arXiv subjects

Dietrich Baade

Publications and source records attributed to Dietrich Baade.

At least 19 recordsLinked to original sources

arXiv2025

Line Polarization of Si II $λ$6355 Å in Type Ia Supernovae: A New Statistical Approach to Probe the Explosion Physics and Diversity

Spectropolarimetry provides a unique probe of ejecta asphericities, offering direct insights into the underlying explosion physics of Type Ia supernovae (SNe Ia). We analyze the statistical properties of pre-maximum spectropolarimetric data for 24 SNe Ia observed with VLT/FORS, focusing on the Si II $λ$6355 Åline. Previous studies have revealed a correlation between the peak Si II polarization degree and the expansion velocity. Here, we combine these observations with multi-dimensional non-LTE radiative transfer simulations. We consider two asphericity classes: (i) lopsided abundance distributions produced by off-center delayed-detonation transitions in near-$M_{Ch}$ white dwarfs or, for example, WD collisions (Class I), and (ii) global, axisymmetric density asphericities such as those arising from explosions of rapidly rotating WDs or mergers (Class II). Our model grid spans normal to subluminous SNe Ia and successfully reproduces the observed Si II velocity-polarization trend, with higher velocities associated with stronger asphericities. Consistent with observations, transitional SNe Ia and the faint end of the normal SNe Ia population show the highest Si II polarization and are best explained by Class I scenarios. In contrast, subluminous SNe Ia are dominated by Class II asphericities, characterized by lower Si II polarization but significant continuum polarization. The observed distribution of Si II polarization depends on both the observer's viewing angle $θ$ and the intrinsic asphericity. Statistical analysis of these spectropolarimetric snapshots enables the separation of Class I and Class II contributions and highlights the intrinsic diversity among SNe Ia. Our results imply viewing-angle-dependent luminosities in our local sample, which may have implications when using high-redshift SNe Ia as evidence for the need of non-standard cosmology.

astro-ph.HE↗

arXiv2025

An axisymmetric shock breakout indicated by prompt polarized emission from the type II supernova 2024ggi

The death of massive stars is triggered by an infall-induced bounce shock that disrupts the star. How such a shock is launched and propagates through the star is a decade-long puzzle. Some models assume that the shock can be reenergized by absorbing neutrinos, leading to highly aspherical explosions. Other models involve jet-powered shocks that lead to bipolar explosions reflected in the geometry of the shock-breakout emission. We report measurement of the geometry of the shock breakout through unprecedentedly early spectropolarimetry of the nearby type II supernova 2024ggi starting ~1.2 days after the explosion. The measurement indicates a well-defined symmetry axis of the shock breakout, which is also shared by the hydrogen-rich envelope that emerged after the circumstellar matter was engulfed by the ejecta, revealing a persisting and prominent symmetry axis throughout the explosion. These findings suggest that the physical mechanism driving the explosion of massive stars manifests a well-defined axial symmetry and acts on large scales.

astro-ph.SR↗

arXiv2024

Newly Formed Dust within the Circumstellar Environment of SNIa-CSM 2018evt

Dust associated with various stellar sources in galaxies at all cosmic epochs remains a controversial topic, particularly whether supernovae (SNe) play an important role in dust production. We report evidence of dust formation in the cold, dense shell behind the ejecta-circumstellar medium (CSM) interaction in the Type Ia-CSM SN 2018evt three years after the explosion, characterized by a rise in the mid-infrared (MIR) emission accompanied by an accelerated decline in the optical radiation of the SN. Such a dust-formation picture is also corroborated by the concurrent evolution of the profiles of the Ha emission line. Our model suggests enhanced CSM dust concentration at increasing distances from the SN as compared to what can be expected from the density profile of the mass loss from a steady stellar wind. By the time of the last MIR observations at day +1041, a total amount of 1.2+-0.2x10^{-2} Msun of new dust has been formed by SN 2018evt, making SN 2018evt one of the most prolific dust factories among SNe with evidence of dust formation. The unprecedented witness of the intense production procedure of dust may shed light on the perceptions of dust formation in cosmic history.

astro-ph.HE↗

arXiv2023

The CHARA Array interferometric program on the multiplicity of classical Be stars: new detections and orbits of stripped subdwarf companions

Rapid rotation and nonradial pulsations enable Be stars to build decretion disks, where the characteristic line emission forms. A major but unconstrained fraction of Be stars owe their rapid rotation to mass and angular-momentum transfer in a binary. The faint, stripped companions can be helium-burning subdwarf OB-type stars (sdOBs), white dwarfs (WDs), or neutron stars. We present optical/near-IR CHARA interferometry of 37 Be stars selected for spectroscopic indications of low-mass companions. From multi-epoch $H$- and/or $K$-band interferometry plus radial velocities and parallaxes collected elsewhere, we constructed 3D orbits and derived flux ratios and absolute dynamical masses of both components for six objects, quadrupling the number of anchor points for evolutionary models. In addition, a new wider companion was identified for the known Be + sdO binary 59 Cyg, while auxiliary VLTI/GRAVITY spectrointerferometry confirmed circumstellar matter around the sdO companion to HR 2142. On the other hand, we failed to detect any companion to the six Be stars with $γ$ Cas-like X-ray emission, with sdOB and main-sequence companions of the expected spectroscopic mass being ruled out for the X-ray-prototypical stars $γ$ Cas and $π$ Aqr, leaving the elusive WD companions as the most likely companions, as well as a likely explanation of the X-rays. No low-mass main-sequence close companions were identified in the other stars.

astro-ph.SR↗

arXiv2023

An independent determination of the distance to supernova SN 1987A by means of the light echo AT 2019xis

Accurate distance determination to astrophysical objects is essential for the understanding of their intrinsic brightness and size. The distance to SN 1987A has been previously measured by the expanding photosphere method, and by using the angular size of the circumstellar rings with absolute sizes derived from light curves of narrow UV emission lines, with reported distances ranging from 46.77 kpc to 55 kpc. In this study, we independently determined the distance to SN 1987A using photometry and imaging polarimetry observations of AT 2019xis, a light echo of SN 1987A, by adopting a radiative transfer model of the light echo developed in Ding et al. (2021). We obtained distances to SN 1987A in the range from 49.09 $\pm$ 2.16 kpc to 59.39 $\pm$ 3.27 kpc, depending on the interstellar polarization and extinction corrections, which are consistent with the literature values. This study demonstrates the potential of using light echoes as a tool for distance determination to astrophysical objects in the Milky Way, up to kiloparsec level scales.

astro-ph.HE↗

arXiv2023

Spectropolarimetry of the type IIP supernova 2021yja: an unusually high continuum polarization during the photospheric phase

We present six epochs of optical spectropolarimetry of the Type IIP supernova (SN) 2021yja ranging from $\sim$ 25 to 95 days after the explosion. An unusually high continuum linear polarization of $p \sim 0.9\%$ is measured during the early photospheric phase, followed by a steady decrease well before the onset of the nebular phase. This behavior has not been observed before in Type IIP supernovae (SNe IIP). The observed continuum polarization angle does not change significantly during the photospheric phase. We find a pronounced axis of symmetry in the global ejecta that is shared in common with the H$α$ and Ca II near-infrared triplet lines. These observations are consistent with an ellipsoidal geometry. The temporal evolution of the continuum polarization is also compatible with the SN ejecta interacting with aspherical circumstellar matter, although no spectroscopic features that may be associated with strong interaction can be identified. Alternatively, we consider the source of the high polarization to be an extended hydrogen envelope that is indistinguishable from low-density circumstellar matter.

astro-ph.HE↗

arXiv2023

The Core Normal Type Ia Supernova 2019np: An Overall Spherical Explosion with an Aspherical Surface Layer and an Aspherical 56Ni Core

Optical spectropolarimetry of the normal thermonuclear supernova SN2019np from -14.5 to +14.5 days relative to B-band maximum detected an intrinsic continuum polarization, p(cont), of 0.21+-0.09% at the first epoch. Between days -11.5 to +05, p(cont) remained about 0 and by day +14.5 was again significant at 0.19+-0.10%. Not considering the first epoch, the dominant axis of SiII(6355A) was roughly constant, staying close to the continuum until both rotated in opposite directions on day +14.5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch p(const) of about 0.2% nevertheless suggests a separate structure with an axis ratio of about 2 in the outer carbon-rich 3...5E-3 Mo. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the CaII near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-center delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The p(cont) and the absorptions due to Si II(6355A) and the CaII NIR3 form in the same region of the extended photosphere, with an interplay between line occultation and thermalization producing p. Small-scale polarization features may be due to small-scale structures, but many could be related to atomic patterns of the quasi-continuum; the hardly have an equivalent in the total-flux spectrum. We compare SN2019np to other SNe and develop future objectives and strategies for SNIa spectropolarimetry.

astro-ph.SR↗

arXiv2022

The effects of stellar rotation along the main sequence of the 100 Myr old massive cluster NGC 1850

Young star clusters enable us to study the effects of stellar rotation on an ensemble of stars of the same age and across a wide range in stellar mass and are therefore ideal targets for understanding the consequences of rotation on stellar evolution. We combine MUSE spectroscopy with HST photometry to measure the projected rotational velocities (Vsini) of 2,184 stars along the split main sequence and on the main sequence turn-off (MSTO) of the 100 Myr-old massive (10^5 M_sun) star cluster NGC 1850 in the Large Magellanic Cloud. At fixed magnitude, we observe a clear correlation between Vsini and colour, in the sense that fast rotators appear redder. The average Vsini values for stars on the blue and red branches of the split main sequence are ~100 km/s and ~200 km/s, respectively. The values correspond to about 25-30% and 50-60% of the critical rotation velocity and imply that rotation rates comparable to those observed in field stars of similar masses can explain the split main sequence. Our spectroscopic sample contains a rich population of ~200 fast rotating Be stars. The presence of shell features suggests that 23% of them are observed through their decretion disks, corresponding to a disk opening angle of 15 degrees. These shell stars can significantly alter the shape of the MSTO, hence care should be taken when interpreting this photometric feature. Overall, our findings impact our understanding of the evolution of young massive clusters and provide new observational constraints for testing stellar evolutionary models.

astro-ph.SR↗

arXiv2022

Diversity of dust properties in external galaxies confirmed by polarization signals from Type II supernovae

Investigating interstellar (IS) dust properties in external galaxies is important not only to infer the intrinsic properties of astronomical objects but also to understand the star/planet formation in the galaxies. From the non-Milky-Way-like extinction and interstellar polarization (ISP) observed in reddened Type Ia supernovae (SNe), it has been suggested that their host galaxies contain dust grains whose properties are substantially different from the Milky-Way (MW) dust. It is important to investigate the universality of such non-MW-like dust in the universe. Here we report spectropolarimetry of two highly-extinguished Type II SNe (SN 2022aau and SN 2022ame). SN 2022aau shows a polarization maximum at a shorter wavelength than MW stars, which is also observed in some Type Ia SNe. This is clear evidence for the existence of non-MW-like dust in its host galaxy (i.e., NGC 1672). This fact implies that such non-MW-like dust might be more common in some environments than expected, and thus it might affect the picture of the star/planet formation. On the other hand, SN 2022ame shows MW-like ISP, implying the presence of MW-like dust in its host galaxy (i.e., NGC 1255). Our findings confirm that dust properties of galaxies are diverse, either locally or globally. The present work demonstrates that further investigation of IS dust properties in external galaxies using polarimetry of highly-reddened SNe is promising, providing a great opportunity to study the universality of such non-MW-like dust grains in the universe.

astro-ph.GA↗

arXiv2022

The Interaction of Supernova 2018evt with a Substantial Amount of Circumstellar Matter -- An SN1997cy-like Event

A rare class of supernovae (SNe) is characterized by strong interaction between the ejecta and several solar masses of circumstellar matter (CSM) as evidenced by strong Balmer-line emission. Within the first few weeks after the explosion, they may display spectral features similar to overluminous Type Ia SNe, while at later phase their observation properties exhibit remarkable similarities with some extreme case of Type IIn SNe that show strong Balmer lines years after the explosion. We present polarimetric observations of SN2018evt obtained by the ESO Very Large Telescope from 172 to 219 days after the estimated time of peak luminosity to study the geometry of the CSM. The nonzero continuum polarization decreases over time, suggesting that the mass loss of the progenitor star is aspherical. The prominent H$α$ emission can be decomposed into a broad, time-evolving component and an intermediate-width, static component. The former shows polarized signals, and it is likely to arise from a cold dense shell (CDS) within the region between the forward and reverse shocks. The latter is significantly unpolarized, and it is likely to arise from shocked, fragmented gas clouds in the H-rich CSM. We infer that SN2018evt exploded inside a massive and aspherical circumstellar cloud. The symmetry axes of the CSM and the SN appear to be similar. SN\,2018evt shows observational properties common to events that display strong interaction between the ejecta and CSM, implying that they share similar circumstellar configurations. Our preliminary estimate also suggests that the circumstellar environment of SN2018evt has been significantly enriched at a rate of $\sim0.1$ M$_\odot$ yr$^{-1}$ over a period of $>100$ yr.

astro-ph.HE↗

arXiv2022

Constraints on Cosmological Parameters with a Sample of Type Ia Supernovae from JWST

We investigate the potential of using a sample of very high-redshift ($2\lesssim z \lesssim6$) (VHZ) Type Ia supernovae (SNe~Ia) attainable by the James Webb Space Telescope (JWST) on constraining cosmological parameters. At such high redshifts, the age of the universe is young enough that the VHZ SNIa sample comprises the very first SNe~Ia of the universe, with progenitors among the very first generation of low mass stars that the universe has made. We show that the VHZ SNe~Ia can be used to disentangle systematic effects due to the luminosity distance evolution with redshifts intrinsic to SNIa standardization. Assuming that the systematic evolution can be described by a linear or logarithmic formula, we found that the coefficients of this dependence can be determined accurately and decoupled from cosmological models. Systematic evolution as large as 0.15 mag and 0.45 mag out to $z=5$ can be robustly separated from popular cosmological models for the linear and logarithmic evolution, respectively. The VHZ SNe~Ia will lay the foundation for quantifying the systematic redshift evolution of SNIa luminosity distance scales. When combined with SNIa surveys at comparatively lower redshifts, the VHZ SNe~Ia allow for a precise measurement of the history of the expansion of the universe from $z\sim 0$ to the epoch approaching reionization.

astro-ph.CO↗

arXiv2022

Spectropolarimetry of the Thermonuclear Supernova 2021rhu: High Calcium Polarization 79 Days After Peak Luminosity

We report spectropolarimetric observations of the Type Ia supernova (SN) 2021rhu at four epochs: $-$7, +0, +36, and +79 days relative to its $B$-band maximum luminosity. A wavelength-dependent continuum polarization peaking at $3890 \pm 93$ Angstroms and reaching a level of $p_{\rm max}=1.78% \pm 0.02$% was found. The peak of the polarization curve is bluer than is typical in the Milky Way, indicating a larger proportion of small dust grains along the sightline to the SN. After removing the interstellar polarization, we found a pronounced increase of the polarization in the CaII near-infrared triplet, from $\sim$0.3% at day $-$7 to $\sim$2.5% at day +79. No temporal evolution in high-resolution flux spectra across the NaID and CaIIH&K features was seen from days +39 to +74, indicating that the late-time increase in polarization is intrinsic to the SN as opposed to being caused by scattering of SN photons in circumstellar or interstellar matter. We suggest that an explanation for the late-time rise of the CaII near-infrared triplet polarization may be the alignment of calcium atoms in a weak magnetic field through optical excitation/pumping by anisotropic radiation from the SN.

astro-ph.HE↗

arXiv2022

The Type Icn SN 2021csp: Implications for the Origins of the Fastest Supernovae and the Fates of Wolf-Rayet Stars

We present observations of SN 2021csp, the second example of a newly-identified type of supernova (Type Icn) hallmarked by strong, narrow, P Cygni carbon features at early times. The SN appears as a fast and luminous blue transient at early times, reaching a peak absolute magnitude of -20 within 3 days due to strong interaction between fast SN ejecta (v ~ 30000 km/s) and a massive, dense, fast-moving C/O wind shed by the WC-like progenitor months before explosion. The narrow line features disappear from the spectrum 10-20 days after explosion and are replaced by a blue continuum dominated by broad Fe features, reminiscent of Type Ibn and IIn supernovae and indicative of weaker interaction with more extended H/He-poor material. The transient then abruptly fades ~60 days post-explosion when interaction ceases. Deep limits at later phases suggest minimal heavy-element nucleosynthesis, a low ejecta mass, or both, and imply an origin distinct from that of classical Type Ic supernovae. We place SN 2021csp in context with other fast-evolving interacting transients, and discuss various progenitor scenarios: an ultrastripped progenitor star, a pulsational pair-instability eruption, or a jet-driven fallback supernova from a Wolf-Rayet star. The fallback scenario would naturally explain the similarity between these events and radio-loud fast transients, and suggests a picture in which most stars massive enough to undergo a WR phase collapse directly to black holes at the end of their lives.

astro-ph.HE↗

arXiv2021

An imaging polarimetry survey of Type Ia supernovae: are peculiar extinction and polarization properties produced by circumstellar or interstellar matter?

Some highly reddened Type Ia supernovae (SNe Ia) display low total-to-selective extinction ratios ($R_V \lesssim 2$) in comparison to that of typical Milky Way dust ($R_V \approx 3.3$), and polarization curves that rise steeply to blue wavelengths, with peak polarization values at short wavelengths ($λ_{\rm max} < 0.4$ $μ$m) in comparison to the typical Galactic values ($λ_{\rm max} \approx 0.55$ $μ$m). Understanding the source of these properties could provide insight into the progenitor systems of SNe Ia. We aim to determine whether they are the result of the host galaxy's interstellar dust or circumstellar dust. This is accomplished by analysing the continuum polarization of 66 SNe Ia in dust-rich spiral galaxies and 13 SNe Ia in dust-poor elliptical galaxies as a function of normalised galactocentric distance. We find that there is a general trend of SNe Ia in spiral galaxies displaying increased polarization values when located closer to the host galaxies' centre, while SNe Ia in elliptical host galaxies display low polarization. Furthermore, all highly polarized SNe Ia in spiral host galaxies display polarization curves rising toward blue wavelengths, while no evidence of such polarization properties is shown in elliptical host galaxies. This indicates that the source of the peculiar polarization curves is likely the result of interstellar material as opposed to circumstellar material. The peculiar polarization and extinction properties observed toward some SNe Ia may be explained by the radiative torque disruption mechanism induced by the SN or the interstellar radiation field.

astro-ph.GA↗

arXiv2021

Spectropolarimetry of the Type Ia SN 2019ein rules out significant global asphericity of the ejecta

Detailed spectropolarimetric studies may hold the key to probing the explosion mechanisms and the progenitor scenarios of Type Ia supernovae (SNe Ia). We present multi-epoch spectropolarimetry and imaging polarimetry of SN 2019ein, an SN Ia showing high expansion velocities at early phases. The spectropolarimetry sequence spans from $\sim -11$ to $+$10 days relative to peak brightness in the $B$-band. We find that the level of the continuum polarization of SN 2019ein, after subtracting estimated interstellar polarization, is in the range $0.0-0.3\%$, typical for SNe Ia. The polarization position angle remains roughly constant before and after the SN light-curve peak, implying that the inner regions share the same axisymmetry as the outer layers. We observe high polarization ($\sim 1\%$) across both the Si II $\lambda6355$ and Ca II near-infrared triplet features. These two lines also display complex polarization modulations. The spectropolarimetric properties of SN 2019ein rule out a significant departure from spherical symmetry of the ejecta for up to a month after the explosion. These observations disfavour merger-induced and double-detonation models for SN 2019ein. The imaging polarimetry shows weak evidence for a modest increase in polarization after $\sim 20$ days since the $B$-band maximum. If this rise is real and is observed in other SNe Ia at similar phases, we may have seen, for the first time, an aspherical interior similar to what has been previously observed for SNe IIP. Future polarization observations of SNe Ia extending to post-peak epochs will help to examine the inner structure of the explosion.

astro-ph.HE↗

arXiv2021

$ν$ Gem: a hierarchical triple system with an outer Be star

Time series of spectroscopic, speckle-interferometric, and optical long-baseline-interferometric observations confirm that $ν$ Gem is a hierarchical triple system. It consists of an inner binary composed of two B-type stars and an outer classical Be star. Several photospheric spectral lines of the inner components were disentangled, revealing two stars with very different rotational broadening ($\sim$260 and $\sim$140 kms$^{-1}$, respectively), while the photospheric lines of the Be star remain undetected. From the combined spectroscopic and astrometric orbital solution it is not possible to unambiguously cross-identify the inner astrometric components with the spectroscopic components. In the preferred solution based on modeling of the disentangled line profiles, the inner binary is composed of two stars with nearly identical masses of 3.3 M$_\odot$ and the more rapidly rotating star is the fainter one. These two stars are in a marginally elliptical orbit ($e$ = 0.06) about each other with a period of 53.8 d. The third star also has a mass of 3.3 M$_\odot$ and follows a more eccentric ($e$ = 0.24) orbit with a period of 19.1 yr. The two orbits are co-directional and, at inclinations of 79$^{\circ}$ and 76$^{\circ}$ of the inner and the outer orbit, respectively, about coplanar. No astrometric or spectroscopic evidence could be found that the Be star itself is double. The system appears dynamically stable and not subject to eccentric Lidov-Kozai oscillations. After disentangling, the spectra of the components of the inner binary do not exhibit peculiarities that would be indicative of past interactions. Motivations for a wide range of follow-up studies are suggested.

astro-ph.SR↗

arXiv2021

A Multi-observing-technique Study of the Dynamical Evolution of the Viscous Disk around the Be Star $ω$ CMa

The observed emission lines of Be stars originate from a circumstellar Keplerian disk that are generally well explained by the Viscous Decretion Disk model. In an earlier work we performed the modeling of the full light curve of the bright Be star $ω$ CMa (Ghoreyshi et al. 2018) with the 1-D time-dependent hydrodynamics code SINGLEBE and the Monte Carlo radiative-transfer code HDUST. We used the V -band light curve that probes the inner disk through four disk formation and dissipation cycles. This new study compares predictions of the same set of model parameters with time-resolved photometry from the near UV through the mid-infrared, comprehensive series of optical spectra, and optical broad-band polarimetry, that overall represent a larger volume of the disk. Qualitatively, the models reproduce the trends in the observed data due to the growth and decay of the disk. However, quantitative differences exist, e.g., an overprediction of the flux increasing with wavelength, too slow decreases in Balmer emission-line strength that are too slow during disk dissipation, and the discrepancy between the range of polarimetric data and the model. We find that a larger value of the viscosity parameter alone, or a truncated disk by a companion star, reduces these discrepancies by increasing the dissipation rate in the outer regions of the disk.

astro-ph.SR↗

arXiv2020

Short-term variability and mass loss in Be stars VI. Frequency groups in $γ$ Cas detected by TESS

In photometry of $γ$ Cas (B0.5 IVe) from the SMEI and BRITE-Constellation satellites, indications of low-order non-radial pulsation have recently been found, which would establish an important commonality with the class of classical Be stars at large. New photometry with the TESS satellite has detected three frequency groups near 1.0 ($g1$), 2.4 ($g2$), and 5.1 ($g3$) d$^{-1}$, respectively. Some individual frequencies are nearly harmonics or combination frequencies but not exactly so. Frequency groups are known from roughly three quarters of all classical Be stars and also from pulsations of $β$ Cep, SPB, and $γ$ Dor stars and, therefore, firmly establish $γ$ Cas as a non-radial pulsator. The total power in each frequency group is variable. An isolated feature exists at 7.57 d$^{-1}$ and, together with the strongest peaks in the second and third groups ordered by increasing frequency ($g2$ and $g3$), is the only one detected in all three TESS sectors. The former long-term 0.82 d$^{-1}$ variability would fall into $g1$ and has not returned at a significant level, questioning its attribution to rotational modulation. Low-frequency stochastic variability is a dominant feature of the TESS light curve, possibly caused by internal gravity waves excited at the core-envelope interface. These are known to be efficient at transporting angular momentum outward, and may also drive the oscillations that constitute $g1$ and $g2$. The hard X-ray flux of $γ$ Cas is the only remaining major property that distinguishes this star from the class of classical Be stars.

astro-ph.SR↗