Day 1418

Tuesday.

1805.10347
The Betelgeuse Project II: astroseismology
Nance, Sullivan, Diaz, Wheeler

Explore the question of whether the interior state of massive red supergiant supernova progenitors can be effectively probed with astroseismology.  Computed a suite of ten models with ZAMS masses from 15 to 25 m_sun in intervals of 1 m_sun, including the effects of rotation, with the stellar evolutionary code MESA.  Estimate the characteristic frequencies and convective luminosities of convective zones at two illustrative stages, core helium burning and off-center convective carbon burning.  Also estimate the power that might be delivered to the surface to modulate the luminous output considering various efficiencies and dissipation mechanisms.  The inner convective regions should generate waves with characteristic periods of ~20 days in core He burning, ~10 days in He shell burning, and 0.1 to 1 day in shell C burning.  Acoustic waves may avoid both shock and diffusive dissipation relatively early in core He burning through most of the structure.  In shell C burning, years before explosion, the signal generated in the He shell might in some circumstances be weak enough to avoid shock dissipation, but is subject to strong thermal dissipation in the H envelope.  Signals from a convective C-buring shell are very likely to be even more severely damped within the envelope.  In the most optimistic case, early in core He burning, waves arriving close to the surface could represent luminosity fluctuations of a few millimagnitudes, but the conditions in the very outer reaches of the envelope suggest severe thermal damping there.


1805.10630
Predicted microlensing events from analysis of Gaia Data release 2
Bramich

Astrometric microlensing can be used to make precise measurements of the masses of lens stars that are independent of their assumed internal physics.  Such direct mass measurements, obtained purely by observing the gravitational effects of the stars on external objects, are crucial for validating theoretical stellar models.  Specifically, astrometric microlensing allows direct mass measurements of single stars for which so few measurements exist.  To use the astrometric solutions and photometric measurements of ~1.7 billion stars from Gaia DR2 to predict microlensing events during the nominal Gaia mission and beyond.  This will enable astronomers to observe the entirety of each event with appropriate observing resources.  The data will allow precise lens mass measurements for white dwarfs and low-mass main sequence stars helping to constrain stellar evolutionary models.  Search for source-lens pairs in GDR2 that could lead to events between 25/7/3014 and 25/07/2026.  Estimate lens masses using GDR2 photometry and parallaxes, and appropriate model isochrones.  Combined with source and lens parallax measurement from GDR2, this allows the Einstein radius to be computed for each pair.  By considering the paths on the sky, calculate the microlensing signals that are to be expected.  Present a list of 76 predicted microlensing events.  9 and 5 astrometric events will be caused by LAWD37 and Stein2051B.  9 events will exhibit detectable photometric and astrometric signatures.  Of the remaining events, ten will exhibit astrometric signals with amplitudes above 0.5 mas, while the rest are low-amplitude astrometric events with amplitudes between 0.131 and 0.5 mas.  5 and 2 events will reach their peaks during 2018 and 2019.  5 of the photometric events have the potential to eveolve into high-magnitfication events, which may also probe for planetary companions to the lenses.


1805.10944
Impact of filaments on galaxy formation in their residing dark matter haloes
Liao, Gao

Make use of a high-resolution zoom-in hydrodynamical simulation to investigate the impact of filaments on galaxy formation in their residing DM haloes.  A method based on the density field and the Hoshen-Kopelman algorithm is developed to identify filaments.  Show that cold and dense gas preprocessed by DM filaments can be further accreted into residing individual low-mass haloes in directions along the filaments.  Consequently, comparing with field haloes, gas accretion is very anisotropic for filament haloes.  About 30-40 percent of the accreted gas of a residing filament halo was preprocessed by filaments, leading to 2 different thermal histories for the gas in filament haloes.  Filament haloes have higher baryon and stellar fractions when comparing with their field counterparts.  The results suggest that filaments assist gas cooling and enhance star formation in their residing DM haloes.