Astronomy & Celestial Mechanics
the sky, computed
Concept

Lyman-alpha — where it appears

The strongest spectral line of hydrogen, at 121.6 nanometres, from the transition between its first excited level and the ground. Stellar chromospheres emit it strongly, and very small columns of neutral gas absorb it. Its core is usually lost, so most measurements rest on its flanks.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

A loss rate that stops following the starlight. The rate at which a hot Jupiter of 1.38 Jupiter radii and 0.69 Jupiter masses loses its atmosphere, in grams per second, against the ionising flux it receives, in erg per square centimetre per second, both on logarithmic axes. The dashed line is the energy-limited rate with a heating efficiency of 0.3: linear in the flux, 1.04·10¹⁰ g s⁻¹ at 10³. The solid curve follows it at low flux and turns over near 10⁴, where the base of the wind becomes nearly fully ionised and each recombination radiates its energy as Lyman-α instead of heating the flow; above that the rate grows only as the square root of the flux. At 10⁶ erg cm⁻² s⁻¹, the flux such a planet receives from a young, active star, the energy-limited formula overestimates the loss by a factor of 11. A single efficiency cannot describe both ends of the curve: fitted at one flux, it is wrong at the other.

An escape that stops following the starlight

The standard estimate of how fast a planet loses its atmosphere assumes the loss grows in proportion to the ultraviolet light it absorbs. It does, up to a point. Beyond about ten thousand erg per square centimetre per second the escaping gas is fully ionised, its heat leaves as Lyman-α light instead of lifting anything, and the loss grows only as the square root of the flux — so the one efficiency every calculation needs is really a function of where the planet sits.

exoplanets · Atmospheric escape A second absorber where one should be enough. A star's Lyman-α emission line in velocity, its light after interstellar hydrogen of 1 × 10¹⁸ atoms per square centimetre at 11 km/s width, and after an added hot low-column absorber of 3 × 10¹⁴ atoms at 70 km/s width and −45 km/s. The interstellar core is black. The added component removes up to 40 per cent of the unabsorbed peak on one flank, an edge the interstellar line alone cannot make.

A wind measured by the wall it piles up

The wind of a star like the Sun is far too thin to see. Its mass-loss rate is about 2 × 10⁻¹⁴ of a solar mass a year, too faint to produce any spectral feature of its own, though it controls how fast the star spins down and what happens to the atmospheres of its planets. The only measurements come from somewhere else: the place, far out, where that wind runs into interstellar gas and stacks hot hydrogen into a wall, which leaves a faint notch in the star's own Lyman-α light.

starlight · Stellar winds

Named alongside it

The objects these essays reach for when they reach for this one.

AstrosphereAtmospheric escapeCharge exchangeEnergy-limited escapeHeating efficiencyHot jupiterMass lossParker windPhoton-limited escapeRecombination-limited escapeStellar windSub-neptune

All concepts