New Astronomy Reviews最新文献

When astrophysical jets were discovered one hundred years ago, the field of numerical simulations did not yet exit. Since the arrival of programmable computers though, numerical simulations have increasingly become an indispensable tool for dealing with “tough nut” problems which involve complex dynamic and non-linear phenomena. Astrophysical jets are an ideal example of such a tough nut, where multi-scale plasma physics, radiative and non-thermal processes, turbulence and relativity combine to present a formidable challenge to researchers.

Highlighting major achievements obtained through numerical simulations concerning the validity and nature of the Blandford–Znajek mechanism, the launching, collimation, acceleration and stability of jets, their interaction with the surrounding plasma, jet-galaxy feedback mechanisms etc., we trace how the field developed from its first tentative steps into the age of “maturity”. We also give a brief and personal outlook on how the field may evolve in the foreseeable future.

The major revolution in modern astronomy recognizing the universe as teeming with exoplanets, the discovery of liquid water in solar moons, and the continuing focus on Mars exploration, all accelerate the re-evaluation of potential biomarkers for extraterrestrial life. Based on life on planet Earth which relies heavily on chiral molecules and especially on homochiral families, the detection of molecules with these structural properties appears in all road-maps as prime indicators of extraterrestrial life. This review analyzes the strengths, bounds and potential weaknesses of relying on chirality and on homochirality as biomarkers, along with recommendations of how to practically use it. Some of the main issues presented, discussed and answered include: what is the extent to which chirality can be expected to be a universal feature of life; is detection of chirality enough or do we need also to detect homochirality; how justified is it to view life on Earth as purely homochiral; what are the weaknesses of the need to invent an arbitrary label of handedness (needed to define homochirality) and what are the pitfalls that emerge from these weaknesses; what stands behind a detected specific value of enantiomeric excess and what affects its values as we consider old, extinct life, just emerging embryonic life, or extant but rare life; how can one quantify the degree of homochirality; and, what are relevant experimental approached for detecting chirality on-ground and from distance? Finally, a summary with a concise list of recommendations is provided, along with a brief outlook.

Novae and supernovae play a key role in many fields of Astrophysics and Cosmology. Despite their importance, an accurate description of which objects explode and why and how they explode is still lacking. One of the main characteristics of such explosions is that they are the main suppliers of newly synthesized chemical elements in the Galaxy. Since some of these isotopes are radioactive, it is possible to use the corresponding gamma-rays as a diagnostic tool of the explosion thanks to their independence on the thermal state of the debris. The drawback is the poor sensitivity of detectors in the MeV energy domain. As a consequence, the radioactive lines have only been detected in one core collapse supernova (SN 1987A), one Type Ia supernova (SN 2014J), and one supernova remnant (Cas A). Nevertheless these observations have provided and are providing important information about the explosion mechanisms. Unfortunately, novae are still eluding detection. These results emphasize the necessity to place as soon as possible a new instrument in orbit with enough sensitivity to noticeably enlarge the sample of detected events.

The measurement and astrophysical interpretation of characteristic $\gamma$-ray lines from nucleosynthesis was one of the prominent science goals of the INTEGRAL mission and in particular its spectrometer SPI. Emission from ${}^{26}$Al and from ${}^{60}$Fe decay lines, due to their My decay times, originates from accumulated ejecta of nucleosynthesis sources, and appears diffuse in nature. ${}^{26}$Al and ${}^{60}$Fe are believed to originate mostly from massive star clusters. The radioactive decay $\gamma$-ray observations open an interesting window to trace the fate and flow of nucleosynthesis ejecta, after they have left the immediate sources and their birth sites, and on their path to mix with ambient interstellar gas. The ${}^{26}$Al emission image obtained with INTEGRAL confirms earlier findings of clumpiness and an extent along the entire plane of the Galaxy, supporting its origin from massive-star groups. INTEGRAL spectroscopy resolved the line and found Doppler broadenings and systematic shifts with longitude, originating from large-scale galactic rotation. But an excess velocity of  200 km s⁻¹ suggests that ${}^{26}$Al decays preferentially within large superbubbles that extend in forward directions between spiral arms. The detection of ${}^{26}$Al line emission from the nearby Orion clusters in the Eridanus superbubble supports this interpretation. Positrons from $\beta$⁺ decays of ${}^{26}$Al and other nucleosynthesis ejecta have been found to not explain the morphology of positron annihilation $\gamma$-rays at 511 keV that have been measured by INTEGRAL. The ${}^{60}$Fe signal measured by INTEGRAL is diffuse but too weak for an imaging interpretation, an origin from point-like/concentrated sources is excluded. The ${}^{26}$Al /${}^{60}$Fe ratio is constrained to a ra