Sources Of Radial Velocity Variability
In the past two decades, We have been able to detect Several interesting planetary sys-tems. Apart from the general properties of a spectra and spectral lines, other barrier that weface in exoplanet detection is stellar activity which includes several atmospheric phenomenonand is very difficult to overcome. It’s long known that magnetic fields are produced by thedynamo process in stellar interior. In Sun-like stars, Radiative and convective zone consti-tutes majority of the stellar interior and a thin layer called Tacholine separates them. Themain source of energy is Nuclear fusion and this energy is transferred from radiative zoneto outward regions in the form of radiations. Transition takes place in this region due tostrong radial shearing between uniformly rotating radiative zone and deferentially rotatingconvective zone. This shearing is responsible for magnetic dynamo which gives rise to stellaractivity. Activity involves acoustic oscillations which evolves within minutes, upto magneticcycles that lasts decades. It spans a broad range of spatial and temporal scales and effectsall the atmospheric layers of star.
P-mode oscillations, Flares and Coronal Mass Ejections. Stars are restless bodies and their internal pressure constantly fluctuates by tiny amountsand produces acoustic waves. So its basically the Acoustic waves trapped inside the star thatgives rise to oscillations observed on the stellar surface. These p-mode oscillations have atypical time period of 5-15 minutes and amplitudes per mode of a few tens of cm/s in radialvelocity. The signal that we observe isa superposition of large number of these modes which causes the RV variation upto severalm/s. Flares are local while CMEs are nothing but the magnetic eruption of energy storedin the active regions. These magnetic gas bubble sometimes may take hour to erupt intospace. They occur simultaneously or sometimes alone but are typically found near the activeregions. These events create sharp and sudden increase in the brightness of star. As a result, they create spikes of several ten of m/s in the mean RV variationof the star.
Granulation
These are small cellular features that originates from convective taking place under thesurface. It covers almost entire star except for the regions covered by spots. Hot fluid cells rise3up from interior to surface forming large patches and sinks inward once they have cooled. It’san unending process as old granules are replace by the new ones and their typical timescaleis about 10 minutes for sun. This flow of hot fluid can reach upto supersonic speeds creatingsonic booms or other noise that generates waves on the surface. This leads to the variationin brightness and eventually affects radial velocity measurements upto 2 m/s.
Magnetic Activity: Starspots and Faculae
Spots are basically dark areas on stellar surface in which magneto-convective interaction sig-nificantly suppresses convective energy transport over an area large enough that a structureforms that is cool and dark relative to the surrounding photosphere. Theirtemperature varies from 500k to 2000k and as they have lower temperature compared totheir surroundings, they appear darker. When the star rotates than the flux emitted by theapproaching half is blueshited while the receeding half is redshifted, as a result they cancelout each other if there’s no feature on star’s surface. While the presence of spot on star sur-face creates imbalance in the blueshifted and redshifted halves of the star. This asymmetrycreates a small shifting in the centroid of the line profile. This small shifting translates intoRV vairation of the order of 1m/s for sunspots while for starspots it could be even more higher if they’re more active and rapidly rotatingstar. Faculae are small bright areas on the stellar surface that are usually found near theedge of the disk. Their temperature is around 100k higher than photosphere and they alwayssurrounds spots. During Sunspot cycle, they outlive the sunspot and appear before spotsmaking the sun look brighter at spots. They have a lifetime of few hours but a group of themcan remain for weeks on stellar surface and eventually will last for several cycles. These inhomogenetic features like faculae, spots induce semi periodic RVs signal thatcan mimic planetary signal.
Magnetic Cycles
It’s long known that Sun goes through an 11 year activity cycle and during that period thenumber of active regions on stellar surface dramatically changes. With the commencement ofcycle, more activity is observed as cycles are accompanied by the sunspots, faculae and morevoilent events like CMEs, prominences. But at solar minimum, no spots are seen at all on thesurface. Besides this, there might be some other trends as well because the average surface ofstar covered with spots changes as well. As a result of which spectral lines might show reducedconvective blueshifts, thereby, creating a long-term drift in RV measurements.
