Microturbulence

{{short description|Turbulence that varies over small distance scales}}

Microturbulence is a form of turbulence that varies over small distance scales. (Large-scale turbulence is called macroturbulence.)

Stellar

Microturbulence is one of several mechanisms that can cause broadening of the absorption lines in the stellar spectrum.{{cite journal

| last =De Jager | first =C.

| title=High-energy Microturbulence in the Solar Photosphere

| journal=Nature | date=1954 | volume=173

| issue =4406 | pages=680–1

| doi=10.1038/173680b0 |bibcode = 1954Natur.173..680D | s2cid =4188420

}} Stellar microturbulence varies

with the effective temperature and the surface gravity.{{cite journal

| title=The Effect of the microturbulence parameter on the Color-Magnitude Diagram

| journal=Reports on Progress in Physics | date=1999 | volume=61

| issue=S239

| pages=77–115

| bibcode=2007IAUS..239..166M

| doi=10.1017/S1743921307000361 | doi-access=free }}

The microturbulent velocity is defined as the microscale

non-thermal component of the gas velocity in the region of spectral

line formation.{{cite journal

| last=Cantiello | first=M. et al. (2008)

| title= On the origin of Microturbulence in hot stars

| date=15 February 2024

| url=http://www.astro.uu.nl/~cantiell/articles/liege1.pdf }}

Convection is the mechanism believed to be responsible for the observed turbulent velocity field, both in low mass stars and massive stars.

When examined by a spectroscope, the velocity of the convective gas along the line of sight produces Doppler shifts in the absorption bands. It is the distribution of these velocities along the line of sight that produces the microturbulence broadening of the absorption lines in low mass stars that have convective envelopes. In massive stars convection can be present only in small regions below the surface; these sub-surface convection zones can excite turbulence at the stellar surface through the emission of acoustic and gravity waves.{{cite journal

| last1=Cantiello | first1=M. et al. (2009)

| title= Sub-surface convection zones in hot massive stars and their observable consequences

| bibcode=2009A&A...499..279C

| doi=10.1051/0004-6361/200911643

| date=2009

| last2=Langer

| first2=N.

| last3=Brott

| first3=I.

| last4=De Koter

| first4=A.

| last5=Shore

| first5=S. N.

| last6=Vink

| first6=J. S.

| last7=Voegler

| first7=A.

| last8=Lennon

| first8=D. J.

| last9=Yoon

| first9=S.-C.

| journal=Astronomy and Astrophysics

| volume=499

| issue=1

| pages=279

| arxiv=0903.2049

| s2cid=55396719

}}

The strength of the microturbulence (symbolized by ξ, in units of km s⁻¹) can be determined by comparing the broadening of strong lines versus weak lines.{{cite web | last =Briley | first =Michael | date =July 13, 2006 | url =http://www.phys.uwosh.edu/mike/exercises/lines/lines.html | title =Stellar Properties from Spectral Lines: Introduction | publisher =University of Wisconsin | access-date =2007-05-21 | url-status =dead | archive-url =https://web.archive.org/web/20071123121917/http://www.phys.uwosh.edu/mike/exercises/lines/lines.html | archive-date =November 23, 2007 }}