This award will support an observational study of neutral hydrogen (HI) in the envelopes of a type of highly-evolved low- to intermediate-mass star known as an asymptotic giant branch (AGB) star. Such stars have very large outer envelopes and are losing mass into the interstellar medium; they also are copious producers of dust. The mechanism and rates of mass loss in such stars are poorly known, and this study will measure radio emission and absorption in the circumstellar hydrogen to probe the conditions and kinematics in the gas. The project aims for an improved knowledge of the total mass and extent of circumstellar envelopes, of the duration and geometry of mass loss, and of the formation of planetary nebulae.
The project is expected to provide improved understanding of the physical conditions in the outer layers of evolved stars. It will also provide research opportunities for undergraduates, and the results from this study will be incorporated into public lectures and podcasts.
The 21-cm radiation emitted by neutral hydrogen ("HI") atoms has long been used as a tracer of the interstellar medium---the tenuous gas that fills the space between the stars throughout our Galaxy. In this project, observations of HI 21-cm radio emission were used to obtain novel insights into the workings of stars themselves as well as their interactions with their interstellar surroundings. Part 1 of this project involved the study of HI emission from the circumstellar envelopes of red giants. Red giants are old stars that have begun to deplete their supply of nuclear fuel. At this stage, the stellar core begins to contract while the star's outer layers expand and cool, until the star swells to as much as 100 times its original size. Red giants also undergo significant mass loss through stellar winds, leading to the formation of extensive circumstellar envelopes of gas and dust. Red giants are important players in the Galactic "ecosystem" since the mass they shed is eventually recycled to form new generations of stars and planets. Past observations have provided a highly incomplete picture of red giant mass loss. These observations were typically sensitive only to molecular gas very close to the star or else targeted far-infrared emission from dust which cannot supply information on the motions of the gas that comprises the bulk of the circumstellar matter. In contrast, HI observations provide information on gas motions and permit tracing circumstellar material to large distances from the star, thereby providing a more complete picture of the stellar mass-loss history. This program used HI 21-cm observations obtained with the Very Large Array (VLA) radio telescope and the Green Bank Telescope to study a sample nearby red giants. The circumstellar HI envelopes of the detected stars exhibit a diverse range of morphologies. Some stars are surrounded by gas shells that are formed as the outflowing stellar wind is "shocked" and abruptly slowed by its interaction with the surrounding interstellar medium. It was discovered that the famous red supergiant Betelgeuse is surrounded by such a shell. Other stars in the sample were seen to have cometary-like shapes, with long gaseous tails trailing behind them. Most of the stars exhibiting HI tails are moving through space at large velocities (>50 km/s), causing outflowing gas in their winds to be stripped and swept behind the star, analogous to a wake trailing a ship. These "tails" are typically more than a light year long and provide an archaeological record of more than 100,000 years of stellar mass-loss history. This study was among the first to reveal dramatic signatures of red giants interacting with their interstellar surroundings. Results have been published in the scientific literature and are documented in a podcast titled "Trailing Red Giants", part of a new Radio Stars podcast series that was developed as part of this grant in collaboration with the Science Media Group at the Harvard-Smithsonian Center for Astrophysics. Part 2 of this program extended the use of HI 21-cm observations to the study of Cepheid variable stars. Cepheids undergo steady pulsations, causing their brightness to regularly vary. Because the period of these brightness fluctuations is tightly linked with the stellar luminosity, measurements of a Cepheid's period and apparent brightness provide a direct way to gauge its distance. For this reason, Cepheids have been used as a yardstick for measuring the scale of the universe for more than a century. However, for years a "Cepheid mass discrepancy" has confounded astronomers: the masses of Cepheids derived from their mass-dependent pulsation period do not agree with masses derived from models of stellar evolution. It has long been suspected that a key part of the answer to this puzzle is that Cepheids are losing mass through stellar winds. However, numerous past attempts to detect evidence of mass loss from Cepheids produced only inconclusive results. The current study addressed this problem by attempting to trace the mass loss through 21-cm radio emission for the very first time. Recently, infrared observations with the Spitzer Space Telescope revealed evidence for nebulosity surrounding several Cepheids, including the archetype δ Cephei. Follow-up HI 21-cm observations of this star using the VLA revealed that δ Cephei is surrounded by an enormous nebula of gas more than 3 light years across. The head-tail structure of this nebula implies that it comprises a wake of debris shed by a wind from the star. Together the Spitzer and VLA data provide some of the most compelling evidence to date that at least some Cepheids are losing mass. This work underscored how looking at familiar objects in a new light (in this case, at infrared and radio wavelengths) can reveal a whole new picture. These results were documented in the scientific literature and in a podcast aimed at general audiences entitled "Secrets of Cepheids" (part of the Radio Stars series).
