12:00-12:30pm

Nicholas Erickson (University of Colorado Boulder)

EUV/FUV Sounding Rocket Observations of Epsilon CMa to Better Constrain the B Star Contribution to Ionization in the Universe

Abstract: Hot star models are foundational for understanding the ionizing outputs of star forming galaxies and the contribution of stars to ionization in the Universe across cosmic time. When tested against 505-730A EUVE observations of B star Epsilon CMa, hot star models appear to underpredict hot star EUV flux by a factor of 10-20×, calling into question their accuracy in predicting the strength of the B star contribution to ionizing flux in the Universe. I have obtained the first-ever observation of a hot, ionizing star (Epsilon CMa) in the critical 730-900A bandpass in which neutral hydrogen is most sensitive to ionization. By comparing this new, highly-ionizing spectrum to stellar models generated using the range of published stellar parameters of Epsilon CMa, I test if and to what extent these models fail to correctly predict the most ionizing fluxes of massive hot stars, and whether the role of B stars as sources of large-scale ionization in the Universe needs to subsequently be revised. I constructed, calibrated, and launched the DEUCE sounding rocket to obtain this first-ever spectrum of Epsilon CMa near the Lyman Limit and allow the predictive power of hot star models to be more comprehensively appraised in the EUV. In this talk I discuss the motivations for this observation, the construction and calibration of the the DEUCE instrument,  the DEUCE launches and their results, and the accuracy of hot star models when compared to these new data in the EUV.

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12:30-1:00pm

Wenbin Liu (CalTech)

On the formation of GW190814

Abstract: The LIGO-Virgo collaboration recently reported a puzzling event, GW190814, with component masses of 23 and 2.6 Msun. Motivated by the relatively small rate of such a coalescence and the fact that the mass of the secondary is close to the total mass of known binary neutron star (BNS) systems, we propose that GW190814 was a second-generation merger from a hierarchical triple system, i.e., the remnant from the BNS coalescence was able to merge again with the 23 Msun black hole tertiary. We show that this occurs with a sufficiently high probability provided that the semimajor axis of the outer orbit is less than a few AU at the time of BNS coalescence. Such tight triple systems may be commonly realized in low-metallicity (less than 0.1 solar) environments. Our model provides many testable predictions, the most important ones being: (1) The secondary in GW190814-like systems have spins of about 0.7; (2) The component mass distribution from a large LIGO-Virgo sample should have a narrow peak around 2.6 Msun (provided that stellar core collapse does not generate black holes in the “lower mass gap” between 3 and 5 Msun).