B. , Rosotti G. P. 
Published: January 2023

Context. The hot molecular core phase of massive star formation shows emission from complex organic molecules. However, these species are only detected toward a fraction of high-mass protostars. In particular, there is a spread of ~2 orders of magnitude in methanol emission intensity from high-mass protostars.
Aims. The goal of this work is to answer the question of whether high-mass disks can explain the lack of methanol emission from some massive protostellar systems.
Methods. We considered an envelope-only and an envelope-plus-disk model and used the code RADMC-3D to calculate the methanol emission. High and low millimeter (mm) opacity dust (representing large and small dust distributions) were considered for both models separately, and the methanol abundance was parameterized. Viscous heating was included due to the high accretion rates of these objects in the disk.
Results. In contrast with low-mass protostars, the presence of a disk does not significantly affect the temperature structure and methanol emission. The shadowing effect of the disk is not as important for high-mass objects, and the disk midplane is hot because of viscous heating, which is effective due to the high accretion rates. The methanol emission is lower for models with high mm opacity dust because the dust attenuation blocks the emission in the envelope and hides it in the disk through continuum oversubtraction, but the disk needs to be large for this to become effective. A minimum disk size of ~2000–2500 au is needed (at L = 104 L⊙) with high mm opacity dust for drop of a factor of about one order of magnitude in the methanol emission compared with the envelope-only models with low mm opacity dust. Consistent with observations of infrared absorption lines toward high-mass protostars, we find a vertical temperature inversion, that is, higher temperatures in the disk midplane than the disk surface, at radii ≲50 au for models with L = 104 L⊙ and high mm opacity dust as long as the envelope mass is ≳550 M⊙ (Ṁ = 3.6 × 10−3 M⊙ yr−1).
Conclusions. The large observed scatter in methanol emission from massive protostars can be mostly explained toward lower-luminosity objects (~103 L⊙) with the envelope-plus-disk models including low and high mm opacity dust. The methanol emission variation toward sources with high luminosities (≳104 L⊙) cannot be explained by models with or without a disk with a relatively high gas-phase abundance of methanol. However, the luminosity-to-mass ratios of these objects suggest that they might be associated with hypercompact or ultracompact HII regions. Therefore, the low methanol emission toward the high-luminosity sources can be explained by them hosting an HII region in which methanol is absent.
Tidal perturbations and eclipse mapping in the pulsations in the hierarchical triple system U Gru
Authors: Johnston C. , Tkachenko A. , Van Reeth T. , Bowman D. M. , Pavlovski K. , Sana H. , Sekaran S. 
Published: January 2023

Context. Unambiguous examples of the influence of tides on self-excited, free stellar pulsations have recently been observationally detected in space-based photometric data.
Aims. We aim to investigate U Gru and contextualise it within the growing class of tidally influenced pulsators. An initial analysis of U Gru revealed frequencies spaced by the orbital frequency that are difficult to explain by currently proposed tidal mechanisms.
Methods. We reinvestigate the TESS photometry for U Gru alongside new UVES spectroscopy. We analyse the UVES spectroscopy with least-squares deconvolution and spectral disentangling techniques, and perform an atmospheric analysis. We remove the binary signature from the light curve using an effective model in order to investigate the pulsation signal in the residuals. We track the amplitudes and phases of the residual pulsations as a function of the orbital period to reveal their tidal influence.
Results. We establish that U Gru is likely a hierarchical triple system. We identify a single p-mode oscillation that exhibits amplitude and phase variation over the binary orbit. We propose a toy model to demonstrate that the series of frequencies separated by the orbital frequency can be reproduced by eclipse mapping. We find no evidence of modulation to the other independent oscillation modes.
Conclusions. We demonstrate that U Gru hosts at least one tidally perturbed pulsation. Additionally, we argue that eclipse mapping of the dominant, tidally perturbed mode can produce the series of frequencies separated by the observed orbital frequency. Our simulations show that the effects of eclipse mapping are mode dependent, and are not expected to produce an observable signature for all pulsation modes in an eclipsing binary.
Massive pre-main-sequence stars in M17
Authors: Backs F. , Poorta J. , Rab Ch. , Derkink A. R. , de Koter A. , Kaper L. , Ramírez-Tannus M. C. , Kamp I. 
Published: January 2023

Context.The young massive-star-forming region M17 contains optically visible massive pre-main-sequence stars that are surrounded by circumstellar disks. Such disks are expected to disappear when these stars enter the main sequence. The physical and dynamical structure of these remnant disks are poorly constrained, especially the inner regions where accretion, photo-evaporation, and companion formation and migration may be ongoing.Aims.We aim to constrain the physical properties of the inner parts of the circumstellar disks of massive young stellar objects B243 (6M⊙) and B331 (12M⊙), two systems for which the central star has been detected and characterized previously despite strong dust extinction.Methods.Two-dimensional radiation thermo-chemical modelling with PRODIMOof double-peaked hydrogen lines of the Paschen and Brackett series observed with X-shooter was used to probe the properties of the inner disk of the target sources. The model was modified to treat these lines. Additionally, the dust structure was studied by fitting the optical and near-infrared spectral energy distribution.Results.B243 features a hot gaseous inner disk with dust at the sublimation radius at ~3 AU. The disk appears truncated at roughly 6.5 AU; a cool outer disk of gas and dust may be present, but it cannot be detected with our data. B331 also has a hot gaseous inner disk. A gap separates the inner disk from a colder dusty outer disk starting at up to ~100 AU. In both sources the inner disk extends to almost the stellar surface. Chemistry is essential for the ionization of hydrogen in these disks.Conclusions.The lack of a gap between the central objects and these disks suggests that they accrete through boundary-layer accretion. This would exclude the stars having a strong magnetic field. Their structures suggest that both disks are transitional in nature, that is to say they are in the process of being cleared, either through boundary-layer accretion, photo-evaporation, or through companion activity.
Physical conditions for dust grain alignment in Class 0 protostellar cores
Authors: Le Gouellec V. J. M. , Maury A. J. , Hull C. L. H. 
Published: January 2023

Context. High angular resolution observations of Class 0 protostars have produced detailed maps of the polarized dust emission in the envelopes of these young embedded objects. Interestingly, the improved sensitivity brought by ALMA has revealed wide dynamic ranges of polarization fractions, with specific locations harboring surprisingly large amounts of polarized dust emission.
Aims. Our aim is to characterize the grain alignment conditions and dust properties responsible for the observed polarized dust emission in the inner envelopes (≤1000 au) of Class 0 protostars.
Methods. We analyzed the polarized dust emission maps obtained with ALMA and compared them to molecular line emission maps of specific molecular tracers, mainly C2H, which allowed us to probe one of the key components in dust grain alignment theories: the irradiation field.
Results. We show that C2H peaks toward outflow cavity walls, where the polarized dust emission is also enhanced. Our analysis provides a tentative correlation between the morphology of the polarized intensity and C2H emission, suggesting that the radiation field impinging on the cavity walls favors both the grain alignment and the warm carbon chain chemistry in these regions. We propose that shocks happening along outflow cavity walls could potentially represent an additional source of photons contributing to dust grain alignment. However, some parts of the cores, such as the equatorial planes, exhibit enhanced polarized flux, although no radiation driven chemistry is observed, for example where radiative torques are theoretically not efficient enough. This suggests that additional physical conditions, such as source geometry and dust grain evolution, may play a role in grain alignment.
Conclusions. Comparing chemical processes with grain alignment physics opens a promising avenue to develop our understanding of the dust grain evolution (i.e., their origin, growth, and structure) in the interior of Class 0 protostars. The source geometry and evolution can represent important factors that set the environmental conditions of the inner envelope, determining whether the radiation field strength and spectrum can drive efficient dust grain alignment via radiative torques.
Misalignment of the outer disk of DK Tau and a first look at its magnetic field using spectropolarimetry
Authors: Nelissen M. , McGinnis P. , Folsom C. P. , Ray T. , Vidotto A. A. , Alecian E. , Bouvier J. , Morin J. , Donati J.-F. , Devaraj R. 
Published: January 2023

Context. Misalignments between a forming star’s rotation axis and its outer disk axis, although not predicted by standard theories of stellar formation, have been observed in several classical T Tauri stars (cTTs). The low-mass cTTs DK Tau is suspected of being among them. In addition, it is an excellent subject to investigate the interaction between stellar magnetic fields and material accreting from the circumstellar disk, as it presents clear signatures of accretion.
Aims. The goal of this paper is to study DK Tau’s average line-of-sight magnetic field in both photospheric absorption lines and emission lines linked to accretion, using spectropolarimetric observations, as well as to examine inconsistencies regarding its rotation axis.
Methods. We used data collected with the ESPaDOnS spectropolarimeter, at the Canada-France-Hawaii Telescope, and the NARVAL spectropolarimeter, at the Télescope Bernard Lyot, probing two distinct epochs (December 2010 to January 2011 and November to December 2012), each set spanning a few stellar rotation cycles. We first determined the stellar parameters of DK Tau, such as effective temperature and vsini. Next, we removed the effect of veiling from the spectra, then obtained least-squares deconvolution (LSD) profiles of the photospheric absorption lines for each observation, before determining the average line-of-sight magnetic field from them. We also investigated accretion-powered emission lines, namely the 587.6 nm HeI line and the CaII infrared triplet (at 849.8 nm, 854.2 nm and 866.2 nm), as tracers of the magnetic fields present in the accretion shocks.
Results. We find that DK Tau experiences accretion onto a magnetic pole at an angle of ∼30° from the pole of its rotation axis, with a positive field at the base of the accretion funnels. In 2010 we find a magnetic field of up to 0.95 kG (from the CaII infrared triplet) and 1.77 kG (from the HeI line) and in 2012 we find up to 1.15 kG (from the CaII infrared triplet) and 1.99 kG (from the HeI line). Additionally, using our derived values of period, vsini and stellar radius, we find a value of 58° (+18)(−11) for the inclination of the stellar rotation axis, which is significantly different from the outer disk axis inclination of 21° given in the literature.
Conclusion. We find that DK Tau’s outer disk axis is likely misaligned compared to its rotation axis by 37°.
Non-local thermodynamic equilibrium analysis of the methylidyne radical molecular lines in metal-poor stellar atmospheres
Authors: Popa S. A. , Hoppe R. , Bergemann M. , Hansen C. J. , Plez B. , Beers T. C. 
Published: January 2023

Aims. An analysis of the methylidyne (CH) radical in non-local thermodynamic equilibrium (NLTE) is performed for the physical conditions of cool stellar atmospheres typical of red giants (log ɡ = 2.0, Teff = 4500 K) and the Sun. The aim of the present work is to explore whether the G band of the CH molecule, which is commonly used in abundance diagnostics of carbon-enhanced metal-poor stars, is sensitive to NLTE effects.
Methods. LTE and NLTE theoretical spectra were computed with the MULTI code. We used one-dimensional (1D) LTE hydrostatic MARCS model atmospheres with parameters representing eleven red giant stars with metallicities ranging from [Fe/H] = −4.0 to [Fe/H] = 0.0 and carbon-to-iron ratios of [C/Fe] = 0.0, +0.7, +1.5, and +3.0. The CH molecule model was represented by 1981 energy levels, 18 377 radiative bound-bound transitions, and 932 photo-dissociation reactions. The rates due to transitions caused by collisions with free electrons and hydrogen atoms were computed using classical recipes.
Results. Our calculations suggest that NLTE effects in the statistical equilibrium of the CH molecule are significant and cannot be neglected for precision spectroscopic analysis of C abundances. The NLTE effects are mostly driven by radiative over-dissociation, owing to the very low dissociation threshold of the molecule and significant resonances in the photo-dissociation cross-sections. The NLTE effects in the G band increase with decreasing metallicity. When comparing the C abundances determined from the CH G band in LTE and in NLTE, we show that the C abundances are always under-estimated if LTE is assumed. The NLTE corrections to C abundance inferred from the CH feature range from +0.04 dex for the Sun to +0.21 dex for a red giant with metallicity [Fe/H] = −4.0.
Conclusions. Departures from the LTE assumption in the CH molecule are non-negligible, and NLTE effects have to be taken into account in the diagnostic spectroscopy based on the CH lines. We show here that the NLTE effects in the optical CH lines are non-negligible for the Sun and red giant stars, but further calculations are warranted to investigate the effects in other regimes of stellar parameters.
Effects of rotation on the spectroscopic observables of massive stars
Authors: Abdul-Masih M. 
Published: January 2023

Context. Rotation is ubiquitous among massive stars and with rotation comes a deformation to the surface geometry. This, in turn leads to alterations in the distribution of parameters across the surface including surface gravity, temperature, and ionization balance of the surface elements. These 3D effects are often neglected when analyzing the spectra of rapidly rotating massive stars.
Aims. We aim to determine whether neglecting the 3D deformations resulting from rapid rotation has an impact on the final spectroscopic observables, and if so to what degree.
Methods. Using the SPAMMS code, we generated a grid of synthetic spectra that account for the 3D geometry of rapidly rotating stars and compared them to synthetic spectra generated assuming spherical geometry. Using equivalent width (EW) and full width half maximum (FWHM) measurements as proxies, we determined how the measured temperature, helium abundance, and projected rotation rates of individual lines in different ionization states vary with rotation rates and inclinations.
Results. We find that the 3D geometry can have a significant impact on the measured parameters. We show that the temperature is highly dependent on both the rotation rate and the inclination, and that the same system viewed at different inclinations can have measured temperatures that differ by as much as 10%. We also find that the helium abundance can be underestimated by as much as 60% and that lines in different ionization states can have measurable differences in rotation rates. We demonstrate that these differences in rotation rates can be seen in observed data and show that this could allow for an inclination-independent measurement of the rotational velocity.
Conclusions. Our results indicate that neglecting the 3D effects of rotation can cause significant biases in the measured spectroscopic parameters and that in many cases, the measured values are more than 3σ away from the true values.
Aminotetrazole Synthesis from Secondary Amides by C–C Bond Nitrogenation
Authors: Jiao Ning , Zhang Cheng , Liu Jianzhong , Cheng Zengrui , Li Junhua , Song Song 
Published: January 2023

AbstractThe development of novel methods for the preparation of aminotetrazoles is of long-standing interest to chemists due to the great importance of these compounds in chemistry and biology. Here, we report an efficient method for the preparation of aminotetrazoles from secondary amides by selective C–C bond cleavage. Compared with the conventional laborious and cumbersome approaches to aminotetrazoles, this chemistry provides a highly efficient nitrogenation strategy, with the installation of four nitrogen atoms into a secondary amide in one step.