Monitoring Earendel, the Lensed z 6 Star
Abstract
Three strongly magnified individual stars have been discovered in z~1 galaxies thanks to temporary boosts from microlensing. All were discovered as transient events that have since faded. Now a more persistent and distant lensed star has been discovered at z~6, named "Earendel" ("morning light"). Its extremely high magnification has persisted for 3.5 years between HST observations, consistent with microlensing simulations of this object. But HST monitoring of the z~1 stars revealed things weren't always as they seemed.
Here we propose the first repeat monitoring of Earendel in the same WFC3/IR filter, with 4 more epochs F110W imaging. These data will yield Earendel's baseline flux to better constrain its magnification, and thus luminosity and mass. Current HST data suggest it is likely very massive > 50 M_sun. And at z~6, it is likely low metallicity < 0.2 Z_sun. Very massive low metallicity stars are extremely rare in the local group and thus hard to study locally. Such stars are the likely progenitors of ~30 M_sun black holes measured by LIGO gravitational wave detections. But models of their evolutionary tracks are highly uncertain. Remarkably, this isolated z~6 star may offer the best opportunity to pinpoint the giant branch of a low metallicity massive star. Approved JWST GO observations will constrain the star's temperature, putting a z~6 star on the H-R diagram with a luminosity (and mass) precision significantly improved by this HST proposal. To support the various communities interested in these results, we waive any proprietary period to the data, in the same spirit as the previous HST observations and approved JWST observations of this exciting target.- Publication:
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HST Proposal
- Pub Date:
- June 2021
- Bibcode:
- 2021hst..prop16668C