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Stellar Astrophysics

You can find a simpler description of stellar astrophysics on the National Schools' Observatory website.

Rapid/Timely Follow-up

The profile of time domain astrophysics will rise immensely in the coming decades, since temporal work will be a key component of many new ground- and space-based facilities. From the ground, huge numbers of optical transients are being discovered by new ‘synoptic surveys’, such as iPTF/ZTF and Pan-STARRS. The key new survey astronomy facility, the Legacy Survey of Space and Time (LSST) at the Vera Rubin Observatory, will offer cadence and depth, probing a new ‘faint and fast’ discovery regime. We anticipate a wide variety of variable sources to be discovered, including supernovae (see supernovae section) and previously unknown sources. For this we require a flexible telescope facility and first and second generation instrumentation designed to study these new and exciting sources.


The Rubin Observatory will join other transient discovery facilities in the Southern Hemisphere such as Public ESO Spectroscopic Survey for Transient Objects (PESSTO), Son of X-Shooter (SoXS) and SkyMapper. Although the NRT will be in the Northern hemisphere there is still a considerable area of sky overlap, with 60% of the main Rubin survey field being observable by NRT with an airmass <2. The high observing efficiency of NRT will afford the best opportunity to efficiently spectroscopically follow-up LSST sources, essential for delivering astrophysical insight.


The Figure above shows the sky coverage for the NRT (yellow) and VRO (pink) along with the sky overlap (orange). This overlap results in ~60% of the main Rubin survey field being observable by NRT with an airmass <2.

Northern Hemisphere facilities such as the ZTF offer sensitivity and field-of-view improvements over their predecessors. There is a critical need for follow-up capacity to provide the spectroscopic classifications and multi-band light curves required for exploitation of the survey telescopes’ discoveries. Dedicated follow-up telescopes are vital as the rate of targets-of-opportunity will be too high to rely on priority overrides on existing facilities (which would interrupt their existing science programmes). The PESSTO programme demonstrated the value of dedicating large amounts of telescope time to transient follow-up. In the new era, the New Technology Telescope (NTT) and the Son of X-Shooter (SoXS) instrument will fill this role in the Southern hemisphere, and NRT will be the ideal Northern counterpart.

The Gaia mission is due to complete before the NRT reaches first light, however, there will still be a large catalogue of local Galactic objects to follow-up. The LT is currently active at conducting follow-up of Gaia sources such as dwarf carbon and binary stars, conducting photometric monitoring and spectroscopic classification. The NRT will provide flexible instrumentation for follow-up, whilst the increased mirror size improves efficiency, allowing  the facility to classify ~10,000 objects per year, an increase of a factor of 5 over the current combined worldwide effort of all observatories (2000/year). 

Gaia will build a near-complete sample of white dwarfs within a distance of 20 pc from the Sun. This census is important to reconstruct the history of the star formation in our Galaxy, the kinematics, binarity and the presence of planetary systems. This sample allows for numerous follow-up programmes dedicated to spectroscopic studies of such candidates, to classify them within the white dwarf taxonomy, to follow-up particular objects looking for transiting planets, to take spectra analysing possible binarity and measuring gravitational redshift. A 4 metre telescope with an intermediate resolution spectrograph, such as the NRT, is ideal to observe the whole range of magnitudes of Gaia white dwarf stars.

One of the Liverpool Telescope's core science cases is that of Novae, this legacy will continue with the New Robotic Telescope. Classical novae (CNe) are characterised by explosive events on the surface of a white dwarf (WD) following accretion of an envelope from an evolved low-mass (or sub-giant) companion main sequence star that has filled its Roche lobe. When the gas pressure in the accreted envelope reaches a critical value, dependent on the WD mass, CNO thermonuclear reactions are ignited. This results in violent ejections of the accreted material at velocities in the range of ~10^2 to 10^4 km/s. 

A number of extreme physical phenomena can be studied in each outburst, such as strong shocks, thermonuclear runaway, nuclear burning, nucleosynthesis, and dust formation. Simultaneous observations in different wavelength regimes explore various optical depths, and thus physical phenomena. Classical nova systems are a viable pathway to supernova type Ia events. Recent advancements in this field show evolutions on timescales much shorter than previously thought. Including evidence for shock breakout within the first few minutes to hours of a nova’s rising journey to maximum. Spectroscopic studies in the optical of the early evolving complex P-Cygni profiles of these events lends important diagnostic information of the underlying physics related to the early shaping, optical depth effects, stellar winds, compact objects and thermonuclear processes of CNe, and related events that do not evolve on human timescales (e.g., planetary nebulae, symbiotic novae), more rare events (e.g., Luminous Blue Variables, Fast Radio Bursts) and gaining the knowledge of the environment of a subset of supernovae of type IIn.

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