Finding low-mass planets around solar-type stars requires understanding the physical variability of the host star, which greatly exceeds the planet-induced radial-velocity modulation. Different solar photospheric absorption lines have slightly disparate responses to stellar activity, which should permit us to disentangle the wavelength shifts induced by exoplanets from those originating in stellar atmospheres. Changing area coverages of magnetic active-region granulation (faculae and plage) cause radial-velocity fluctuations of the disk-integrated solar spectrum, whose precise modeling requires active-region spectral line profiles. Hydrodynamic 3D modeling of granulation in magnetic fields extends previous nonmagnetic studies, revealing different line profiles and altered convective velocity shifts. Different types of lines in the visual and near-infrared are examined in synthetic hyper-high-resolution spectra (łambda/Deltałambda ∼900,000), comparing nonmagnetic areas with those with strongly magnetic (240 mT = 2400 G) granulation. Magnetic fields inhibit convective motions, decrease the energy flow, produce more symmetric lines, and remove the common blueshift with its familiar C-shaped bisectors. Unexpectedly, magnetic granulation displays convective redshifts. Their origin is traced to contributions from small areas, where hot and bright down-moving elements are created through shocks and adiabatic compression when rising gas is forced over into magnetically channeled downflows. Understanding line formation in also stellar active regions is needed to simulate full-disk spectra toward exoEarth detections. Detailed shapes of spectral lines carry significant information, suggesting that hyper-high spectral resolution may ultimately be required.

Abstract Electromagnetically induced transparency (EIT) can greatly reduce the group velocity of light, which has extensive applications in optical sensing, optical storage, etc, and it has been widely studied in visible and microwave wavelengths. In the X-ray regime, EIT was realized in an X-ray thin-film planar cavity, while the detection of the transparency window in the reflection direction is not easy. Due to the interference between the bare cavity reflection and the nuclear resonance, the spectral line profile can be manipulated by adjusting the phases and amplitudes of different channels. By varying the thickness of the top layer, we demonstrate that in the X-ray regime, the spectral line can be transformed from a dip to a peak at the resonant window while maintaining the slow light effect. The slow light ability is evaluated by calculating the group velocity, the interaction strength, and the clarification of EIT and Autler Townes splitting (ATS). As the interaction strength increases, the X-ray cavity transitions from EIT to EIT-ATS crossover regime, which is consistent with the variation of group velocity. The present work would facilitate the detection of slow X-ray light using the cavity system and be helpful for the EIT and ATS clarification studies.

Transmission spectroscopy has advanced our understanding of exoplanet atmospheres, but it can be hindered by contamination originating in stellar heterogeneities, mainly coming from line-of-sight effects. Therefore, probing how stellar spectra vary across the stellar surface is essential to accurately disentangling stellar and planetary spectral contributions in transit observations. Such observations can actually be used to reconstruct the local stellar spectra behind the planet's transit chord. These methodologies can help us learn more about the physics of stellar surface and how to tackle line-of-sight effects. In this paper, we study the centre-to-limb variations of line profiles across the surface of HD 189733 using the ESPRESSO spectrograph. We build on other works by analysing the same sets of lines, allowing for a direct comparison of results and an assessment of the feasibility of applying the Doppler shadow technique with ESPRESSO. We gain a better understanding of the variations in line profiles, while also making a comparison between the data of HD 189733 and synthetic spectra and solar data. Fe I We analysed spectra collected by ESPRESSO during two transits of HD 189733 b as separate sets of data. We performed a cross-correlation of each individual spectrum with two different masks made of selected spectral lines for a total of four sets of cross-correlation functions (CCFs) and employed a Doppler shadow methodology to retrieve profiles for local regions of the stellar surface. We then compared the results with previous works and with solar disc-resolved observations from IAG ATLAS. Finally, we compared the data with two separate transit simulations made using SOAPv4 with Turbospectrum synthetic spectra computed with MARCS stellar atmosphere models under local thermal equilibrium (LTE) and non-LTE (NLTE) conditions. Fe I For the profile depth of three sets of CCFs, we verified a statistically significant increase in line depth from the stellar limb to the centre. This variation was expected from simulations with MARCS models, although the solar data present a smaller gradient in the variation of line depth. In the case of the width of the line profiles, we verified that the profile width decreases from stellar limb to stellar centre for a set of CCFs. This result is consistent with the behaviour observed in solar data, but not reproduced by the simulations. Fe I Fe I These results highlight the abilities of ESPRESSO in providing the necessary precision and resolution to study centre-to-limb variations of spectral line profiles on the surface of other stars with the use of CCFs. The local CCF profiles of HD 189733 agree with the IAG ATLAS data, but disagree with simulations on line widths, indicating that important physical processes are missing and must be included to recover accurate profile widths.

Precision measurements of molecular hydrogen and its isotopologues provide critical tests of molecular theory, including quantum electrodynamics (QED) and fundamental constants. However, advancing the experimental accuracy of hydrogen overtone transitions remains challenging due to unexplained asymmetries in spectral line profiles. We present the Lamb dip measurement at a temperature as low as 12 K, where the spectral features exhibit enhanced resolution. By applying a lineshape centroid extrapolation method – which is independent of the lineshape model – we determine the transition frequency of the R(0) line in the (2-0) overtone of HD to be 214 905 335 170 ( 27 ) stat ( 124 ) sys kHz. While the low-temperature Lamb-dip measurements challenge conventional models of spectral line shapes, our results are in excellent agreement with the latest theoretical results.

Investigation of effective line strengths of N2O4 at 7.8μm using a continuous-wave quantum cascade laser spectrometer.

Stark broadening of spectral lines is an important tool for diagnostics of laboratory and astrophysical plasmas. In particular, in studies of laser-plasma interactions, advanced methods of the diagnostics, based on the Stark broadening of spectral lines, revealed the rich physics of processes at the surface of the critical density including the development of the electrostatic plasma turbulence. At the relatively high amplitudes E0 of the laser field at the frequency ω, at the surface of the critical density (or the relativistic critical density), there was diagnosed spectroscopically the combination of a Low-frequency Electrostatic Plasma Turbulence (LEPT), such as e.g., ion acoustic waves, and the “high-frequency” quasimonochromatic electric field of Langmuir waves. In the present paper we consider the situation where the amplitude F0 and/or the frequency ω of the laser field is smaller than in the above experiments, so that the radiating hydrogen atom (the radiator) does not “feel” the dynamical nature of the laser field: the radiator perceives not only the LEPT as quasistatic (which is practically always the case), but also the laser field as quasistatic. We focused on the situation where the plasma ion microfield is not quasistatic, but rather produces the dynamical Stark broadening, just as the plasma electrons. In this situation the quasistatic fields in the plasma are either the linearly-polarized laser field F and the colinear field E1 of the LEPT, or the circularly-polarized laser field F and the coplanar field E1 of the LEPT We calculated analytically the distribution of the total quasistatic field E = F + E1 and provided the illustration. Then we calculated analytically the Stark profiles of the Ly-beta line (with the allowance for the dynamic Stark broadening by the plasma microfield) for various combinations of the dynamical Stark halfwidth γ and the ratio of the laser field amplitude F0 to the root-mean-square LEPT field E0. We studied the evolution of the Ly-beta profiles as the ratio f = F0/E0 changes at fixed γ, and as γ changes at fixed f. We believe that our results can be important for the spectroscopic studies of the laser-plasma interactions.

Abstract Spectral line analysis is essential for investigating the solar atmospheric dynamic processes. Modeling the line profiles provides key insights into plasma motions and variations in temperature and density. This study aims to enhance the parameter estimation of spectral line profiles observed by the Interface Region Imaging Spectrograph through the application of an ensemble approach within the Bayesian Markov Chain Monte Carlo (MCMC) framework. We analyze silicon ion lines (Si IV 1394 and 1403 Å) for samples in the plage, flaring/nonflaring active regions, coronal hole, and boundary of coronal hole and quiet Sun, fitting them with single-Gaussian, double-Gaussian, and Voigt models using the Bayesian MCMC approach. The performance of the model is assessed using statistical metrics, including the reduced chi-square ( χ 2 ), Akaike information criterion, and Bayesian information criterion. The Bayesian MCMC method not only provides precise parameter estimates with associated uncertainties but also converges efficiently when initialized with Levenberg–Marquardt least-squares results. Overall, this approach enhances the accuracy of spectral line modeling and model selection, contributing to a more detailed understanding of solar atmospheric dynamics.

Quantifying stellar parameters and magnetic activity for cool stars in double-lined spectroscopic binaries (SB2s) is not straightforward, as both stars contribute to the observed composite spectra and are likely variable. Disentangled component spectra allow a detailed analysis of the magnetic activity of the primary component. We aim to separate the spectra of the two stellar components of the HR,7275 SB2 system. We also aim to obtain a more accurate orbital solution by correcting the observed radial velocities (RV) from activity perturbations of the spotted primary (``RV jitter'') and to derive a surface image of this component. We obtained time-series high- and ultra-high resolution optical spectra and applied two different disentangling methods. We modeled radial velocity (RV) residuals using three-sine function fits and modeled the spectral-line profile of the primary with the Doppler imaging code iMAP. We measured magnetic fields for the primary based on least-squares deconvolved Stokes-V line profiles, and determined chromospheric emission from the line cores of ̧ahk, ̧airt,8542,Å, and Balmer ̋alpha. We first applied our disentangling technique, which allowed us to determine the properties of the system more accurately before performing these analyses. The Doppler image of the primary shows two large cool spots that cover approximately 20% of the visible hemisphere, plus three smaller spots each still covering approximately 13% in size. Between May-June 2022, HR,7275a exhibited an impressive spottedness of roughly 40% of its entire surface. The RV is modulated by the rotation of the primary, with maximum amplitudes of 320, for HR,7275b, which is an extremely low value. and 650, _ for two different modulation behaviors during the 250,d of our observations. This jitter is primarily caused by the varying asymmetries of the apparent disk brightness due to the cool spots. Its removal results in roughly ten times higher precision of the orbital elements. Our snapshot magnetic-field measurements reveal phase-dependent (large-scale) surface fields between +0.6±2.0,G at phase 0.1 and -15.2±2.7,G at phase 0.6, indicating a complex magnetic morphology related to the location of the photospheric spots. We also obtain a logarithmic lithium abundance of 0.58±0.1 for HR,7275a, indicating considerable mixing, and 0.16 +0.23 -0.63

Power broadening refers to the widening of the spectral line profile in a two-state quantum transition as the strength of the driving field increases. This phenomenon commonly arises in continuous-wave driving when the radiation field's intensity exceeds the transition's saturation intensity and it has been extensively studied in spectroscopy. For pulsed-field excitation, the spectral response of the quantum system may differ significantly: while a rectangular-shaped pulse leads to a linear power broadening, pulses with smooth shapes show significantly reduced power broadening, for instance, logarithmic for the Gaussian shape and none for the hyperbolic-secant shape. Recently [Phys. Rev. Lett. 132, 020802 (2024)], in a dramatic paradigm shift, we have demonstrated experimentally that for Lorentzian-shaped pulses, the opposite effect---power narrowing---takes place: the width of the spectral profile decreases when the driving pulse amplitude increases, with a narrowing factor of as much as 10 observed. While in high-resolution spectroscopy the push is for eliminating or even inverting the power broadening, there are applications where it is used to an advantage for it facilitates off-resonance excitation. Here, we present a number of shaped pulses that exhibit power broadening much greater than that of the rectangular pulse of the same pulse area. They are grouped in two families of pulse shapes. In particular, in regard to the width of the second Rabi oscillation maximum, the quadratic pulse family shows a spectral increase by a factor of 3.3 whereas the power-law pulse family exhibits an increase by a factor of more than 3.5.

Abstract We present a sample of low H i mass ( M H I < 10 8 M ⊙ ) dwarf galaxies detected by The FAST All Sky H i Survey (FASHI) project. Due to the faint and irregular morphology of these galaxies, the default photometry is often inaccurate. Therefore, we utilized The Dark Energy Camera Legacy Survey data to perform careful photometric measurements, and find that the low H i mass galaxies have similar stellar mass densities to dwarf elliptical galaxies. Compared to other dwarf galaxy populations, the H i -selected dwarfs exhibit higher stellar mass densities than ultradiffuse galaxies, and similar densities to H i -selected low-surface-brightness galaxies, albeit with lower stellar masses, suggesting a possible evolutionary connection among these populations. By classifying the galaxies according to their H i spectral-line profiles, we show that the double-peaked sources conform closely to the Tully–Fisher relation, whereas the single-peaked sources follow the Faber–Jackson relation but with large scatter. This indicates that the single-peaked systems are likely dispersion dominated and that the relationship between stellar mass and halo mass in such systems may remain consistent across both low- and high-mass regimes. These findings suggest that H i -selected dwarf galaxies with single-peaked H I profiles may share a similar dynamical state with massive ellipticals, offering new insights into their structural evolution and the diversity of formation pathways for low-mass galaxies.

Context . The interplay between surface magnetic topology and chromospheric heating in active M dwarfs remains poorly constrained, limiting our understanding of their magnetic cycles and high-energy environments. Aims . We aim to test whether detailed Zeeman–Doppler imaging (ZDI) maps of AD Leo can be used to spatially anchor a multi-component chromospheric model and thus validate the link between magnetic flux distribution and emission-line formation. Methods . We analyze high-resolution CARMENES spectra of H α and the Ca II infrared triplet, together with detailed ZDI maps. The RH1.5D non-local thermodynamic equilibrium (non-LTE) radiative transfer code is employed to synthesize spectral lines with two active atmospheric components (low-latitude near the equator and polar near the pole) and a quiet background. Their relative filling factors and temperature structures are optimized per epoch. The ZDI maps serve as a qualitative reference for the large-scale magnetic topology but are not used as input to the optimization procedure. Results . Our model reproduces the spectral line profiles over multiple epochs. The low-latitude active region shows notable variability – accounting for approximately 55−86% of the emission, while the polar active region remains relatively constant in area (12−17%) but exhibits temperature variations over time, particularly during the periods of increased activity. The spatial locations of the active regions derived from spectroscopy are in good agreement with the radial magnetic field distribution obtained from ZDI. Conclusions . Our results indicate that combining spectroscopic modeling with magnetic field maps is an effective approach for mapping magneto-chromospheric structures in M dwarfs. This framework deepens our understanding of stellar magnetic cycles and chromospheric dynamics, paving the way for detailed time-resolved studies in active low-mass stars.

Disc winds driven by thermal and magnetic processes are thought to play a critical role in protoplanetary disc evolution. However, the relative contribution of each mechanism remains uncertain, particularly in light of their observational signatures. We investigate whether spatially resolved emission and synthetic spectral line profiles can be used to distinguish between thermally and magnetically driven winds in protoplanetary discs. We modelled three disc wind scenarios with different levels of magnetisation: a relatively strongly magnetised wind (β4), a rather weakly magnetised wind (β6), and a purely photoevaporative wind ( Using radiative transfer post-processing, we generated synthetic emission maps and line profiles for and and compared them with observational trends in the literature. We find that the β4 model generally produces broader and more blueshifted low-velocity components across all tracers, consistent with compact emission regions and steep velocity gradients. The β6 and models yield narrower profiles with smaller blueshifts, in better agreement with most observed narrow low-velocity components (NLVCs). We also find that some line profile diagnostics, such as the inclination at maximum centroid velocity, are not robust discriminants. However, the overall blueshift and full width at half maximum of the low-velocity components provide reliable constraints. The β4 model reproduces the most extreme blueshifted NLVCs in observations, while most observed winds are more consistent with the β6 and models. Our findings reinforce previous conclusions that most observed NLVCs are compatible with weakly magnetised or purely photo-evaporative flows. The combination of line kinematics and emission morphology offers meaningful constraints on wind-driving physics, and synthetic line modelling remains a powerful tool for probing disc wind mechanisms.

Massive stars play a fundamental role in galactic evolution through their strong stellar winds, chemical enrichment, and feedback mechanisms. Accurate modelling of their atmospheres and winds is critical for understanding their physical properties and evolutionary pathways. Traditional spectroscopic analyses often rely on the β-law approximation for wind-velocity profiles, which may not capture the complexity of observed phenomena. This study aims to introduce and validate the grId of Stellar atmOSphere and hydrodynamiC modELs for massivE Stars (ISOSCELES), a grid-based framework for the quantitative spectroscopic analysis of massive stars. The project leverages hydrodynamic wind solutions derived from the m-CAK theory, including both fast and δ-slow solutions, to improve the accuracy of derived stellar and wind parameters. We constructed a comprehensive grid of models based on hydrodynamic wind solutions from the Hydwind code and synthetic spectral line profiles generated by the Fastwind code. The grid spans a broad parameter space covering OBA-type stars with solar metallicity. A semi-automatic fitting procedure was developed to analyse key spectral lines and derive the stellar and wind parameters. Applying ISOSCELES to six stars demonstrates its ability to reproduce observed spectral profiles with high fidelity. The δ-slow solution proved effective for two early-type B supergiants. The grid also highlights the difference of using the β-law in modelling stellar winds compared with the m-CAK wind solutions. The ISOSCELES database represents a step forward in quantitatively analysing massive stars, offering an alternative to the β-law approximation. Future work will address the inclusion of UV lines and metallicity effects to further refine its applicability across diverse stellar populations.

Context. Helioseismology aims to infer the properties of the solar interior by analyzing observations of acoustic oscillations. Interpreting helioseismic data, however, is complicated by the non-trivial relationship between helioseismic observables and the physical perturbations associated with acoustic modes as well as by various instrumental effects. Aims. We aim to improve our understanding of the signature of acoustic modes measured in the Helioseismic and Magnetic Imager (HMI) continuum intensity and Doppler velocity observables by accounting for radiative transfer, solar background rotation, and spacecraft velocity. Methods. We started with a background model atmosphere that accurately reproduces solar limb darkening and the Fe I 6173Å spectral line profile. We employed first-order perturbation theory to model the effect of acoustic oscillations on inferred intensity and velocity. By solving the radiative transfer equation in the atmosphere, we synthesized the spectral line, convolved it with the six HMI spectral windows, and deduced the continuum intensity (hmi.Ic_45s) and Doppler velocity (hmi.V_45s) according to the HMI algorithm. Results. We analytically derived the relationship between mode displacement in the atmosphere and the HMI observables and show that both the intensity and velocity deviate significantly from simple approximations. Specifically, the continuum intensity does not simply reflect the true continuum value, while the line-of-sight velocity does not correspond to a straightforward projection of the velocity at a fixed height in the atmosphere. Our results indicate that these deviations are substantial, with amplitudes of approximately 10% and phase shifts of around 10° across the detector for both observables. Moreover, these effects are highly dependent on the acoustic mode under consideration and the position on the solar disk. To achieve accurate modeling of the observables, it is important to account for the impact of radiative transfer on oscillation velocities and perturbations in atmospheric thermodynamic quantities, which influence the line profile. Conclusions. The combination of these effects leads to non-trivial systematic errors (in amplitude and phase) across the detector that must be taken into account to understand the observables. This framework can be used to study mode visibility across the solar disk and for asteroseismology applications.

Abstract Massive stars in the Hertzsprung gap are a mixed population of objects in short-lived evolutionary phases: yellow supergiants (YSGs) evolving toward the red supergiant (RSG) phase, partially stripped post-RSGs, and other, rarer outcomes of stellar evolution. Studies of sufficiently large samples of these objects can constrain massive star structure and evolution during these poorly understood phases. As part of our ongoing program searching for post-RSGs, we characterized the spectral line profiles of 32 YSGs in the Large Magellanic Cloud using high-resolution spectra obtained with the Magellan Inamori Kyocera Echelle spectrograph on the Magellan 2/Clay telescope at Las Campanas Observatory. We find that the line profiles are strongly broadened by turbulent photospheric motion. After fitting the profiles to measure microturbulent and macroturbulent velocities, we identify two groups within our sample that are separated by the ratio of the two velocity scales. In both groups, the macroturbulent velocity ζ RT scales with stellar properties such as effective temperature. Additionally, we find statistically significant correlations between the macroturbulent velocity and other possible probes of large-scale photospheric motions: line profile asymmetry, as well as the amplitude and quality factor of the stochastic low frequency variability measured from Transiting Exoplanet Survey Satellite lightcurves. These correlations differ between the two groups of YSGs. Finally, we construct 1D evolutionary models of YSGs in both pre- and post-RSG phases, and find reasonable agreement between the convective velocities in these models and our measured microturbulent velocities. However, the macroturbulent velocities are much higher than the convective velocities in the models.

Aims. Our aim was to construct a comprehensive global multi-scale kinematic equilibrium radiative-transfer model for the pretransitional disc of DoAr 44 (Haro 1-16, V2062 Oph) in the Ophiuchus star-forming region. This model integrates diverse observational datasets to describe the system, spanning from the accretion region to the outer disc. Methods. Our analysis utilised a large set of observational data, including ALMA continuum complex visibilities, VLTI/GRAVITY continuum squared visibilities, closure phases, and triple products, as well as VLT/UVES and VLT/X-shooter Hα spectra. Additionally, we incorporated absolute flux measurements from ground-based optical observatories, Spitzer, IRAS, the Submillimeter Array, the IRAM, or the ATCA radio telescopes. These datasets were used to constrain the structure and kinematics of the object through radiative-transfer modelling. Results. Our model reveals that the spectral line profiles are best explained by an optically thin spherical inflow or outflow within the co-rotation radius of the star, exhibiting velocities exceeding 380 km/s. The VLTI near-infrared interferometric observations are consistent with an inner disc extending from 0.1 to 0.2 au. The ALMA sub-millimetre observations indicate a dust ring located between 36 and 56 au, probably related to the CO2 condensation line. The global density and temperature profiles derived from our model provide insight into an intermediate disc, located in the terrestrial planet-forming zone, which has not yet been spatially resolved.

Flux emergence is ubiquitous in the Sun's lower atmosphere. The emerging flux can reconnect with the pre-existing magnetic field. We aim to investigate plasmoid formation and the resulting multithermal emissions during the three-dimensional reconnection process in the lower solar atmosphere. We conducted 3D radiation magnetohydrodynamic (RMHD) simulations using the MURaM code, which incorporates solar convection and radiation. We simulated the emergence of a flat magnetic flux sheet that was introduced into the convection zone. For comparison with results previously reported from observations, we employed the RH1.5D code to synthesize Hα and Si IV spectral line profiles and we synthesized the ultraviolet images using the optical thin methods. Flux emergence took place as part of the imposed flux tube crossed the photosphere. In the lower solar atmosphere, magnetic reconnection occurred and formed thin, elongated current sheets. Plasmoid-like features appear as part of the reconnection process; this results in many small twisted magnetic flux ropes, which are expelled toward the two ends of the reconnection region. Consequently, hot plasma with a temperature exceeding 20,000 K and much cooler plasmas with a temperature below 10,000 K can coexist in the reconnection region. Synthesized images and spectral line profiles through the reconnection region display typical characteristics of reconnection occuring in the lower solar atmosphere, such as Ellerman bombs (EBs) and UV bursts. The cooler plasmas that show characteristics of EBs can be found above hot plasma and reach altitudes more than 2Mm above the solar surface. Meanwhile, some hot plasma that features characteristics of UV bursts can extend downward to the lower chromosphere, approximately 0.7Mm above the solar surface. Our simulation results indicate that the turbulent reconnection mediated with plasmoid instability can occur in small-scale reconnection events such as EBs and UV bursts. The coexistence of hot and much cooler plasmas in such a turbulent reconnection process can well explain the temporal and spatical connection of UV bursts with EBs.

The behavior of laser-TIG hybrid welding plasma is closely related to the welding process. In order to obtain the behavior of plasma more quickly, this paper proposes a new method for in situ monitoring of plasma by acquiring the line spectrum of plasma radiation. By applying narrow-band filtering, the time-varying intensity profiles of specific Mg-I spectral lines (470, 518, and 552 nm) in the plasma radiation generated during the hybrid welding of 5083 aluminum alloy are selectively obtained. These optical signals are then denoised using wavelet packet transform and Savitzky–Golay smoothing filters. Further analysis is performed using empirical mode decomposition, fast Fourier transform, and Hilbert transform to extract comprehensive time, frequency, and time–frequency features. The results demonstrate that, compared to full-spectrum acquisition, the selectively captured line spectra exhibit more pronounced variations in signal intensity and frequency, making them more representative of plasma behavior. Among them, the Mg-I 518 nm line—with the highest intensity—exhibits the strongest sensitivity to changes in plasma dynamics. This targeted spectral approach significantly enhances the accuracy and responsiveness of real-time monitoring. The findings provide a promising strategy for the intelligent in situ monitoring of laser-TIG hybrid welding processes, contributing to improved weld quality and process stability.

We present an analysis of the optical and near-infrared properties of SN 2022lxg, a bright (Mg peak = −19.41 mag) and rapidly evolving supernova (SN). It was discovered within a day of explosion, and rose to peak brightness in ∼10 d. Two distinct phases of circumstellar interaction are evident in the data. The first is marked by a steep blue continuum (T > 15 000 K) with flash-ionisation features due to hydrogen and He II. The second, weaker phase is marked by a change in the colour evolution accompanied by changes in the shapes and velocities of the spectral line profiles. Narrow P-Cygni profiles (∼150 km s−1) of He I further indicate the presence of slow-moving, unshocked material and suggest partial stripping of the progenitor. The fast decline of the light-curve from the peak (3.48 ± 0.26 mag (50 d)−1 in g band) implies that the ejecta mass must be low. Spectroscopically, until +35 d there are similarities with some Type IIb SNe but then there is a transition to spectra that are more reminiscent of an interacting SN II. However, metal lines are largely absent in the spectra, even at epochs of ∼80 d. Its remote location (∼4.6 kpc projected offset) from the presumed host galaxy, a dwarf with MB ∼ −14.4 mag, is consistent with our metallicity estimate – close to the values of the Small Magellanic Cloud – obtained from scaling relations. Furthermore, several lines of evidence (including intrinsic polarisation of p ∼ (0.5 − 1.0)%) point to deviations from spherical symmetry. We suggest that a plausible way of uniting the observational clues is to consider a binary system that underwent case C mass transfer. This failed to remove the entire H envelope of the progenitor before it underwent core collapse. In this scenario, the progenitor itself would be more compact and perhaps straddle the boundary between blue and yellow supergiants, which ties in with the early spectroscopic similarity to Type IIb SNe.

Study on point defects and La doping in KH2PO4 crystal from combined first-principles