Astrophysics

Most supernova explosions accompany the death of a massive star. These explosions give birth to neutron stars and black holes and eject solar masses of heavy elements. However, determining the mechanism of explosion has been a half-century journey of great complexity. In this paper, we present our perspective of the status of this theoretical quest and the physics and astrophysics upon which its resolution seems to depend. The delayed neutrino-heating mechanism is emerging as a robust solution, but there remain many issues to address, not the least of which involves the chaos of the dynamics, before victory can unambiguously be declared. It is impossible to review in detail all aspects of this multi-faceted, more-than-half-century-long theoretical quest. Rather, we here map out the major ingredients of explosion and the emerging systematics of the observables with progenitor mass, as we currently see them. Our discussion will of necessity be speculative in parts, and many of the ideas may not survive future scrutiny. Some statements may be viewed as informed predictions concerning the numerous observables that rightly exercise astronomers witnessing and diagnosing the supernova Universe. Importantly, the same explosion in the inside, by the same mechanism, can look very different in photons, depending upon the mass and radius of the star upon explosion. A 10 51 -erg (one "Bethe") explosion of a red supergiant with a massive hydrogen-rich envelope, a diminished hydrogen envelope, no hydrogen envelope, and, perhaps, no hydrogen envelope or helium shell all look very different, yet might have the same core and explosion evolution.

We present the discovery and follow-up observations of two CCSNe that occurred in the luminous infrared galaxy (LIRG), NGC3256. The first, SN2018ec, was discovered using the ESO HAWK-I/GRAAL adaptive optics seeing enhancer, and was classified as a Type Ic with a host galaxy extinction of A V = 2.1 +0.3 ??.1 mag. The second, AT2018cux, was discovered during the course of follow-up observations of SN2018ec, and is consistent with a sub-luminous Type IIP classification with an A V =2.1±0.4 mag of host extinction. A third CCSN, PSNJ10275082-4354034 in NGC3256, has previously been reported in 2014, and we recovered the source in late time archival HST imaging. Based on template light-curve fitting, we favour a Type IIn classification for it with modest host galaxy extinction of A V = 0.3 +0.4 ??.3 mag. We also extend our study with follow-up data of the recent Type IIb SN2019lqo and Type Ib SN2020fkb that occurred in the LIRG system Arp299 with host extinctions of A V = 2.1 +0.1 ??.3 and A V = 0.4 +0.1 ??.2 mag, respectively. Motivated by the above, we inspected, for the first time, a sample of 29 CCSNe located within a projected distance of 2.5 kpc from the host galaxy nuclei in a sample of 16 LIRGs. We find that, if star formation within these galaxies is modelled assuming a global starburst episode and normal IMF, there is evidence of a correlation between the starburst age and the CCSN subtype. We infer that the two subgroups of 14 H-poor (Type IIb/Ib/Ic/Ibn) and 15 H-rich (Type II/IIn) CCSNe have different underlying progenitor age distributions, with the H-poor progenitors being younger at 3 ? significance. However, we do note that the available sample sizes of CCSNe and host LIRGs are so far small, and the statistical comparisons between subgroups do not take into account possible systematic or model errors related to the estimated starburst ages. (abridged)

The question why the solar corona is much hotter than the visible solar surface still puzzles solar researchers. Most theories of the coronal heating involve a tight coupling between the coronal magnetic field and the associated thermal structure. This coupling is based on two facts: (i) the magnetic field is the main source of the energy in the corona and (ii) the heat transfer preferentially happens along the magnetic field, while is suppressed across it. However, most of the information about the coronal heating is derived from analysis of EUV or soft X-ray emissions, which are not explicitly sensitive to the magnetic field. This paper employs another electromagnetic channel -- the sunspot-associated microwave gyroresonant emission, which is explicitly sensitive to both the magnetic field and thermal plasma. We use nonlinear force-free field reconstructions of the magnetic skeleton dressed with a thermal structure as prescribed by a field-aligned hydrodynamics to constrain the coronal heating model. We demonstrate that the microwave gyroresonant emission is extraordinarily sensitive to details of the coronal heating. We infer heating model parameters consistent with observations.

We present an in-depth characterization of the polarimetric channel of the Large-Angle Spectrometric COronagraph/LASCO-C3 onboard SOHO. The polarimetric analysis of the white-light images makes use of polarized sequences composed of three images obtained through three polarizers oriented at +60 ∘ , 0 ∘ , and -60 ∘ , complemented by a neighboring unpolarized image. However, the degradation of the 0 ∘ polarizer noticed in 1999 compelled us to reconstruct the corresponding images from the other ones thereafter. The analysis closely follows the method developed for LASCO-C2 (Lamy, et al. Solar Physics 295, 89, 2020 and arXiv:2001.05925) and implements the formalism of Mueller, albeit with additional difficulties notably the presence of a non-axially symmetric component of stray light. Critical corrections were derived from a SOHO roll sequence and from consistency criteria (e.g., the tangential direction of polarization). The quasi-uninterrupted photopolarimetric analysis of the outer corona over two complete Solar Cycles 23 and 24 was successfully achieved and our final results encompass the characterization of its polarization, of its polarized radiance, of the two-dimensional electron density, and of the K-corona. Comparison between the C3 and C2 results where their field of view overlaps shows an overall agreement. The C3 results are further in agreement with those of eclipses and radio ranging measurements to an elongation of about 10 solar radii but tend to diverge further out. Whereas the coronal polarization out to 20 solar radii is still highly correlated with the temporal variation of the total magnetic field, this divergence probably results from the increasing polarization of the F-corona.

The Extreme-ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory (SDO) detects the solar EUV spectra with high temporal cadence and spectral resolution. The wavelength shifts of emission lines provide key information of the dynamics of the Sun. However, some of EVE spectral observations are influenced by the non-uniformly distributed irradiance on the Sun, which may prevent us from correctly understanding the physical processes happened in the solar corona. Here, based on the only published on-orbit calibration data of EVE He II 30.38 nm line on 27 Jan 2011 (Chamberlin, 2016), we develop a method to correct the He II 30.38 nm line by using AIA 304 imaging data. This correction method is then applied to EVE He II 30.38 nm data from 29 Oct 2010 to 3 Mar 2011 to study the Doppler oscillations of the solar He II 30.38 nm line, in which we show that the half-month periodic Doppler oscillation is caused by non-uniformly distributed irradiance mainly due to the presence of active regions. Other EVE coronal lines also present similar Doppler oscillations, suggesting that an appropriate correction must be implemented before interpret the oscillation phenomena appearing in these lines.

In this work, we use the spectroscopy-based stellar color regression (SCR) method with ~ 0.7 million common stars between LAMOST DR7 and Gaia EDR3 to acquire color corrections in G - GRP and GBP - GRP. A sub-mmag precision is achieved. Our results demonstrate that improvements in the calibration process of the EDR3 have removed the color term in GBP - GRP and eliminated the discontinuity caused by the changes of instrument configurations to a great extent. However, modest systematic trends with G magnitude are still detected. The corresponding color correction terms as a function of G are provided for 9.5 < G < 17.5 mag and compared with other determinations. We conclude that the corrections given in this work are particularly suited for cases where the color-color investigations are required while for color-magnitude investigations other corrections may be better due to systematic with reddening. Possible applications of our results are discussed.

The second Gaia data release (DR2) delivers accurate and homogeneous photometry data of the whole sky to an exquisite quality, reaching down to the unprecedented milli-magnitude (mmag) level for the G, GRP, and GBP passbands. However, the presence of magnitude-dependent systematic effects at the 10 mmag level limits its power in scientific exploitation. In this work, using about half-million stars in common with the LAMOST DR5, we apply the spectroscopy-based stellar color regression method to calibrate the Gaia G-GRP and GBP-GRP colors. With an unprecedented precision of about 1 mmag, systematic trends with G magnitude are revealed for both colors in great detail, reflecting changes in instrument configurations. Color dependent trends are found for the GBP-GRP color and for stars brighter than G~11.5 mag. The calibration is up to 20 mmag in general and varies a few mmag/mag. A revised color-color diagram of Gaia DR2 is given, and some applications are briefly discussed.

In this letter, we have carried out an independent validation of the Gaia EDR3 photometry using about 10,000 Landolt standard stars from Clem & Landolt (2013). Using a machine learning technique, the UBVRI magnitudes are converted into the Gaia magnitudes and colors and then compared to those in the EDR3, with the effect of metallicity incorporated. Our result confirms the significant improvements in the calibration process of the Gaia EDR3. Yet modest trends up to 10 mmag with G magnitude are found for all the magnitudes and colors for the 10 < G < 19 mag range, particularly for the bright and faint ends. With the aid of synthetic magnitudes computed on the CALSPEC spectra with the Gaia EDR3 passbands, absolute corrections are further obtained, paving the way for optimal usage of the Gaia EDR3 photometry in high accuracy investigations.

Waiting time distributions of solar flares and {\sl coronal mass ejections (CMEs)} exhibit power law-like distribution functions with slopes in the range of α ? ??.4??.2 , as observed in annual data sets during 4 solar cycles (1974-2012). We find a close correlation between the waiting time power law slope α ? and the {\sl sunspot number (SN)}, i.e., α ? = 1.38 + 0.01 ? SN. The waiting time distribution can be fitted with a Pareto-type function of the form N(?)= N 0 ( ? 0 +? ) ??α ? , where the offset ? 0 depends on the instrumental sensitivity, the detection threshold of events, and pulse pile-up effects. The time-dependent power law slope α ? (t) of waiting time distributions depends only on the global solar magnetic flux (quantified by the sunspot number) or flaring rate, independent of other physical parameters of {\sl self-organized criticality (SOC)} or {\sl magneto-hydrodynamic (MHD)} turbulence models. Power law slopes of α ? ??.2??.6 were also found in solar wind switchback events, as observed with the {\sl Parker Solar Probe (PSP)}. We conclude that the annual variability of switchback events in the heliospheric solar wind is modulated by flare and CME rates originating in the photosphere and lower corona.

Aims. We present a database of 43,340 atmospheric models ( ∼ 80,000 models at the conclusion of the project) for stars with stellar masses between 9 and 120 M ⊙ , covering the region of the OB main-sequence and Wolf-Rayet (W-R) stars in the Hertzsprung--Russell (H--R) diagram. Methods. The models were calculated using the ABACUS I supercomputer and the stellar atmosphere code CMFGEN. Results. The parameter space has six dimensions: the effective temperature T eff , the luminosity L , the metallicity Z , and three stellar wind parameters: the exponent β , the terminal velocity V ∞ , and the volume filling factor F cl . For each model, we also calculate synthetic spectra in the UV (900-2000 A), optical (3500-7000 A), and near-IR (10000-40000 A) regions. To facilitate comparison with observations, the synthetic spectra can be rotationally broadened using ROTIN3, by covering vsin(i) velocities between 10 and 350 km s −1 with steps of 10 km s −1 . Conclusions. We also present the results of the reanalysis of ϵ Ori using our grid to demonstrate the benefits of databases of precalculated models. Our analysis succeeded in reproducing the best-fit parameter ranges of the original study, although our results favor the higher end of the mass-loss range and a lower level of clumping. Our results indirectly suggest that the resonance lines in the UV range are strongly affected by the velocity-space porosity, as has been suggested by recent theoretical calculations and numerical simulations.