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Papers by Kate Evans

Research paper thumbnail of The Red Supergiant Content of the Local Group

Proceedings of the International Astronomical Union, 2016

We summarize here recent work in identifying and characterizing red supergiants (RSGs) in the gal... more We summarize here recent work in identifying and characterizing red supergiants (RSGs) in the galaxies of the Local Group.

Research paper thumbnail of The Red Supergiant Content of M31

The Astrophysical Journal, 2016

We investigate the red supergiant (RSG) population of M31, obtaining radial velocities of 255 sta... more We investigate the red supergiant (RSG) population of M31, obtaining radial velocities of 255 stars. These data substantiate membership of our photometrically-selected sample, demonstrating that Galactic foreground stars and extragalactic RSGs can be distinguished on the basis of B-V, V-R two-color diagrams. In addition, we use these spectra to measure effective temperatures and assign spectral types, deriving physical properties for 192 RSGs. Comparison with the solar-metallicity Geneva evolutionary tracks indicates astonishingly good agreement. The most luminous RSGs in M31 are likely evolved from 25-30M ⊙ stars, while the vast majority evolved from stars with initial masses of 20M ⊙ or less. There is an interesting bifurcation in the distribution of RSGs with effective temperatures that increases with higher luminosities, with one sequence consisting of early K-type supergiants, and with the other consisting of M-type supergiants that become later (cooler) with increasing luminosities. This separation is only partially reflected in the evolutionary tracks, although that might be due to the mis-match in metallicities between the solar Geneva models and the higher-than-solar metallicity of M31. As the luminosities increase the median spectral type also increases; i.e., the higher mass RSGs spend more time at cooler temperatures than do those of lower luminosities, a result which is new to this study. Finally we discuss what would be needed observationally to successfully build a luminosity function that could be used to constrain the mass-loss rates of RSGs as our Geneva colleagues have suggested. * Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. This paper uses data products produced by the OIR Telescope Data Center, supported by the Smithsonian Astrophysical Observatory.

Research paper thumbnail of A Runaway Red Supergiant in M31

The Astronomical Journal, 2015

A significant percentage of OB stars are runaways, so we should expect a similar percentage of th... more A significant percentage of OB stars are runaways, so we should expect a similar percentage of their evolved descendants to also be runaways. However, recognizing such stars presents its own set of challenges, as these older, more evolved stars will have drifted further from their birthplace, and thus their velocities might not be obviously peculiar. Several Galactic red supergiants (RSGs) have been described as likely runaways, based upon the existence of bow shocks, including Betelgeuse. Here we announce the discovery of a runaway RSG in M31, based upon a 300 km s −1 discrepancy with M31's kinematics. The star is found about 21 ′ (4.6 kpc) from the plane of the disk, but this separation is consistent with its velocity and likely age (∼10 Myr). The star, J004330.06+405258.4, is an M2 I, with M V = −5.7, log L/L ⊙ =4.76, an effective temperature of 3700 K, and an inferred mass of 12-15M ⊙. The star may be a high-mass analog of the hypervelocity stars, given that its peculiar space velocity is probably 400-450 km s −1 , comparable to the escape speed from M31's disk.

Research paper thumbnail of The Red Supergiant Content of the Local Group

Proceedings of the International Astronomical Union, 2016

We summarize here recent work in identifying and characterizing red supergiants (RSGs) in the gal... more We summarize here recent work in identifying and characterizing red supergiants (RSGs) in the galaxies of the Local Group.

Research paper thumbnail of The Red Supergiant Content of M31

The Astrophysical Journal, 2016

We investigate the red supergiant (RSG) population of M31, obtaining radial velocities of 255 sta... more We investigate the red supergiant (RSG) population of M31, obtaining radial velocities of 255 stars. These data substantiate membership of our photometrically-selected sample, demonstrating that Galactic foreground stars and extragalactic RSGs can be distinguished on the basis of B-V, V-R two-color diagrams. In addition, we use these spectra to measure effective temperatures and assign spectral types, deriving physical properties for 192 RSGs. Comparison with the solar-metallicity Geneva evolutionary tracks indicates astonishingly good agreement. The most luminous RSGs in M31 are likely evolved from 25-30M ⊙ stars, while the vast majority evolved from stars with initial masses of 20M ⊙ or less. There is an interesting bifurcation in the distribution of RSGs with effective temperatures that increases with higher luminosities, with one sequence consisting of early K-type supergiants, and with the other consisting of M-type supergiants that become later (cooler) with increasing luminosities. This separation is only partially reflected in the evolutionary tracks, although that might be due to the mis-match in metallicities between the solar Geneva models and the higher-than-solar metallicity of M31. As the luminosities increase the median spectral type also increases; i.e., the higher mass RSGs spend more time at cooler temperatures than do those of lower luminosities, a result which is new to this study. Finally we discuss what would be needed observationally to successfully build a luminosity function that could be used to constrain the mass-loss rates of RSGs as our Geneva colleagues have suggested. * Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. This paper uses data products produced by the OIR Telescope Data Center, supported by the Smithsonian Astrophysical Observatory.

Research paper thumbnail of A Runaway Red Supergiant in M31

The Astronomical Journal, 2015

A significant percentage of OB stars are runaways, so we should expect a similar percentage of th... more A significant percentage of OB stars are runaways, so we should expect a similar percentage of their evolved descendants to also be runaways. However, recognizing such stars presents its own set of challenges, as these older, more evolved stars will have drifted further from their birthplace, and thus their velocities might not be obviously peculiar. Several Galactic red supergiants (RSGs) have been described as likely runaways, based upon the existence of bow shocks, including Betelgeuse. Here we announce the discovery of a runaway RSG in M31, based upon a 300 km s −1 discrepancy with M31's kinematics. The star is found about 21 ′ (4.6 kpc) from the plane of the disk, but this separation is consistent with its velocity and likely age (∼10 Myr). The star, J004330.06+405258.4, is an M2 I, with M V = −5.7, log L/L ⊙ =4.76, an effective temperature of 3700 K, and an inferred mass of 12-15M ⊙. The star may be a high-mass analog of the hypervelocity stars, given that its peculiar space velocity is probably 400-450 km s −1 , comparable to the escape speed from M31's disk.