Photoevaporation

File:Eagle nebula pillars.jpg pillars being photoevaporated]]

One of the most obvious manifestations of astrophysical photoevaporation is seen in the eroding structures of molecular clouds that luminous stars are born within.{{cite journal|bibcode=1996AJ....111.2349H|title=Hubble Space Telescope WFPC2 Imaging of M16: Photoevaporation and Emerging Young Stellar Objects|journal=Astronomical Journal |volume=111|pages=2349|last1=Hester|first1=J. J.|last2=Scowen|first2=P. A.|last3=Sankrit|first3=R.|last4=Lauer|first4=T. R.|last5=Ajhar|first5=E. A.|last6=Baum|first6=W. A.|last7=Code|first7=A.|last8=Currie|first8=D. G.|last9=Danielson|first9=G. E.|last10=Ewald|first10=S. P.|last11=Faber|first11=S. M.|last12=Grillmair|first12=C. J.|last13=Groth|first13=E. J.|last14=Holtzman|first14=J. A.|last15=Hunter|first15=D. A.|last16=Kristian|first16=J.|last17=Light|first17=R. M.|last18=Lynds|first18=C. R.|last19=Monet|first19=D. G.|last20=O'Neil|first20=E. J.|last21=Shaya|first21=E. J.|last22=Seidelmann|first22=P. K.|last23=Westphal|first23=J. A.|year=1996|doi=10.1086/117968|url=https://authors.library.caltech.edu/53670/1/1996AJ____111_2349H.pdf}}

{{main|Evaporating gaseous globule}}Evaporating gaseous globules or EGGs were first discovered in the Eagle Nebula. These small cometary globules are being photoevaporated by the stars in the nearby cluster. EGGs are places of ongoing star-formation.{{Cite journal|last1=Hester|first1=J. J.|last2=Scowen|first2=P. A.|last3=Sankrit|first3=R.|last4=Lauer|first4=T. R.|last5=Ajhar|first5=E. A.|last6=Baum|first6=W. A.|last7=Code|first7=A.|last8=Currie|first8=D. G.|last9=Danielson|first9=G. E.|last10=Ewald|first10=S. P.|last11=Faber|first11=S. M.|date=June 1996|title=Hubble Space Telescope WFPC2 Imaging of M16: Photoevaporation and Emerging Young Stellar Objects|journal= The Astronomical Journal|language=en|volume=111|pages=2349|doi=10.1086/117968|issn=0004-6256|bibcode=1996AJ....111.2349H|url=https://authors.library.caltech.edu/53670/1/1996AJ____111_2349H.pdf}}

A planet can be stripped of its atmosphere (or parts of the atmosphere) due to high energy photons and other electromagnetic radiation. If a photon interacts with an atmospheric molecule, the molecule is accelerated and its temperature increased. If sufficient energy is provided, the molecule or atom may reach the escape velocity of the planet and "evaporate" into space. The lower the mass number of the gas, the higher the velocity obtained by interaction with a photon. Thus hydrogen is the gas which is most prone to photoevaporation.

Photoevaporation is the likely cause of the small planet radius gap.{{cite journal | last1=Owen | first1=James E. | last2=Wu | first2=Yanqin|author2-link= Yanqin Wu | title=The Evaporation Valley in the Kepler Planets | journal=The Astrophysical Journal | publisher=American Astronomical Society | volume=847 | issue=1 | date=2017-09-20 | issn=1538-4357 | doi=10.3847/1538-4357/aa890a | page=29| arxiv=1705.10810 | doi-access=free | bibcode=2017ApJ...847...29O }}

Examples of exoplanets with an evaporating atmosphere are HD 209458 b, HD 189733 b and Gliese 3470 b. Material from a possible evaporating planet around WD J0914+1914 might be responsible for the gaseous disk around this white dwarf.

Image:Sig06-023.jpg

Protoplanetary disks can be dispersed by stellar wind and heating due to incident electromagnetic radiation. The radiation interacts with matter and thus accelerates it outwards. This effect is only noticeable when there is sufficient radiation strength, such as coming from nearby O and B type stars or when the central protostar commences nuclear fusion.

The disk is composed of gas and dust. The gas, consisting mostly of light elements such as hydrogen and helium, is mainly affected by the effect, causing the ratio between dust and gas to increase.

Radiation from the central star excites particles in the accretion disk. The irradiation of the disk gives rise to a stability length scale known as the gravitational radius ($r\_g$). Outside of the gravitational radius, particles can become sufficiently excited to escape

the gravity of the disk, and evaporate. After 10⁶ – 10⁷ years,

the viscous accretion rates fall below the photoevaporation rates at $r\_g$.

A gap then opens around $r\_g$, the inner disk drains onto the central star,

or spreads to $r\_g$ and evaporates. An inner hole extending to $r\_g$

is produced. Once an inner hole forms, the outer disk is very rapidly cleared.

The formula for the gravitational radius of the disk is{{cite journal|bibcode=2003PASA...20..337L|title=The Gravitational Radius of an Irradiated Disk|journal=Publications of the Astronomical Society of Australia|volume=20|issue=4|pages=337–339|last1=Liffman|first1=Kurt|year=2003|doi=10.1071/AS03019|doi-access=free}}

:$r\_g\; =\; \backslash frac\{\backslash left(\backslash gamma\; -\; 1\backslash right)\}\{2\backslash gamma\}\backslash frac\{GM\backslash mu\}\{k\_B\; T\}$

\approx 2.15 \frac{\left(M/M_\odot\right)}{\left(T/10^4 \ {\rm K} \right)} \ {\rm AU},\!

where $\backslash gamma$ is the ratio of specific heats (= 5/3 for a monatomic gas), $G$ the universal gravitational constant, $M$ the mass of the central star, $M\_\backslash odot$ the mass of the Sun,

$\backslash mu$ the mean weight of the gas, $k\_B$ Boltzmann constant,

$T$ is the temperature of the gas and AU the Astronomical Unit.

If we denote the coefficient in the above equation by the Greek letter $\backslash kappa$ then

         $\backslash kappa\; =\; \backslash frac\{(\backslash gamma\; -\; 1)\}\{2\; \backslash gamma\}\; =\; \backslash frac\{1\}\{(2+f)\}$  ,                                           .                                                                    

where $f$ is the number of degrees of freedom and we have used the formula: $\backslash gamma\; =\; 1\; +\; \backslash frac\{2\}\{f\}$.

For an atom, such as a hydrogen atom, then $f\; =\; 3$, because an atom can move in three different, orthogonal directions. Consequently, $\backslash kappa\; =\; 0.2$. If the hydrogen atom is ionized, i.e., it is a proton, and is in a strong magnetic field then $f\; =\; 2$, because the proton can move along the magnetic field and rotate around the field lines. In this case, $\backslash kappa\; =\; 0.25$. A diatomic molecule, e.g., a hydrogen molecule, has $f\; =\; 5$ and $\backslash kappa\; =\; 1/7\; \backslash approx\; 0.143$. For a non-linear triatomic molecule, such as water, $f\; =\; 6$ and $\backslash kappa\; =\; 0.125$. If $f$ becomes very large, then $\backslash kappa$ approaches zero. This is summarised in the Table 1 , where we see that different gases may have different gravitational radii.

Table 1: Gravitational radius coefficient as a function of the degrees of freedom.

Because of this effect, the presence of massive stars in a star-forming region is thought to have a great effect on planet formation from the disk around a young stellar object, though it is not yet clear if this effect decelerates or accelerates it.

The most famous region containing photoevaporated protoplanetary disks is the Orion Nebula. They were called bright proplyds and since then the term was used for other regions to describe photoevaporation of protoplanetary disks. They were discovered with the Hubble Space Telescope.{{Cite journal|last1=O'dell|first1=C. R.|last2=Wen|first2=Zheng|last3=Hu|first3=Xihai|date=June 1993|title=Discovery of New Objects in the Orion Nebula on HST Images: Shocks, Compact Sources, and Protoplanetary Disks|journal= The Astrophysical Journal|language=en|volume=410|pages=696|doi=10.1086/172786|issn=0004-637X|bibcode=1993ApJ...410..696O|doi-access=free}} There might even be a planetary-mass object in the Orion Nebula that is being photoevaporated by θ ¹ Ori C.{{Cite journal|last1=Fang|first1=Min|last2=Kim|first2=Jinyoung Serena|last3=Pascucci|first3=Ilaria|author3-link=Ilaria Pascucci|last4=Apai|first4=Dániel|last5=Manara|first5=Carlo Felice|date=2016-12-12|journal=The Astrophysical Journal|language=en|volume=833|issue=2|pages=L16|doi=10.3847/2041-8213/833/2/l16|issn=2041-8213|title=A Candidate Planetary-Mass Object with a Photoevaporating Disk in Orion|bibcode=2016ApJ...833L..16F|arxiv=1611.09761|s2cid=119511524 |doi-access=free }} Since then HST did observe other young star clusters and found bright proplyds in the Lagoon Nebula,{{Cite journal|last1=Stecklum|first1=B.|last2=Henning|first2=T.|last3=Feldt|first3=M.|last4=Hayward|first4=T. L.|last5=Hoare|first5=M. G.|last6=Hofner|first6=P.|last7=Richter|first7=S.|date=February 1998|title=The Ultracompact H II Region G5.97−1.17: An Evaporating Circumstellar Disk in M8|journal=The Astronomical Journal|language=en|volume=115|issue=2|pages=767|doi=10.1086/300204|issn=1538-3881|bibcode=1998AJ....115..767S|doi-access=free}} the Trifid Nebula,{{Cite journal|last1=Yusef-Zadeh|first1=F.|last2=Biretta|first2=J.|last3=Geballe|first3=T. R.|date=September 2005|title=Hubble Space Telescopeand United Kingdom Infrared Telescope Observations of the Center of the Trifid Nebula: Evidence for the Photoevaporation of a Proplyd and a Protostellar Condensation|journal=The Astronomical Journal|language=en|volume=130|issue=3|pages=1171–1176|doi=10.1086/432095|issn=0004-6256|bibcode=2005AJ....130.1171Y|arxiv=astro-ph/0505155|s2cid=324270}} Pismis 24{{Cite journal|last1=Fang|first1=M.|last2=Boekel|first2=R. van|last3=King|first3=R. R.|last4=Henning|first4=Th|last5=Bouwman|first5=J.|last6=Doi|first6=Y.|last7=Okamoto|first7=Y. K.|last8=Roccatagliata|first8=V.|last9=Sicilia-Aguilar|first9=A.|date=2012-03-01|title=Star formation and disk properties in Pismis 24|journal=Astronomy & Astrophysics|language=en|volume=539|pages=A119|doi=10.1051/0004-6361/201015914|issn=0004-6361|bibcode=2012A&A...539A.119F|arxiv=1201.0833|s2cid=73612793}} and NGC 1977.{{Cite journal|last1=Kim|first1=Jinyoung Serena|last2=Clarke|first2=Cathie J.|last3=Fang|first3=Min|last4=Facchini|first4=Stefano|date=2016-07-20|journal=The Astrophysical Journal|language=en|volume=826|issue=1|pages=L15|doi=10.3847/2041-8205/826/1/l15|issn=2041-8213|title=Proplyds Around a B1 Star: 42 Orionis in NGC 1977|bibcode=2016ApJ...826L..15K|arxiv=1606.08271|s2cid=118562469 |doi-access=free }} After the launch of the Spitzer Space Telescope additional observations revealed dusty cometary tails around young cluster members in NGC 2244, IC 1396 and NGC 2264. These dusty tails are also explained by photoevaporation of the proto-planetary disk.{{Cite journal|last1=Balog|first1=Zoltan|last2=Rieke|first2=G. H.|last3=Su|first3=Kate Y. L.|last4=Muzerolle|first4=James|last5=Young|first5=Erick T.|date=2006-09-25|title=SpitzerMIPS 24 μm Detection of Photoevaporating Protoplanetary Disks|journal=The Astrophysical Journal|language=en|volume=650|issue=1|pages=L83–L86|doi=10.1086/508707|issn=0004-637X|bibcode=2006ApJ...650L..83B|arxiv=astro-ph/0608630|s2cid=18397282}} Later similar cometary tails were found with Spitzer in W5. This study concluded that the tails have a likely lifetime of 5 Myrs or less.{{Cite journal|last1=Koenig|first1=X. P.|last2=Allen|first2=L. E.|author2-link=Lori Allen (astronomer)|last3=Kenyon|first3=S. J.|last4=Su|first4=K. Y. L.|last5=Balog|first5=Z.|date=2008-10-03|title=Dusty Cometary Globules in W5|journal=The Astrophysical Journal|language=en|volume=687|issue=1|pages=L37–L40|doi=10.1086/593058|issn=0004-637X|bibcode=2008ApJ...687L..37K|arxiv=0809.1993|s2cid=14049581}} Additional tails were found with Spitzer in NGC 1977, NGC 6193{{Cite web|url=http://casa.colorado.edu/~skinners/grenoble_abst.pdf|title=SPITZER OBSERVATIONS OF THE YOUNG STELLAR CLUSTER NGC6193 IN THE ARA OB1 ASSOCIATION.|last1=Skinner|first1=StephenL.|last2=Kimberly|first2=R.Sokal|date=|website=Stephen L. Skinner: CASA, U. of Colorado|archive-url=|archive-date=|access-date=2019-12-12|last3=Damineli|first3=Augusto|last4=Palla|first4=Francesco|last5=Zhekov|first5=Svet}} and Collinder 69.{{Cite journal|last1=Thévenot|first1=Melina|last2=Doll|first2=Katharina|last3=Durantini Luca|first3=Hugo A.|date=2019-07-15|title=Photoevaporation of Two Proplyds in the Star Cluster Collinder 69 Discovered with Spitzer MIPS|journal=Research Notes of the AAS|language=en|volume=3|issue=7|pages=95|doi=10.3847/2515-5172/ab30c5|issn=2515-5172|bibcode=2019RNAAS...3...95T |doi-access=free }} Other bright proplyd candidates were found in the Carina Nebula with the CTIO 4m and near Sagittarius A* with the VLA.{{Cite journal|last1=Smith|first1=Nathan|last2=Bally|first2=John|last3=Morse|first3=Jon A.|date=2003-03-24|title=Numerous Proplyd Candidates in the Harsh Environment of the Carina Nebula|journal=The Astrophysical Journal|language=en|volume=587|issue=2|pages=L105–L108|doi=10.1086/375312|issn=0004-637X|bibcode=2003ApJ...587L.105S|doi-access=free}}{{Cite journal|last1=Yusef-Zadeh|first1=F.|last2=Roberts|first2=D. A.|last3=Wardle|first3=M.|last4=Cotton|first4=W.|last5=Schödel|first5=R.|last6=Royster|first6=M. J.|date=2015-03-11|journal=The Astrophysical Journal|language=en|volume=801|issue=2|pages=L26|doi=10.1088/2041-8205/801/2/l26|issn=2041-8213|title=Radio Continuum Observations of the Galactic Center: Photoevaporative Proplyd-Like Objects Near SGR A|bibcode=2015ApJ...801L..26Y|arxiv=1502.03109|s2cid=119112454}} Follow-up observations of a proplyd candidate in the Carina Nebula with Hubble revealed that it is likely an evaporating gaseous globule.{{Cite journal|last1=Sahai|first1=R.|last2=Güsten|first2=R.|last3=Morris|first3=M. R.|date=2012-11-30|journal=The Astrophysical Journal|language=en|volume=761|issue=2|pages=L21|doi=10.1088/2041-8205/761/2/l21|issn=2041-8205|title=Are Large, Cometary-Shaped Proplyds Really (Free-Floating) Evaporating Gas Globules?|bibcode=2012ApJ...761L..21S|arxiv=1211.0345|s2cid=118387694}}

Objects in NGC 3603 and later in Cygnus OB2 were proposed as intermediate massive versions of the bright proplyds found in the Orion Nebula.{{Cite journal|last1=Brandner|first1=Wolfgang|last2=Grebel|first2=Eva K.|last3=Chu|first3=You-Hua|last4=Dottori|first4=Horacio|last5=Brandl|first5=Bernhard|last6=Richling|first6=Sabine|last7=Yorke|first7=Harold W.|last8=Points|first8=Sean D.|last9=Zinnecker|first9=Hans|date=January 2000|title=HST/WFPC2 and VLT/ISAAC Observations of Proplyds in the Giant H II Region NGC 3603|journal=The Astronomical Journal|language=en|volume=119|issue=1|pages=292–301|doi=10.1086/301192| arxiv=astro-ph/9910074|bibcode=2000AJ....119..292B |s2cid=15502401|issn=0004-6256}}{{Cite journal|last1=Wright|first1=Nicholas J.|last2=Drake|first2=Jeremy J.|last3=Drew|first3=Janet E.|last4=Guarcello|first4=Mario G.|last5=Gutermuth|first5=Robert A.|last6=Hora|first6=Joseph L.|last7=Kraemer|first7=Kathleen E.|date=2012-02-01|journal=The Astrophysical Journal|language=en|volume=746|issue=2|pages=L21|doi=10.1088/2041-8205/746/2/l21|issn=2041-8205|title=Photoevaporating Proplyd-Like Objects in Cygnus Ob2|bibcode=2012ApJ...746L..21W|arxiv=1201.2404|s2cid=16509383}}

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Category:Concepts in stellar astronomy