This webpage hosts the recorded talks of the weekly colloquium we organize at the Department of Astronomy, Universidad de Concepcion. If you are interested in giving a Seminar presentation please contact Nathan Leigh (nleigh [at] amnh.org). Talks should be no longer than 45 min, and present exciting new results.
Determining the orbital history of satellite galaxies in observed galaxy groups and clusters is fundamental to distinguish the environmental and internal processes that shaped them. Here, I present our orbital study of a sample of Milky Way satellites that are located near the virial radius of the Milky Way. We find that Phoenix is on its first infall, while Leo T, Eridanus II and Cetus also have backsplash solutions, i.e. satellites that could have entered and left the Milky Way halo in the past, being now on their second infall. Furthermore, we use this orbital information to constrain hydro-dynamical wind tunnel simulations to study the effects of ram pressure stripping on the internal dynamics of dwarf galaxy satellites, where we focus on Leo T by comparing with HI observations.
Nearly a quarter of all massive stars will merge with a companion during their lifetimes. This coalescence is preceded by a contact phase, which, while expected to be common, is poorly understood. This is due to a lack of observational constraints: less than ten O-type overcontact binaries are currently known. Recent theoretical studies have postulated that these massive overcontact binaries could be progenitors of several exotic classes of objects including magnetic massive stars, Be/Oe stars, LBVs and gravitational wave events. In this talk, I will discuss the current state of the field of massive overcontact binary research, with a specific focus on the chemically homogeneous evolution pathway. I will discuss the theoretical predictions as well as what the observational data tells us, and how these compare and contrast with one another. I will also describe a new spectroscopic analysis technique specifically designed to analyze these highly deformed systems and I will discuss how accounting for the 3D geometry can change our understanding of these objects.
Low mass stars below about 0.35 solar mass or spectral class M3.5 are fully convective. According to stellar structure and evolution models the transition to full convection is abrupt in that the minimum size of a stable radiative core in the lightest partially convective stars is roughly 40% of stellar radius. This is interesting from a dynamo-theoretic perspective because fully convective stars do not have a tachocline, a layer of strong shear at the interface of radiative and convective zones, which plays a crucial role in some models of the solar dynamo. This could mean that the dynamos in fully convective stars are fundamentally different from those in partially convective stars. However, no unambiguous break in magnetic activity indicators has been observed across the transition to full convection to date. Thus it is of great interest for stellar dynamo theory to study the magnetic field generation in fully convective stars in comparison to corresponding partially convective stars. In this talk I will present results from a set of modest resolution simulations of a 0.2 solar mass M5 dwarf made with an updated version of the star-in-a-box model introduced in Dobler et al. (2006, ApJ, 638, 336). This setup allows the full star to be modeled in Cartesian coordinates without having to deal with coordinate singularities. The set of runs covers rotation periods between 4.3 and 430 days corresponding to Coriolis (inverse Rossby) numbers between 0.7 and more than 300. The current results show that transitions of differential rotation and large-scale dynamo solutions occur very similarly as in partially convective cases which could suggests a common origin of magnetism in fully and partially convective stars.
Finding a lonely black hole in the Milky Way is not an easy task as the black holes are pitch black. One of the promising methods is gravitational microlensing, in which a background source gets brighter temporarily due to its light being bent and magnified by a foreground object. Gaia space mission provides unique data which can be used for discovering black holes thanks to its all-sky monitoring and superb-accuracy time-series astrometric measurements. However, in order to efficiently use Gaia data, long-term monitoring and follow-up of candidate microlensing events are necessary. I will describe the products of Gaia Science Alerts and our programme of coordinated time-domain observations of candidates for lensing events, including long-term photometry collected by a global network of robotic and manual telescopes, as well as spectroscopic and astrometric follow-up using the largest telescopes in the world. I will also describe the forthcoming results to be part of Gaia Data Release 3, containing candidates for microlensing events due to black holes.
Machine learning provides a diversity of architectures that goes from simple fully connected neural layers to state-of-the-art transformer architectures, that allow us to solve problems in a supervised, semi-supervised and unsupervised manner. Astronomy has become one of the favorites fields for data scientists, because of the huge amount of public data, this has created a new hybrid branch of astronomers from the computer science world and inspired several collaborations to overcome the challenges of the new telescopes. Therefore I will try to give an intuition of how different branches of machine learning works and how these have been used to approach different astronomical problems.
In this talk I provide an overview of my recent major research programs, moving from small to large scale. At the planetary scale topic is the testing of a hypothesis for the formation of meteoritic chondrules in protostellar disks by non-ideal magnetohydrodynamics processes. I’ll then briefly describe our demonstration that chondrule accretion onto planetesimals formed by streaming instability reproduces the asteroid belt’s primordial size distribution. I’ll then turn to the scale of star formation where I’ll demonstrate that interstellar turbulence alone cannot reproduce the size-velocity dispersion relation in molecular clouds, but gravitational collapse does, suggesting that molecular clouds are short-lived, dynamic objects that are ultimately destroyed by feedback. We have further begun exploring the dynamical properties of the resultant stellar groups. At the galactic scale, models of entire galaxies provide insight into the distribution of dense clouds formed by gravitational instability, while study of magnetic fields in supernova-driven turbulence shows that small-scale dynamos quickly grow fields in such environments. Finally, I focus on how stellar or intermediate mass black holes in the disks of gas around supermassive black holes can be modeled by analogy with planets in protostellar disks, demonstrating that these environments may be the optimal location for forming the black hole binaries whose mergers are detected by LIGO.
The recent gravitational wave detections by LIGO/Virgo revolutionized the way we sense our Universe. However, it remains challenging to explain the formation channels of these sources. Motivated by these challenges, recent studies have emphasized the significant contribution of dynamical formation channels in dense stellar environments to the overall gravitational wave signals. Focusing on the dense stellar clusters surrounding supermassive black holes at the center of galaxies, I will outline stellar binaries' evolution from birth up to possible gravitational wave mergers. The supermassive black hole can induce collisions between binary members, while the frequent interactions with the neighbors in this dense environment can sometimes tend to unbind the binary. I will highlight some exotic outcomes, including gravitational-wave emission, for this dynamical evolution channel. I will show how this channel can leave a clear signature on the gravitational wave signals, allowing differentiation between different merger mechanisms. The Laser Interferometer Space Antenna (LISA) can potentially be used to distinguish between channels.
About 30% of pulsating red giant and supergiant stars show additional variability with a period ranging from several months to several years - typically ten times longer than the pulsation period of the same star. Numerous authors have explored various scenarios for the origin of LSPs, but were unable to give a final solution to this problem. I will present known properties of LSP variables and show new results proving that the physical mechanism responsible for LSPs is binarity. Namely, the LSP light changes are due to the presence of a dusty cloud orbiting the red giant together with the brown-dwarf companion and obscuring the star once per orbit. In this scenario, the low-mass companion is a former planet that accreted a significant amount of mass from the envelope of its host star and grown into a brown dwarf.
Binary systems are indispensable tools for studying stellar evolution, as they offer a straightforward way of measuring masses of stars, once the orbits are determined. With these, we can test models of stellar evolution for a wide variety of stellar types. Decades of studies have revealed that most stars are located in binary systems. Perplexingly, until very recently only one RR Lyrae variable star was known to reside in a binary system, despite large surveys (OGLE, Gaia, Pan-STARRS) finding hundreds of thousands of this class of stars in the Milky Way. In this talk, I will present our latest search for RR Lyrae variables in binary systems, using the light-travel time effect. Our candidate sample is the first one large enough to draw firm conclusions on the statistics of the distribution of orbital parameters for RR Lyrae variables. Our efforts toward the radial velocity confirmation of these systems, as well the reasons behind their apparent paucity will also be discussed.
Magnetic fields have long been thought to play key roles in driving accretion onto black holes and launching their relativistic jets. I will present spatially resolved long-baseline interferometry observations of linearly polarized synchrotron radiation from near the event horizons of two weakly accreting supermassive black holes, Sgr A* and M87. I will show how the data can be used to infer properties of the radiating plasma, and map the magnetic field structure. The data imply the presence of dynamically important magnetic fields in the emission region in both cases, which can alter the structure of the accretion flow and launch powerful jets.
During hydrogen reionization the UV radiation from the first luminous sources injected vast amount of energy into the intergalactic medium, photo-heating the gas to tens of thousands of degree Kelvin. This increase in temperature has left measurable `imprints' in the thermal history of the cosmic gas: a peak in the temperature evolution at the mean density and a smoothing out of the gas in the physical space by the increased gas pressure following reionization (i.e. Jeans smoothing effect). The structures of the HI Lyman-alpha forest at high redshift are sensitive to both these effects and therefore represent a powerful tool to understand when and how reionization happened. I will present the most recent constraints on the thermal history of the intergalactic medium obtained using the Lyman-alpha forest flux power spectrum at z>5. I will show how these results can be used to obtain information on the timing and the sources of the reionization process and I will discuss their consistency with different possible reionization scenarios.
Stellar mass black holes (BHs) orbiting around supermassive black holes (SMBH) in active galactic nucleus (AGN) disks are possible sources for binary stellar mass black hole (BBH) mergers detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) and extreme mass ratio inspirals (EMRIs) that could be detected by the Laser Interferometer Space Antenna (LISA). AGN disks are promising locations for BBH mergers because the powerful influence of the gas drives orbital evolution, makes encounters dissipative, and leads to migration. In this talk, I will first share results from simulations of BHs orbiting in the prograde direction in AGN disks performed using an augmented N-body code that includes drag and migration forces exerted by the gas. In these simulations, BBHs form rapidly as they migrate to regions of the disk where the positive and negative torque cancels out, called migration traps. Like a majority of reported LIGO detections thus far, a large fraction of the BBH mergers in these simulations have similar component masses, total mass below 100 solar masses, and small effective dimensionless spins (\chi_eff). However, these simulations also produce high total mass and uneven mass ratio mergers resembling two unusual LIGO detections GW190521 and GW190412 at a rate of ∼0.1 Gpc-3 yr-1, and ∼0.3 Gpc-3 yr-1, respectively. Additionally, some mergers could have mis-aligned spins, or negative \chi_eff. Second, I will discuss the impact of BHs that orbit in the retrograde direction in AGN disks on BBH mergers. An analytic calculation of the orbital evolution of these retrograde orbiters suggests they may not have a large effect on the overall merger rates of BHs in AGN disks and are likely to become EMRIs.
In the past decade, mounting evidence has cast doubt on the standard Chandrasekhar-mass Type Ia supernova progenitor scenario. In this talk, I will discuss a new scenario, in which an explosion is triggered as two white dwarfs begin to merge. In this dynamically driven double-degenerate double-detonation (D6) scenario, the exploding white dwarf is significantly below the Chandrasekhar mass. I will present our hydrodynamical and radiation transport simulations of these explosions and show that they reproduce most of the observable features of Type Ia supernovae. I will also discuss the discovery of hypervelocity survivors that provide overwhelming evidence that the D6 scenario occurs in nature.
Turbulence is ubiquitous in the universe and plays an important role in diverse astrophysical processes, from star formation to galaxy evolution. Measuring turbulence in astrophysics, however, is not always easy. In this talk, I will discuss novel techniques my group recently developed in the analysis of turbulence in the interstellar medium (ISM) and the intracluster medium (ICM). We study the motions of young stars in the Orion Molecular Cloud Complex, using the full 6-dimensional measurements of positions and velocities provided by the APOGEE-2 and Gaia DR2 surveys. We compute the velocity structure functions (VSFs) of the stars in six different groups within the Orion Complex, and find that the motions of stars in all diffuse groups exhibit strong characteristics of turbulence. Our VSFs also show features supporting local energy injection from supernovae. We have also analyzed the motions of multiphase gas in the ICM of three nearby galaxy clusters: Perseus, Abell 2597 and Virgo. We show that the motions of the filamentary cool clouds are turbulent, and the turbulence is driven by feedback from the SMBHs in the centers of these clusters. The smallest scales we probed are comparable to the mean free path in the hot ICM. The detection of turbulence on these scales provides strong evidence that isotropic viscosity is suppressed in the hot plasma.
The observation of gravitational waves has opened a new, unexplored window onto the Universe. Among the sources of gravitational wave transients, compact objects such as neutron stars (NSs) and black holes (BHs) play the most important role. In this talk, I will focus on the expected gravitational wave signal when two compact objects (NS-NS and NS-BH) in a binary merge. These events are believed to be accompanied by a strong electromagnetic signature in gamma-rays, followed by longer-wavelength radiation. I will discuss what can be learned from the complementary observations of the electromagnetic and the gravitational wave signals during these events, with particular focus on the source GW170817/GRB170817.
The detection of fully-grown supermassive black holes in active galactic nuclei at high redshift, when the Universe was young, challenges the theories of black holes growth, requiring long periods of high accretion, most likely above the Eddington limit. These objects will be difficult to probe even with future advanced observatories. Ultraluminous X-ray sources (ULXs) are nearby stellar-mass black holes or neutron stars accreting above their Eddington limit. This was understood after the discovery of coherent pulsations and cyclotron lines in some ULXs, indicating that at least a fraction of them hosts neutron stars as compact objects and, finally, our discovery of powerful winds as predicted by theoretical models of super-Eddington accreting black holes and neutron stars. ULX winds carry a huge amount of power owing to their mildly relativistic speeds (~0.2c) and are able to significantly affect the surrounding medium, producing the observed 100s pc super bubbles, and limit the amount of matter that can reach the central accretor. The study of ULX winds is therefore quintessential to understand 1) how much and how fast can matter be accreted by black holes and 2) how strong is their feedback onto the surrounding medium in the regime of high accretion rate such as for quasars and supermassive black holes at their peak of growth. I will provide an overview on this vast phenomenology and discuss how we can use similar techniques on highly-accreting supermassive black holes to understand their fast growth and feedback onto the host galaxy.
Tidal disruption events (TDEs) occurs when a star is tidally disrupted by a supermassive black hole. As TDEs are being detected at an accelerating rate, we will soon enter the era where demographic studies for inactive supermassive black holes are possible via TDEs. Two important characteristic scales, the tidal radius and the energy of the stellar debris, are often estimated on an order-of-magnitude basis without taking into account the star’s internal structure and relativistic effects. Since tidal disruption events take place at shorter distances for higher black hole mass, fully general relativistic calculations of tidal forces are required to study the pericenter-dependence of tidal disruption properties for a wide range of black hole mass. Using fully general relativistic hydrodynamics simulations and MESA-model initial conditions, we determine the physical tidal radii yielding full disruptions and the characteristic energy widths of the stellar debris. Furthermore, I present a new mass inference method called TDEmass, based upon a physical model for optical/UV light emission that incorporates the relativistic correction factor for the debris energy. In this model, the only two inputs are peak luminosity and blackbody temperature at peak. Since only spectral data at peak are used, no assumptions for the temporal trends of lightcurves and their relations to mass fallback rates are made. In the future, this method could be useful to study the population properties of both massive black holes and stars in quiescent galactic nuclei.
The centers of massive galaxies are special in many ways, not least because apparently all of them host supermassive black holes. Since the discovery of a number of relations linking the mass of this central black hole to the large scale properties of the surrounding galaxy bulge it has been suspected that the growth of the central black hole is intimately connected to the evolution of its host galaxy. However, at lower masses, and especially for bulgeless galaxies, the situation is much less clear. Interestingly, these galaxies often host massive star clusters at their centers, and unlike black holes, these nuclear star clusters provide a visible record of the accretion of stars and gas into the nucleus. I will present our ongoing observing programme of the nearest nuclear star clusters, including the ones in our Milky Way and the Sagittarius dwarf galaxy. These observations provide important information on the formation mechanism of nuclear star clusters, allow us to measure potential black hole masses and give clues on how black holes get to the centers of galaxies.
Comets are among the most primitive bodies in the Solar System and still contain records of the physical processes occurred during the Solar System formation. In particular, cometary dust is very useful to reconstruct the history of the Solar System, because dust was the source of all Solar System bodies. The ESA/Rosetta mission (2014-2016) gave a unique opportunity to decipher the comets’ evolution, since it was the first mission to orbit a comet (i.e., 67P/Churyumov-Gerasimenko), escorting it during the main stages of its orbit, from pre-perihelion (when the comet was poorly active) to post-perihelion (when the comet surface and coma were partially renewed from the occurred activity). The seminar presents the works leaded from the speaker during the mission and during the activity of the ISSI (International Space Science Institute) International Team “Characterization of 67P/Churyumov-Gerasimenko cometary activity” (2019-2022, whose the speaker is leader).
The New Horizons flyby of the cold classical Kuiper Belt object MU69 showed it to be a contact binary. The existence of other contact binaries in the 1--10 km range raises the question of how common these bodies are and how they evolved into contact. In this talk I will show, considering that the pre-contact lobes of MU69 formed as a binary embedded in the Solar nebula, the subsequent orbital evolution in the presence of gas drag. While the sub-Keplerian wind of the disk is able to bring the drag timescales for 10 km bodies to under 1 Myr in the asteroid belt, at the distances of the Kuiper belt the same mechanism is rendered ineffective. Instead, a combination of nebular drag and Kozai-Lidov oscillations is a more promising channel for collapse. I will describe a solution to the hierarchical three-body problem with nebular drag that we implemented into a Kozai cycles plus tidal friction model. The permanent quadrupoles of the pre-merger lobes make the Kozai oscillations stochastic, and we find that when gas drag is included the shrinking of the semimajor axis more easily allows the stochastic fluctuations to bring the system into contact. Evolution to contact happens very rapidly (within 10,000 yrs) in the pure, double-average quadrupole, Kozai region between ~85-95 degrees, and within 3 Myr in the drag-assisted region beyond it. The synergy between the permanent quadrupole and gas drag widens the window of contact to 80-100 degrees of initial inclination, over a larger range of semimajor axes than either Kozai or J2 alone. As such, the model predicts a low initial occurrence of binaries in the asteroid belt, and an initial contact binary fraction of about 10% for the cold classicals in the Kuiper belt.
Gaia DR2 provides unprecedented precision in measurements of the distance and kinematics of stars in the solar neighborhood. Through applying hierarchical clustering on 5D data set (3D position + 2D velocity), we identify a number of clusters, associations, and comoving groups within 3 kpc. Through leveraging machine learning techniques, we can estimate the ages of these stars with pseudo-isochrone fitting. Furthermore, supervised learning then allows for identification of isolated pre-main sequence stars that cannot be recovered through clustering. With these efforts combined, we can produce to date the largest catalog of stars with known ages, allowing for investigation of star formation history of the solar neighborhood, such as identifying a ring of stars with ages of up to 40 Myr tracing the outer edges of the Local Bubble that has likely been responsible for the formation of the Gould's belt. Most of the young stars are commonly found to be filamentary or string-like populations, oriented in parallel to the Galactic plane, and some span hundreds of parsec in length. Most likely, these strings are primordial, tracing the morphology of filamentary clouds that produced them, rather than the result of tidal stripping or dynamical processing. The youngest strings (younger than 100 Myr) tend to be orthogonal to the Local Arm. Stars in a string tend to persist as comoving for time scales of ~300 Myr, after which most dissolve into the Galaxy. These data shed a new light on the local galactic structure and a large-scale cloud collapse.
Gaia, an astrometry satellite, has dramatically updated the dynamical structures of the Milky-Way galaxies. However, the information that we can obtain from observations are limited in terms of both time and space. We can observe only the current positions and velocities of stars within a few kpc from the Sun. On the other hand, simulations provide us the entire time evolution of modeled galaxies. We performed fully self-consistent N-body simulations of disk galaxies modeling the Milky-Way. We model galactic disk, bulge, and dark matter halo as N-body systems with maximum 8 billion particles. using the same mass resolution to avoid numerical heating. Our high-resolution simulation allows us to follow the individual orbits of stars which are trapped in the inner Lindblad resonances of the bar. Observed structures in the distribution of stars in the phase space is explained by such resonant orbits. I will also talk about the transient spiral structures of disk galaxies.
The flatness of the solar system led to the notion that the planets formed within a disk around the young Sun. Although this notion is broadly confirmed by images of protoplanetary disks and a population a multi-planet systems, some exoplanets challenge this flatness hypothesis, presenting significant tilts relative to either their host stars’ equators (stellar obliquities) or other planets (mutual inclinations). In this talk, I will discuss what physical processes can be responsible from these tilts and what they teach us about the history of planetary systems. First, I will focus on a population of sub-Neptune planets with large stellar obliquities and argue that their large tilts were likely imprinted early on during the evaporation of their birth disks. Second, I will focus on the question of coplanarity in Kepler planetary systems and argue that most of these systems have been subject to dynamical heating, with the hottest systems ending as ultra-short-period planets in sub-day orbits. Finally, I will discuss the prospects of using astrometric measurements from Gaia to continue providing with new clues on the flatness of exoplanet systems.