Unsolved Problems In Astrophysics Research Articles

Neutrino fast flavor instability in three dimensions for a neutron star merger

The flavor evolution of neutrinos in core collapse supernovae and neutron star mergers is a critically important unsolved problem in astrophysics. Following the electron flavor evolution of the neutrino system is essential for calculating the thermodynamics of compact objects as well as the chemical elements they produce. Accurately accounting for flavor transformation in these environments is challenging for a number of reasons, including the large number of neutrinos involved, the small spatial scale of the oscillation, and the nonlinearity of the system. We take a step in addressing these issues by presenting a method which describes the neutrino fields in terms of angular moments. We apply our moment method to neutron star merger conditions and show it simulates fast flavor neutrino transformation in a region where this phenomenon is expected to occur. By comparing with particle-in-cell calculations we show that the moment method is able to capture the three phases of growth, saturation, and decoherence, and correctly predicts the lengthscale of the fastest growing fluctuations in the neutrino field.

The VLT-FLAMES Tarantula Survey

Context. The origin of massive runaway stars is an important unsolved problem in astrophysics. Two main scenarios have been proposed, namely: dynamical ejection or release from a binary at the first core collapse. However, their relative contribution remains heavily debated. Aims. Taking advantage of two large spectroscopic campaigns towards massive stars in 30 Doradus, we aim to provide observational constraints on the properties of the O-type runaway population in the most massive active star-forming region in the Local Group. Methods. We used radial velocity measurements of the O-type star populations in 30 Doradus obtained by the VLT-FLAMES Tarantula Survey and the Tarantula Massive Binary Monitoring to identify single and binary O-type runaways. Here, we discuss the rotational properties of the detected runaways and qualitatively compare the observations with expectations of ejection scenarios. Results. We identified 23 single and one binary O-type runaway objects, most of them located outside the main star-forming regions in 30 Doradus. We find an overabundance of rapid rotators (ve sin i > 200 km s−1) among the runaway population, thus providing an explanation for the observed overabundance of rapidly rotating stars in the 30 Doradus field. Considerations of the projected rotation rates and runaway line-of-sight velocities reveal a conspicuous absence of rapidly rotating (ve sin i > 210 km s−1), fast-moving (vlos > 60 km s−1) runaway stars in our sample, strongly suggesting the presence of two different populations of runaway stars: a population of rapidly spinning but slowly moving runaway stars and a population of fast-moving but slowly rotating ones. These are detected with a ratio close to 2:1 in our sample. Conclusions. We argue that slowly moving but rapidly spinning runaway stars result from binary ejections, while rapidly moving but slowly spinning runaways could result from dynamical ejections. Given that detection biases will more strongly impact the slow-moving runaway population, our results suggest that the binary evolution scenario dominates the current massive runaway star population in 30 Doradus.

Nanoflare Heating of the Solar Corona Observed in X-Rays

The existence of the million-degree corona above the cooler photosphere is an unsolved problem in astrophysics. Detailed study of the quiescent corona that exists regardless of the phase of the solar cycle may provide fruitful hints toward resolving this conundrum. However, the properties of heating mechanisms can be obtained only statistically in these regions due to their unresolved nature. Here, we develop a two-step inversion scheme based on the machine-learning scheme of Upendran & Tripathi (2021a) for the empirical impulsive heating model of Pauluhn & Solanki (2007), and apply it to disk integrated flux measurements of the quiet corona as measured by the X-ray solar monitor on board Chandrayaan-2. We use data in three energy passbands, viz, 1–1.3, 1.3–2.3, and 1–2.3 keV, and estimate the typical impulsive event frequencies, timescales, amplitudes, and the distribution of amplitudes. We find that the impulsive events occur at a frequency of ≈25 events per minute with a typical lifetime of ≈10 minutes. They are characterized by a power-law distribution with a slope α ≤ 2.0. The typical amplitudes of these events lie in an energy range of 1021–1024 erg, with a typical radiative loss of about ≈103 erg cm−2 s−1 in the energy range of 1–2.3 keV. These results provide further constraints on the properties of subpixel impulsive events in maintaining the quiet solar corona.

Characterizing fast radio bursts through statistical cross-correlations

Understanding the origin of fast radio bursts (FRB's) is a central unsolved problem in astrophysics that is severely hampered by their poorly determined distance scale. Determining the redshift distribution of FRB's appears to require arcsecond angular resolution, in order to associate FRB's with host galaxies. In this paper, we forecast prospects for determining the redshift distribution without host galaxy associations, by cross-correlating FRB's with a galaxy catalog such as the SDSS photometric sample. The forecasts are extremely promising: a survey such as CHIME/FRB that measures catalogs of $\sim 10^3$ FRB's with few-arcminute angular resolution can place strong constraints on the FRB redshift distribution, by measuring the cross-correlation as a function of galaxy redshift $z$ and FRB dispersion measure $D$. In addition, propagation effects from free electron inhomogeneities modulate the observed FRB number density, either by shifting FRB's between dispersion measure (DM) bins or through DM-dependent selection effects. We show that these propagation effects, coupled with the spatial clustering between galaxies and free electrons, can produce FRB-galaxy correlations which are comparable to the intrinsic clustering signal. Such effects can be disentangled based on their angular and $(z, D)$ dependence, providing an opportunity to study not only FRB's but the clustering of free electrons.

Linking gravitational waves and X-ray phenomena with joint LISA and Athena observations

The evolution of cosmic structures, the formation and growth of the first black holes and the connection to their baryonic environment are key unsolved problems in astrophysics. The X-ray Athena mission and the gravitational-wave Laser Interferometer Space Antenna (LISA) offer independent and complementary angles on these problems. We show that up to about 10 black hole binaries in the mass range of approximately 105 to 108 solar masses discovered by LISA at redshift below about 3.5 could be detected by Athena in an exposure time up to 100 ks, if prompt X-ray emission of 1–10% of the Eddington luminosity is present. Likewise, if any LISA-detected extreme-mass-ratio inspirals occur in accretion disks, Athena can detect associated electromagnetic emission out to a redshift of about 1. Finally, warned by LISA, Athena can point in advance and stare at stellar-mass binary black hole mergers at redshift less than about 0.1. These science opportunities emphasize the vast discovery space of simultaneous observations from the two observatories, which would be missed if they were operated in different epochs. This Perspective highlights the science opportunities and the vast discovery space of simultaneous observations from the LISA and Athena observatories, which would be missed if they were operated in different epochs .

Finding forming globular clusters at high redshifts

Abstract The formation of globular clusters (GCs) with their multiple stellar populations remains a puzzling, unsolved problem in astrophysics. One way to gather critical insight consists in finding sizable numbers of GC progenitors (GCPs) while still near the peak of their star formation phase, at a look-back time corresponding to GC ages (∼12.5 Gyr, or z ≃ 5). This opportunity is quantitatively explored, calculating how many GCPs could be detected by deep imaging in the optical, near-IR and mid-IR bands. For concreteness, for the imaging camera performances those of NIRCam on board of James Webb Space Telescope are adopted. The number of GCPs that could be detected scales linearly with their mass, i.e. on how much more massive GCPs were compared to their GC progeny, and perspectives look promising. Besides providing direct evidence on GC formation, the detection of GCPs, their clustering, with or without a central galaxy already in place, would shed light on the relative timing of GC formation and galaxy growth and assembly. All this may be the result of dedicated observations as well as a side benefit of deep imaging meant to search for the agents of cosmic reionization.

The suppression of direct collapse black hole formation by soft X-ray irradiation

The origin of supermassive black holes (SMBHs) in galactic nuclei is one of the major unsolved problems in astrophysics. One hypothesis is that they grew from >10^5 M_sun black holes that formed in the `direct collapse' of massive gas clouds that have low concentrations of both metals and molecular hydrogen (H_2). Such clouds could form in the early (z>10) Universe if pre-galactic gas is irradiated by H_2-photodissociating, far-ultraviolet (FUV) light from a nearby star-forming galaxy. In this work, we re-examine the critical FUV flux J_crit that is required to keep H_2 photodissociated and lead to direct collapse. We submit that the same galaxies that putatively supply the extraordinary FUV fluxes required for direct collapse should also produce copious amounts of soft X-rays, which work to offset H_2 photodissociation by increasing the ionization fraction and promoting H_2 formation. Accounting for this effect increases the value of J_crit by a factor of at least 3-10, depending on the brightness temperature of FUV radiation. This enhancement of J_crit suppresses the abundance of potential direct collapse sites at z>10 by several orders of magnitude. Recent studies---without accounting for the soft X-rays from the FUV source galaxies---had already arrived at large values of J_crit that implied that direct collapse may occur too rarely to account for the observed abundance of high-redshift quasars. Our results suggest that J_crit should be even higher than previously estimated, and pose an additional challenge for the direct collapse scenario via strong FUV radiation to explain the high-redshift quasar population.

The Concept of Law and Models in Chemistry

After a brief introduction to the basic concepts including some questions of language, the first part of this paper provides a brief survey of the historical development of laws and models in Chemistry, in particular atomic and molecular models. In the second part this paper deals with the fundamental role of the observation of symmetry violations in physics and chemistry in understanding the most ‘fundamental laws’ and current efforts towards such studies by means of high resolution spectroscopy of molecules. We conclude with a brief discussion of the implications for current unsolved problems in astrophysics and biology.

The collective origin of spiral structures in disk galaxies

After almost fifty years the origin of spiral arms in disk galaxies remains one of the major unsolved problems in astrophysics. Recent attempts have indicated that substructures in the dark matter halos of galaxies could induce spiral patterns in disks by generating localized disturbances that grow by swing amplification. However, there are indications that dark matter substructures orbiting in the inner regions of galaxy halos would be destroyed by dynamical processes such as disk shocking, and hence would not be able to seed the formation of spiral structure. Instead, we use numerical simulations of unprecedented resolution explore the possibility that spiral arms might be generated by the dynamical response of the disk to overdensities corotating within the disk. These perturbations can be identified with fluctuations in the distribution of gas in the interstellar medium of galaxies, such as giant molecular clouds. We develop a new theory for spiral structure formation based on the non-linear effects of swing amplification. Our model makes numerous testable predictions, making it possible to finally confront theory with observations.

Type-Ia Supernova Rates and the Progenitor Problem: A Review

AbstractThe identity of the progenitor systems of type-Ia supernovae (SNe Ia) is a major unsolved problem in astrophysics. SN Ia rates are providing some striking clues. We review the basics of SN rate measurement, preach about some sins of SN rate measurement and analysis, and illustrate one of these sins with an analogy about Martian scientists. We review the recent progress in measuring SN Ia rates in various environments and redshifts, and their use to reconstruct the SN Ia delay-time distribution (DTD)—the SN rate versus time that would follow a hypothetical brief burst of star formation. A good number of DTD measurements, using a variety of methods, appear to be converging. At delays 1 <t<10 Gyr, these measurements show a similar, ∼t−1, power-law shape. The DTD peaks at the shortest delays probed. This result supports the idea of a double-degenerate progenitor origin for SNe Ia. Single-degenerate progenitors may still play a role in producing short-delay SNe Ia, or perhaps all SNe Ia, if the red-giant donor channel is more efficient than is found by most theoretical models. The DTD normalization enjoys fairly good agreement (though perhaps some tension), among the various measurements, with a Hubble time–integrated DTD value of about 2 ± 1 SNe Ia per 1000M⊙ (stellar mass formed with a low–mass turnover initial mass function). The local WD binary population suggests that the WD merger rate can explain the Galactic SN Ia rate, but only if sub-Chandra mergers lead to SN Ia events. We point to some future directions that should lead to progress in the field, including measurement of the bivariate (delay and stretch) SN Ia response function.

Magnetic Braking and the Evolution of the HW Vir-like Binary Stars

HW Vir-like close binaries are thought to be the progenitors of cataclysmic variables. However, how this type of peculiar stars evolve into cataclysmic variables is an unsolved problem in astrophysics. Here we report the variations of the orbital period of HW Vir, the prototype of this group of systems. A long-term period decrease is discovered to be superimposed on the well-known cyclic change reported by previous investigators. It is suggested that the continuous decrease can be plausibly interpreted by angular momentum loss via magnetic braking of the completely convective star. This is driving the shrink of the orbit and will result in mass transfer between both components. We found that the binary will be undergoing mass transfer long before the sdB primary star evolves into a white dwarf. This conclusion suggests that HW Vir-like binaries do not directly evolve into cataclysmic variables through angular momentum loss as astronomers previously thought. On the other hand, if the observed period decrease is only part of a long-period cyclic variation, it may reveal the presence of another substellar companion in a wide orbit. This makes HW Vir an important system for future study of the formation and evolution of substellar objects.

Fundamental Unsolved Problems in Astrophysics

There is given a list and discussion of what are arguably the top 20 unsolved problems in astrophysics today. The list ranges from particle physics to cosmology. Possible resolutions are noted, but without judgement. Perhaps the most remarkable aspect of the discussed problems is that they are closely interrelated. This opens the prospect that a solution to one or a few may lead to a significantly better understanding of modern astrophysics.

Unsolved problems in astrophysics

Unsolved problems in astrophysics

Unsolved Problems in Astrophysics

The field of astrophysics is in the midst of a technologically driven renaissance, as fundamental discoveries are being made with astonishing frequency. In the last decade, new detectors in space, on earth, and deep underground have, when coupled with the computational power of modern computers, revolutionized our knowledge and understanding of the astronomical world. This is a great time for a student of any age to become acquainted with the remarkable universe in which we live. This volume is a collection of essays, originally presented orally to a diverse group of students and professionals, which reveal the most fertile areas for future study of astronomy and astrophysics. The emphasis of this work is on the clear description of the current state of our knowledge as a preparation for the future unraveling of the mysteries of the universe that appear today as most fundamental and most amenable to solution.A stellar group of astronomers and astrophysicists describes the directions and styles of work that they think are most likely to lead to progress. Bibliographical notes at the end of each presentation provide guidance for the reader who wishes to go more deeply into a given subject. Unsolved Problems in Astrophysics is a uniquely stimulating introduction to some of the most important topics in modern astrophysics.

Magnetic Fields and Disks in Star-forming Regions

AbstractThe star-formation process is an outstanding and largely unsolved problem in astrophysics. The role of magnetic fields is unclear but is widely considered to be important at all stages of protostellar evolution, from cloud collapse to ZAMS. For example, in some hydromagnetic models, the field may assist in removing angular momentum, thereby driving accretion and perhaps bipolar outflows.Spectropolarimetry between 8 and 13μm provides information on the direction of the transverse component of a magnetic field through the alignment of dust grains. We present results of 8–13μm spectropolarimetric observations of a number of bipolar molecular outflow sources, and compare the field directions observed with the axes of the outflows and putative disk-like structures observed to be associated with some of the objects. There is a strong correlation, though so far with limited statistics, between the magnetic field and disk orientations. We compare our results with magnetic field configurations predicted by current models for hydromagnetically driven winds from the disks around Young Stellar Objects (YSOs). Our results appear to argue against the Pudritz and Norman model and instead seem to support the Uchida and Shibata model.

Some Unsolved Problems in Astrophysics

The theoretical and observational situation with regard to solar neutrinos is reviewed and some of the implications of the discrepancy between theory and observation are discussed. A definition is suggested for compact luminous objects (CLO's) and various proposals for the energy source of CLO's are reviewed. The observational situation and opportunities with regard to the association of CLO's (or QSO's) with galaxies is outlined. The absorption lines observed in large-redshift CLO's are divided tentatively into two classes whose origin is discussed. A number of critical observations are suggested to test various unorthodox ideas.