SAO: Science Updates 2021

Find out the latest thinking about our universe.
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SAO: Science Updates 2021

Post by bystander » Wed Jan 27, 2021 12:25 am


Smithsonian Astrophysical Observatory

SAO: Science Updates 2021

<< Science Updates 2020
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The Uncertainties in Measuring Cosmic Expansion

Post by bystander » Wed Jan 27, 2021 12:40 am

The Uncertainties in Measuring Cosmic Expansion
SAO Science Updates | 2101 Jan 01
Ninety years after Edwin Hubble discovered the systematic motions of galaxies and George Lemaitre explained them as cosmic expansion from a point using Einstein’s equations of relativity, observational cosmology today is facing a challenge. Values deduced from the two primary methodologies – the properties of galaxies and the cosmic microwave background radiation (CMBR) -- disagree with each other at roughly the ten percent level, yet each one is precise at the level of a few percent. Uncorrected observational errors are possible, but estimates suggest they are too small to account for the differences. As a result, no consistent and precise value of the expansion - Hubble's constant - has been found. The problem is not so much the value itself - the age of the universe will not change by much either way - rather, it is that something unexplained is clearly going on connected with the fact that the CMBR data arise from a vastly different epoch of cosmic time than do the galaxy data. Perhaps new physics is needed.

An exciting new and independent method of measuring the cosmic expansion parameter uses gravitational waves (GW). The observed intensity of the GW provides a measure of the distance since models can infer the intrinsic strength. When the GW results from a binary neutron star merger which has a detected optical counterpart, the host galaxy's cosmic recession velocity (as measured from its light) provides a calibration for the expansion rate. This new method is called the "standard siren." If the accuracy of the standard siren method is better than that of the other methods, it would be able to resolve the discrepancy.

CfA astronomer Hsin-Yu Chen has investigated the uncertainties associated with the standard siren method and finds that two issues complicate the standard siren method and pose major challenges to its resolving the tension. Both are related to the emitted light and the viewing angle of the source. The first problem is that the light is not emitted spherically according to computer simulations, and so the intensity we observe depends on our viewing angle; even the color is angle-dependent. The viewing angle must somehow be estimated and included in the calibration, and this carries an uncertainty. The second is that the merger event is also seen from a particular angle that affects the result; even after observing many sources, a statistical analysis of the sample will still have an uncertain bias. Chen concludes that these two systematic effects will introduce a bias in the standard siren value of Hubble’s constant that results in its having an uncertainty that is as about large as the uncertainty in other methods.

Systematic Uncertainty of Standard Sirens from the Viewing Angle of Binary Neutron Star Inspirals ~ Hsin-Yu Chen
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Coronal Holes During the Solar Maximum

Post by bystander » Wed Jan 27, 2021 2:43 am

Coronal Holes During the Solar Maximum
SAO Science Updates | 2101 Jan 08
Sunspots were first seen by Galileo, and in the eighteenth century Rudolf Wolf concluded from his study of previous observations that there was a roughly eleven-year solar cycle of activity. In 1919 the astronomer George Ellery Hale found a new solar periodicity, the twenty-two year solar magnetic cycle which is composed of two eleven-year cycles and today is referred to as the Hale cycle. The eleven-year cycle is a complex dynamo process in which the Sun's twisted magnetic fields flip to the opposite direction as the result of the combination of the Sun's differential rotation and the convection in its atmosphere. Then, after a second cycle, the original polarity is recovered. The cycle is characterized by periodic changes in solar activity such as the number of sunspots and active regions (ensembles of looped magnetic structures); during the period of maximum activity the number of sunspots reaches a maximum. The number of coronal holes provides another measure of activity, a coronal hole being a darker appearing region of colder gas on the Sun's surface. During maximum activity, coronal holes are found at low latitudes of the Sun with fewer of them at the polar regions.

Energetic events on the Sun like eruptions, flares, and coronal mass ejections peak at or near times of solar maximum; at the same time some structures in the magnetic field weaken to zero strength and then increase but with the opposite sign. A particularly powerful solar wind can escape during these periods of weak magnetic fields and its charged particles can then travel into space and towards the Earth. Coronal holes are key structures that indicate these weakened fields. CfA astronomers Nishu Karna, Steven Saar, and Ed DeLuca and a team of colleagues performed a statistical study of the coronal holes near the equatorial region, and of active regions, during the maximum phase of the last four solar cycles spanning the years from 1979-2015.

The scientists found a strong negative correlation between the numbers of equatorial coronal holes and active regions as well as statistically significant differences in the properties of the two eleven-year cycles of the Hale cycle. For example, they examined the changing distances ("pairings") between equatorial coronal holes and active regions and find more of the close pairings during the peak of activity in one half of the Hale cycle...but not in the other. Most significantly, during these active times the solar wind flow and wind pressure also increase significantly. The results lead to important insights into how solar activity impacts the Earth and highlight important processes that are still not understood like the different behaviors of the two halfs of the Hale cycle.

A Study of Equatorial Coronal Holes during the Maximum Phase of Four Solar Cycles ~ Mahendra Lal Karna et al
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Quasars in the Early Universe

Post by bystander » Wed Jan 27, 2021 2:55 am

Quasars in the Early Universe
SAO Science Updates | 2101 Jan 15
Quasars are perhaps the best-known kinds of active galactic nuclei (AGN), galaxies whose central supermassive black holes are luminous, sometimes brighter than the rest of the galaxy. In an AGN, material accretes onto a surrounding torus of gas and dust, heating it to thousands of degrees and prompting the ejection of jets of charged particles. In the case of quasars, our viewing angle is such that these dusty tori do not obscure the light and the bright core of a quasar dominates the galaxy's emission. The most distant known quasar dates from the era only about 700 million years after the big bang, with dozens more known dating from the first few billion years.

One outstanding puzzle is how the supermassive black holes in these young quasars could have formed in the short time available since the universe existed. The very hot material and the fast-moving particles produce X-ray emission, especially from the inner region of the accretion. Although X-ray emission is difficult to detect from such distant objects, CfA astronomers Bradford Snios, Aneta Siemiginowska, Malgosia Sobolewska, Vinay Kashyap, and Dan Schwarz led a team that has obtained X-ray spectra from fifteen quasars that date from roughly a billion years after the big bang and that individually span a period of about one hundred and fifty million years. The astronomers used the Chandra X-ray Observatory to look at targets selected from a catalog of quasars whose characters and distances were already known from their radio emission and optical emission. In particular, the team selected quasars whose radio emission appears (based on its spectral shape) to arise from a small volume within the galaxy.

The astronomers analyzed the X-ray emission from these quasars with other data to infer how these objects and their emission may have evolved in comparison with quasars in the nearby universe. The most significant conclusion from this ongoing work is that there does not appear to be any clear evolutionary trends during this era. They also identified several outlier quasars, one of them named J1606+3124 with an extremely high gas density along the line-of-sight, only the fourth known quasar in the early universe known to have as much dense material.

X-ray Properties of Young Radio Quasars at z > 4.5 ~ Bradford Snios et al
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Rotating Stars in the Pleiades

Post by bystander » Wed Jan 27, 2021 3:05 am

Rotating Stars in the Pleiades
SAO Science Updates | 2101 Jan 22
The Pleiades star cluster, a brilliant collection of several hundred stars visible in the winter sky near the constellation of Orion, has been admired by people for thousands of years; it is cited in the Bible and the works of Greek authors. It is a relatively young star cluster whose estimated age is only about 100 million years, meaning that it was born long after the Jurassic dinosaurs had passed into history. The Pleiades cluster is also relatively close by, only about 448 light-years away, which is why it is such a visual splendor and also why it is such a tempting target for astronomers.

The brightest stars in the Pleiades cluster have been studied spectroscopically for more than a century. The stellar spectrum not only reveals a star’s character, the positions and shapes of its atomic lines reveal the star’s motions, both its line-of-sight motion in space (its radial velocity) and its rotation. One of the most extensive spectroscopic programs in the last century lasted for nearly 20 years focused on the moderate mass stars, leading to the discovery and characterization of many new binary stars in the cluster. But while it is relatively easy to measure the radial velocities of moderate mass stars, more massive ones are more challenging because they have fewer lines and the line profiles are typically very broad because these stars tend to rotate rapidly, some with rotational velocities in excess of 200 km/sec. As a result, the radial velocities of the more massive stars in the Pleiades are generally of poor quality. Nevertheless, over the years more than a dozen of them have been claimed to be possible binary stars, or variable, and in some cases tentative orbits have even been published, but few of these conclusions have been confirmed.

CfA astronomer Guillermo Torres undertook to investigate some of the unconfirmed claims about these more massive stars, ranging from about two to four solar-masses, using new observations gathered over the course of eleven years of observations with the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5 m Tillinghast Reflector at SAO’s Fred L. Whipple Observatory on Mount Hopkins (AZ). The goal of the survey was a more complete census of the suspected spectroscopic binaries in the cluster, a study that would also refine the estimates of the stellar masses. The target list included thirty three rapidly rotating stars, and a further motivation for the work was to derive accurate and consistent rotational velocities for all the fast rotators.

With one or two exceptions Torres does not confirm any of the previous claims of variability, and he rules out all of the published orbital solutions for the binaries. He reports one previous binary actually has a period of 71.8 days, a second binary has a (preliminary) period estimated to be 8.7 years, and finds a new binary with a period of about ten years. The study supports the conventional understanding that slightly more than one-third of these kinds of massive stars are in binary or higher multiple systems.

Spectroscopic Monitoring of Rapidly Rotating Early-type Stars in the Pleiades Cluster ~ Guillermo Torres
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Re: Rotating Stars in the Pleiades

Post by Ann » Wed Jan 27, 2021 8:27 am

Minor nit:
SAO Science Updates wrote:

It is a relatively young star cluster whose estimated age is only about 100 million years, meaning that it was born long after the Jurassic dinosaurs had passed into history.
The dinosaurs died 65 million years ago. So they would have been able to see the Pleiades.

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Re: Rotating Stars in the Pleiades

Post by bystander » Wed Jan 27, 2021 2:34 pm

Ann wrote:
Wed Jan 27, 2021 8:27 am

Minor nit:
SAO Science Updates wrote:
Wed Jan 27, 2021 3:05 am

It is a relatively young star cluster whose estimated age is only about 100 million years, meaning that it was born long after the Jurassic dinosaurs had passed into history.

The dinosaurs died 65 million years ago. So they would have been able to see the Pleiades.

While the K-t event happened about 66 Mya, ending the Cretaceous Period, the Jurassic Period ended about 145 Mya.
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Modeling Galaxy Formation

Post by bystander » Sat Jan 30, 2021 2:21 pm

Modeling Galaxy Formation
SAO Science Updates | 2101 Jan 22
Understanding the formation and evolution of galaxies is difficult because so many different physical processes besides just gravity are involved, including processes associated with star formation and stellar radiation, the cooling of the gas in the interstellar medium, feedback from accreting black holes, magnetic fields, cosmic rays, and more. Astronomers have used computer simulations of galaxy formation to help understand the interplay of these processes and address questions that cannot yet be answered through observations, like how the first galaxies in the universe formed. Simulations of galaxy formation require the self-consistent modelling of all these various mechanisms at once, but a key difficulty is that each of them operates at a different spatial scale making it nearly impossible to properly simulate them all at the same time. Gas inflow from the intergalactic medium into a galaxy, for example, takes place across millions of light-years, the winds of stars have influence over hundreds of light-years, while black hole feedback from its accretion disc occurs at scales of thousandths of a light-year.

CfA astronomers Rahul Kannan and Lars Hernquist, with their colleagues, have developed a novel computational framework that self-consistently includes all these effects. The computations use a new stellar feedback framework called the Stars and Multiphase Gas in Galaxies (SMUGGLE) which integrates processes involving radiation, dust, molecular hydrogen gas (the dominant component of the interstellar medium) and also includes thermal and chemical modeling. The SMUGGLE feedback is incorporated into the popular AREPO hydrodynamic code that simulates the evolution of structures, and which has an added module to include radiation effects.

The astronomers use a simulation of the Milky Way to test their results, and report very good agreement with observations. They find that the feedback effects from radiation on star formation rates are quite modest, at least in a Milky Way example, where stars are forming at a rate of only two-to-three solar-masses per year. On the other hand, they find that the radiation from stars drastically changes the structure and heating of the interstellar medium by influencing the distribution of the hot, warm, and cold material which diverges from the simple expectation. The code does a good job of simulating the dust temperature distribution with warm dust lying (as expected) near the star-forming regions but with the cold dust, perhaps as low as ten kelvin, distributed farther away. The success of these new simulations motivates the authors to extend their work to simulations at even finer spatial resolution.

Simulating the interstellar medium of galaxies with radiative transfer,
non-equilibrium thermochemistry, and dust
~ Rahul Kannan et al
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