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Extinction (astronomy)

January 13, 2023

For other uses, see Extinction (disambiguation).

In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler.[1][2] However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve,[3] and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system) is roughly 1.8 magnitudes per kiloparsec.[4]

For Earth-bound observers, extinction arises both from the interstellar medium (ISM) and the Earth's atmosphere; it may also arise from circumstellar dust around an observed object. Strong extinction in earth's atmosphere of some wavelength regions (such as X-ray, ultraviolet, and infrared) is overcome by the use of space-based observatories. Since blue light is much more strongly attenuated than red light, extinction causes objects to appear redder than expected, a phenomenon referred to as interstellar reddening.[5]

Interstellar reddening

In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which the object originally emitted. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium. Interstellar reddening is a different phenomenon from redshift, which is the proportional frequency shifts of spectra without distortion. Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons (in the optical, light that is redder), leaving the spectroscopic lines unchanged.

In most photometric systems filters (passbands) are used from which readings of magnitude of light may take account of latitude and humidity among terrestrial factors. Interstellar reddening equates to the "color excess", defined as the difference between an object's observed color index and its intrinsic color index (sometimes referred to as its normal color index). The latter is the theoretical value which it would have if unaffected by extinction. In the first system, the UBV photometric system devised in the 1950s and its most closely related successors, the object's color excess   is related to the object's B−V color (calibrated blue minus calibrated visible) by:

 

For an A0-type main sequence star (these have median wavelength and heat among the main sequence) the color indices are calibrated at 0 based on an intrinsic reading of such a star (± exactly 0.02 depending on which spectral point, i.e. precise passband within the abbreviated color name is in question, see color index). At least two and up to five measured passbands in magnitude are then compared by subtraction: U,B,V,I or R during which the color excess from extinction is calculated and deducted. The name of the four sub-indices (R minus I etc.) and order of the subtraction of recalibrated magnitudes is from right to immediate left within this sequence.

General characteristics

Interstellar reddening occurs because interstellar dust absorbs and scatters blue light waves more than red light waves, making stars appear redder than they are. This is similar to the effect seen when dust particles in the atmosphere of Earth contribute to red sunsets (see: Sunset#Colors).[6]

Broadly speaking, interstellar extinction is strongest at short wavelengths, generally observed by using techniques from spectroscopy. Extinction results in a change in the shape of an observed spectrum. Superimposed on this general shape are absorption features (wavelength bands where the intensity is lowered) that have a variety of origins and can give clues as to the chemical composition of the interstellar material, e.g. dust grains. Known absorption features include the 2175 Å bump, the diffuse interstellar bands, the 3.1 μm water ice feature, and the 10 and 18 μm silicate features.

In the solar neighborhood, the rate of interstellar extinction in the Johnson–Cousins V-band (visual filter) averaged at a wavelength of 540 nm is usually taken to be 0.7–1.0 mag/kpc−simply an average due to the clumpiness of interstellar dust.[7][8][9] In general, however, this means that a star will have its brightness reduced by about a factor of 2 in the V-band viewed from a good night sky vantage point on earth for every kiloparsec (3,260 light years) it is farther away from us.

The amount of extinction can be significantly higher than this in specific directions. For example, some regions of the Galactic Center are awash with obvious intervening dark dust from our spiral arm (and perhaps others) and themselves in a bulge of dense matter, causing as much as more than 30 magnitudes of extinction in the optical, meaning that less than 1 optical photon in 1012 passes through.[10] This results in the so-called zone of avoidance, where our view of the extra-galactic sky is severely hampered, and background galaxies, such as Dwingeloo 1, were only discovered recently through observations in radio and infrared.

The general shape of the ultraviolet through near-infrared (0.125 to 3.5 μm) extinction curve (plotting extinction in magnitude against wavelength, often inverted) looking from our vantage point at other objects in the Milky Way, is fairly well characterized by the stand-alone parameter of relative visibility (of such visible light) R(V) (which is different along different lines of sight),[11][12] but there are known deviations from this characterization.[13] Extending the extinction law into the mid-infrared wavelength range is difficult due to the lack of suitable targets and various contributions by absorption features.[14]

R(V) compares aggregate and particular extinctions. It is A(V)/E(B−V). Restated, it is the total extinction, A(V) divided by the selective total extinction (A(B)−A(V)) of those two wavelengths (bands). A(B) and A(V) are the total extinction at the B and V filter bands. Another measure used in the literature is the absolute extinction A(λ)/A(V) at wavelength λ, comparing the total extinction at that wavelength to that at the V band.

R(V) is known to be correlated with the average size of the dust grains causing the extinction. For our own galaxy, the Milky Way, the typical value for R(V) is 3.1,[15] but is found to vary considerably across different lines of sight.[16] As a result, when computing cosmic distances it can be advantageous to move to star data from the near-infrared (of which the filter or passband Ks is quite standard) where the variations and amount of extinction are significantly less, and similar ratios as to R(Ks):[17] 0.49±0.02 and 0.528±0.015 were found respectively by independent groups.[16][18] Those two more modern findings differ substantially relative to the commonly referenced historical value ≈0.7.[11]

The relationship between the total extinction, A(V) (measured in magnitudes), and the column density of neutral hydrogen atoms column, NH (usually measured in cm−2), shows how the gas and dust in the interstellar medium are related. From studies using ultraviolet spectroscopy of reddened stars and X-ray scattering halos in the Milky Way, Predehl and Schmitt[19] found the relationship between NH and A(V) to be approximately:

 

(see also:[20][21][22]).

Astronomers have determined the three-dimensional distribution of extinction in the "solar circle" (our region of our galaxy), using visible and near-infrared stellar observations and a model of distribution of stars.[23][24] The dust causing extinction mainly lies along the spiral arms, as observed in other spiral galaxies.

Measuring extinction towards an object

To measure the extinction curve for a star, the star's spectrum is compared to the observed spectrum of a similar star known not to be affected by extinction (unreddened).[25] It is also possible to use a theoretical spectrum instead of the observed spectrum for the comparison, but this is less common. In the case of emission nebulae, it is common to look at the ratio of two emission lines which should not be affected by the temperature and density in the nebula. For example, the ratio of hydrogen alpha to hydrogen beta emission is always around 2.85 under a wide range of conditions prevailing in nebulae. A ratio other than 2.85 must therefore be due to extinction, and the amount of extinction can thus be calculated.

The 2175-angstrom feature

One prominent feature in measured extinction curves of many objects within the Milky Way is a broad 'bump' at about 2175 Å, well into the ultraviolet region of the electromagnetic spectrum. This feature was first observed in the 1960s,[26][27] but its origin is still not well understood. Several models have been presented to account for this bump which include graphitic grains with a mixture of PAH molecules. Investigations of interstellar grains embedded in interplanetary dust particles (IDP) observed this feature and identified the carrier with organic carbon and amorphous silicates present in the grains.[28]

Extinction curves of other galaxies

 
Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar.[29] The curves are plotted versus 1/wavelength to emphasize the UV.

The form of the standard extinction curve depends on the composition of the ISM, which varies from galaxy to galaxy. In the Local Group, the best-determined extinction curves are those of the Milky Way, the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC).

In the LMC, there is significant variation in the characteristics of the ultraviolet extinction with a weaker 2175 Å bump and stronger far-UV extinction in the region associated with the LMC2 supershell (near the 30 Doradus starbursting region) than seen elsewhere in the LMC and in the Milky Way.[30][31] In the SMC, more extreme variation is seen with no 2175 Å bump and very strong far-UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing.[32][33][34]

This gives clues as to the composition of the ISM in the various galaxies. Previously, the different average extinction curves in the Milky Way, LMC, and SMC were thought to be the result of the different metallicities of the three galaxies: the LMC's metallicity is about 40% of that of the Milky Way, while the SMC's is about 10%. Finding extinction curves in both the LMC and SMC which are similar to those found in the Milky Way[29] and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC[35] and in the SMC Bar[36] has given rise to a new interpretation. The variations in the curves seen in the Magellanic Clouds and Milky Way may instead be caused by processing of the dust grains by nearby star formation. This interpretation is supported by work in starburst galaxies (which are undergoing intense star formation episodes) which shows that their dust lacks the 2175 Å bump.[37][38]

Atmospheric extinction

Atmospheric extinction gives the rising or setting Sun an orange hue and varies with location and altitude. Astronomical observatories generally are able to characterise the local extinction curve very accurately, to allow observations to be corrected for the effect. Nevertheless, the atmosphere is completely opaque to many wavelengths requiring the use of satellites to make observations.

This extinction has three main components: Rayleigh scattering by air molecules, scattering by particulates, and molecular absorption. Molecular absorption is often referred to as telluric absorption, as it is caused by the Earth (telluric is a synonym for terrestrial). The most important sources of telluric absorption are molecular oxygen and ozone, which strongly absorb radiation near ultraviolet, and water, which strongly absorbs infrared.

The amount of such extinction is lowest at the observer's zenith and highest near the horizon. A given star, preferably at solar opposition, reaches its greatest celestial altitude and optimal time for observation when the star is near the local meridian around solar midnight and if the star has a favorable declination (i.e. similar to the observer's latitude); thus, the seasonal time due to axial tilt is key. Extinction is approximated by multiplying the standard atmospheric extinction curve (plotted against each wavelength) by the mean air mass calculated over the duration of the observation. A dry atmosphere reduces infrared extinction significantly.

References

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  2. ^ Karttunen, Hannu (2003). Fundamental astronomy. Physics and Astronomy Online Library. Springer. p. 289. ISBN 978-3-540-00179-9.
  3. ^ Struve, F. G. W. 1847, St. Petersburg: Tip. Acad. Imper., 1847; IV, 165 p.; in 8.; DCCC.4.211 [1]
  4. ^ Whittet, Douglas C. B. (2003). Dust in the Galactic Environment. Series in Astronomy and Astrophysics (2nd ed.). CRC Press. p. 10. ISBN 978-0750306249.
  5. ^ See Binney and Merrifeld, Section 3.7 (1998, ISBN 978-0-691-02565-0), Carroll and Ostlie, Section 12.1 (2007, ISBN 978-0-8053-0402-2), and Kutner (2003, ISBN 978-0-521-52927-3) for applications in astronomy.
  6. ^ "Interstellar Reddening, Extinction, and Red Sunsets". Astro.virginia.edu. 2002-04-22. Retrieved 2017-07-14.
  7. ^ Gottlieb, D. M.; Upson, W.L. (1969). "Local Interstellar Reddening". Astrophysical Journal. 157: 611. Bibcode:1969ApJ...157..611G. doi:10.1086/150101.
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  12. ^ Valencic, Lynne A.; Clayton, Geoffrey C.; Gordon, Karl D. (2004). "Ultraviolet Extinction Properties in the Milky Way". Astrophysical Journal. 616 (2): 912–924. arXiv:astro-ph/0408409. Bibcode:2004ApJ...616..912V. doi:10.1086/424922. S2CID 119330502.
  13. ^ Mathis, John S.; Cardelli, Jason A. (1992). "Deviations of interstellar extinctions from the mean R-dependent extinction law". Astrophysical Journal. 398: 610–620. Bibcode:1992ApJ...398..610M. doi:10.1086/171886.
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  16. ^ a b Majaess, Daniel; David Turner; Istvan Dekany; Dante Minniti; Wolfgang Gieren (2016). "Constraining dust extinction properties via the VVV survey". Astronomy and Astrophysics. 593: A124. arXiv:1607.08623. Bibcode:2016A&A...593A.124M. doi:10.1051/0004-6361/201628763. S2CID 54218060.
  17. ^ R(Ks) is, mathematically likewise, A(Ks)/E(J−Ks)
  18. ^ Nishyiama, Shogo; Motohide Tamura; Hirofumi Hatano; Daisuke Kato; Toshihiko Tanabe; Koji Sugitani; Tetsuya Nagata (2009). "Interstellar Extinction Law Toward the Galactic Center III: J, H, KS Bands in the 2MASS and the MKO Systems, and 3.6, 4.5, 5.8, 8.0 μm in the Spitzer/IRAC System". The Astrophysical Journal. 696 (2): 1407–1417. arXiv:0902.3095. Bibcode:2009ApJ...696.1407N. doi:10.1088/0004-637X/696/2/1407. S2CID 119205751.
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  20. ^ Bohlin, Ralph C.; Blair D. Savage; J. F. Drake (1978). "A survey of interstellar H I from L-alpha absorption measurements. II". Astrophysical Journal. 224: 132–142. Bibcode:1978ApJ...224..132B. doi:10.1086/156357.
  21. ^ Diplas, Athanassios; Blair D. Savage (1994). "An IUE survey of interstellar H I LY alpha absorption. 2: Interpretations". Astrophysical Journal. 427: 274–287. Bibcode:1994ApJ...427..274D. doi:10.1086/174139.
  22. ^ Güver, Tolga; Özel, Feryal (2009). "The relation between optical extinction and hydrogen column density in the Galaxy". Monthly Notices of the Royal Astronomical Society. 400 (4): 2050–2053. arXiv:0903.2057. Bibcode:2009MNRAS.400.2050G. doi:10.1111/j.1365-2966.2009.15598.x.
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  24. ^ Robin, Annie C.; Reylé, C.; Derrière, S.; Picaud, S. (Oct 2003). "A synthetic view on structure and evolution of the Milky Way". Astronomy and Astrophysics. 409 (2): 523–540. arXiv:astro-ph/0401052. Bibcode:2003A&A...409..523R. doi:10.1051/0004-6361:20031117.
  25. ^ Cardelli, Jason A.; Sembach, Kenneth R.; Mathis, John S. (1992). "The quantitative assessment of UV extinction derived from IUE data of giants and supergiants". Astronomical Journal. 104 (5): 1916–1929. Bibcode:1992AJ....104.1916C. doi:10.1086/116367. ISSN 0004-6256.
  26. ^ Stecher, Theodore P. (1965). "Interstellar Extinction in the Ultraviolet". Astrophysical Journal. 142: 1683. Bibcode:1965ApJ...142.1683S. doi:10.1086/148462.
  27. ^ Stecher, Theodore P. (1969). "Interstellar Extinction in the Ultraviolet. II". Astrophysical Journal. 157: L125. Bibcode:1969ApJ...157L.125S. doi:10.1086/180400.
  28. ^ Bradley, John; Dai, ZR; et al. (2005). "An Astronomical 2175 Å Feature in Interplanetary Dust Particles". Science. 307 (5707): 244–247. Bibcode:2005Sci...307..244B. doi:10.1126/science.1106717. PMID 15653501. S2CID 96858465.
  29. ^ a b Gordon, Karl D.; Geoffrey C. Clayton; Karl A. Misselt; Arlo U. Landolt; Michael J. Wolff (2003). "A Quantitative Comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way Ultraviolet to Near-Infrared Extinction Curves". Astrophysical Journal. 594 (1): 279–293. arXiv:astro-ph/0305257. Bibcode:2003ApJ...594..279G. doi:10.1086/376774. S2CID 117180437.
  30. ^ Fitzpatrick, Edward L. (1986). "An average interstellar extinction curve for the Large Magellanic Cloud". Astronomical Journal. 92: 1068–1073. Bibcode:1986AJ.....92.1068F. doi:10.1086/114237.
  31. ^ Misselt, Karl A.; Geoffrey C. Clayton; Karl D. Gordon (1999). "A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud". Astrophysical Journal. 515 (1): 128–139. arXiv:astro-ph/9811036. Bibcode:1999ApJ...515..128M. doi:10.1086/307010. S2CID 14175478.
  32. ^ Lequeux, J.; Maurice, E.; Prevot-Burnichon, M. L.; Prevot, L.; Rocca-Volmerange, B. (1982). "SK 143 - an SMC star with a galactic-type ultraviolet interstellar extinction". Astronomy and Astrophysics. 113: L15–L17. Bibcode:1982A&A...113L..15L.
  33. ^ Prevot, M. L.; Lequeux, J.; Prevot, L.; Maurice, E.; Rocca-Volmerange, B. (1984). "The typical interstellar extinction in the Small Magellanic Cloud". Astronomy and Astrophysics. 132: 389–392. Bibcode:1984A&A...132..389P.
  34. ^ Gordon, Karl D.; Geoffrey C. Clayton (1998). "Starburst-like Dust Extinction in the Small Magellanic Cloud". Astrophysical Journal. 500 (2): 816–824. arXiv:astro-ph/9802003. Bibcode:1998ApJ...500..816G. doi:10.1086/305774. S2CID 18090417.
  35. ^ Clayton, Geoffrey C.; Karl D. Gordon; Michael J. Wolff (2000). "Magellanic Cloud-Type Interstellar Dust along Low-Density Sight Lines in the Galaxy". Astrophysical Journal Supplement Series. 129 (1): 147–157. arXiv:astro-ph/0003285. Bibcode:2000ApJS..129..147C. doi:10.1086/313419. S2CID 11205416.
  36. ^ Valencic, Lynne A.; Geoffrey C. Clayton; Karl D. Gordon; Tracy L. Smith (2003). "Small Magellanic Cloud-Type Interstellar Dust in the Milky Way". Astrophysical Journal. 598 (1): 369–374. arXiv:astro-ph/0308060. Bibcode:2003ApJ...598..369V. doi:10.1086/378802. S2CID 123435053.
  37. ^ Calzetti, Daniela; Anne L. Kinney; Thaisa Storchi-Bergmann (1994). "Dust extinction of the stellar continua in starburst galaxies: The ultraviolet and optical extinction law". Astrophysical Journal. 429: 582–601. Bibcode:1994ApJ...429..582C. doi:10.1086/174346. hdl:10183/108843.
  38. ^ Gordon, Karl D.; Daniela Calzetti; Adolf N. Witt (1997). "Dust in Starburst Galaxies". Astrophysical Journal. 487 (2): 625–635. arXiv:astro-ph/9705043. Bibcode:1997ApJ...487..625G. doi:10.1086/304654. S2CID 2055629.

Further reading

  • Binney, J. & Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 978-0-691-00402-0.
  • Howarth, I. D. (1983). "LMC and galactic extinction". Monthly Notices of the Royal Astronomical Society. 203 (2): 301–304. Bibcode:1983MNRAS.203..301H. doi:10.1093/mnras/203.2.301.
  • King, D. L. (1985). "Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma". RGO/La Palma Technical Note. 31.
  • McCall, M. L. (2004). "On Determining Extinction from Reddening". The Astronomical Journal. 128: 2144–2169. http://adsabs.harvard.edu/abs/2004AJ....128.2144M
  • Rouleau, F.; Henning, T.; Stognienko, R. (1997). "Constraints on the properties of the 2175Å interstellar feature carrier". Astronomy and Astrophysics. 322: 633–645. arXiv:astro-ph/9611203. Bibcode:1997A&A...322..633R.

January 13, 2023
extinction, astronomy, other, uses, extinction, disambiguation, astronomy, extinction, absorption, scattering, electromagnetic, radiation, dust, between, emitting, astronomical, object, observer, interstellar, extinction, first, documented, such, 1930, robert,. For other uses see Extinction disambiguation In astronomy extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler 1 2 However its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve 3 and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth extinction in the visual band of frequencies photometric system is roughly 1 8 magnitudes per kiloparsec 4 For Earth bound observers extinction arises both from the interstellar medium ISM and the Earth s atmosphere it may also arise from circumstellar dust around an observed object Strong extinction in earth s atmosphere of some wavelength regions such as X ray ultraviolet and infrared is overcome by the use of space based observatories Since blue light is much more strongly attenuated than red light extinction causes objects to appear redder than expected a phenomenon referred to as interstellar reddening 5 Contents 1 Interstellar reddening 2 General characteristics 3 Measuring extinction towards an object 4 The 2175 angstrom feature 5 Extinction curves of other galaxies 6 Atmospheric extinction 7 References 8 Further readingInterstellar reddening EditIn astronomy interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which the object originally emitted Reddening occurs due to the light scattering off dust and other matter in the interstellar medium Interstellar reddening is a different phenomenon from redshift which is the proportional frequency shifts of spectra without distortion Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons in the optical light that is redder leaving the spectroscopic lines unchanged In most photometric systems filters passbands are used from which readings of magnitude of light may take account of latitude and humidity among terrestrial factors Interstellar reddening equates to the color excess defined as the difference between an object s observed color index and its intrinsic color index sometimes referred to as its normal color index The latter is the theoretical value which it would have if unaffected by extinction In the first system the UBV photometric system devised in the 1950s and its most closely related successors the object s color excess E B V displaystyle E B V is related to the object s B V color calibrated blue minus calibrated visible by E B V B V observed B V intrinsic displaystyle E B V B V textrm observed B V textrm intrinsic For an A0 type main sequence star these have median wavelength and heat among the main sequence the color indices are calibrated at 0 based on an intrinsic reading of such a star exactly 0 02 depending on which spectral point i e precise passband within the abbreviated color name is in question see color index At least two and up to five measured passbands in magnitude are then compared by subtraction U B V I or R during which the color excess from extinction is calculated and deducted The name of the four sub indices R minus I etc and order of the subtraction of recalibrated magnitudes is from right to immediate left within this sequence General characteristics EditInterstellar reddening occurs because interstellar dust absorbs and scatters blue light waves more than red light waves making stars appear redder than they are This is similar to the effect seen when dust particles in the atmosphere of Earth contribute to red sunsets see Sunset Colors 6 Broadly speaking interstellar extinction is strongest at short wavelengths generally observed by using techniques from spectroscopy Extinction results in a change in the shape of an observed spectrum Superimposed on this general shape are absorption features wavelength bands where the intensity is lowered that have a variety of origins and can give clues as to the chemical composition of the interstellar material e g dust grains Known absorption features include the 2175 A bump the diffuse interstellar bands the 3 1 mm water ice feature and the 10 and 18 mm silicate features In the solar neighborhood the rate of interstellar extinction in the Johnson Cousins V band visual filter averaged at a wavelength of 540 nm is usually taken to be 0 7 1 0 mag kpc simply an average due to the clumpiness of interstellar dust 7 8 9 In general however this means that a star will have its brightness reduced by about a factor of 2 in the V band viewed from a good night sky vantage point on earth for every kiloparsec 3 260 light years it is farther away from us The amount of extinction can be significantly higher than this in specific directions For example some regions of the Galactic Center are awash with obvious intervening dark dust from our spiral arm and perhaps others and themselves in a bulge of dense matter causing as much as more than 30 magnitudes of extinction in the optical meaning that less than 1 optical photon in 1012 passes through 10 This results in the so called zone of avoidance where our view of the extra galactic sky is severely hampered and background galaxies such as Dwingeloo 1 were only discovered recently through observations in radio and infrared The general shape of the ultraviolet through near infrared 0 125 to 3 5 mm extinction curve plotting extinction in magnitude against wavelength often inverted looking from our vantage point at other objects in the Milky Way is fairly well characterized by the stand alone parameter of relative visibility of such visible light R V which is different along different lines of sight 11 12 but there are known deviations from this characterization 13 Extending the extinction law into the mid infrared wavelength range is difficult due to the lack of suitable targets and various contributions by absorption features 14 R V compares aggregate and particular extinctions It is A V E B V Restated it is the total extinction A V divided by the selective total extinction A B A V of those two wavelengths bands A B and A V are the total extinction at the B and V filter bands Another measure used in the literature is the absolute extinction A l A V at wavelength l comparing the total extinction at that wavelength to that at the V band R V is known to be correlated with the average size of the dust grains causing the extinction For our own galaxy the Milky Way the typical value for R V is 3 1 15 but is found to vary considerably across different lines of sight 16 As a result when computing cosmic distances it can be advantageous to move to star data from the near infrared of which the filter or passband Ks is quite standard where the variations and amount of extinction are significantly less and similar ratios as to R Ks 17 0 49 0 02 and 0 528 0 015 were found respectively by independent groups 16 18 Those two more modern findings differ substantially relative to the commonly referenced historical value 0 7 11 The relationship between the total extinction A V measured in magnitudes and the column density of neutral hydrogen atoms column NH usually measured in cm 2 shows how the gas and dust in the interstellar medium are related From studies using ultraviolet spectroscopy of reddened stars and X ray scattering halos in the Milky Way Predehl and Schmitt 19 found the relationship between NH and A V to be approximately N H A V 1 8 10 21 atoms cm 2 mag 1 displaystyle frac N H A V approx 1 8 times 10 21 mbox atoms mbox cm 2 mbox mag 1 see also 20 21 22 Astronomers have determined the three dimensional distribution of extinction in the solar circle our region of our galaxy using visible and near infrared stellar observations and a model of distribution of stars 23 24 The dust causing extinction mainly lies along the spiral arms as observed in other spiral galaxies Measuring extinction towards an object EditTo measure the extinction curve for a star the star s spectrum is compared to the observed spectrum of a similar star known not to be affected by extinction unreddened 25 It is also possible to use a theoretical spectrum instead of the observed spectrum for the comparison but this is less common In the case of emission nebulae it is common to look at the ratio of two emission lines which should not be affected by the temperature and density in the nebula For example the ratio of hydrogen alpha to hydrogen beta emission is always around 2 85 under a wide range of conditions prevailing in nebulae A ratio other than 2 85 must therefore be due to extinction and the amount of extinction can thus be calculated The 2175 angstrom feature EditOne prominent feature in measured extinction curves of many objects within the Milky Way is a broad bump at about 2175 A well into the ultraviolet region of the electromagnetic spectrum This feature was first observed in the 1960s 26 27 but its origin is still not well understood Several models have been presented to account for this bump which include graphitic grains with a mixture of PAH molecules Investigations of interstellar grains embedded in interplanetary dust particles IDP observed this feature and identified the carrier with organic carbon and amorphous silicates present in the grains 28 Extinction curves of other galaxies Edit Plot showing the average extinction curves for the MW LMC2 LMC and SMC Bar 29 The curves are plotted versus 1 wavelength to emphasize the UV The form of the standard extinction curve depends on the composition of the ISM which varies from galaxy to galaxy In the Local Group the best determined extinction curves are those of the Milky Way the Small Magellanic Cloud SMC and the Large Magellanic Cloud LMC In the LMC there is significant variation in the characteristics of the ultraviolet extinction with a weaker 2175 A bump and stronger far UV extinction in the region associated with the LMC2 supershell near the 30 Doradus starbursting region than seen elsewhere in the LMC and in the Milky Way 30 31 In the SMC more extreme variation is seen with no 2175 A bump and very strong far UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing 32 33 34 This gives clues as to the composition of the ISM in the various galaxies Previously the different average extinction curves in the Milky Way LMC and SMC were thought to be the result of the different metallicities of the three galaxies the LMC s metallicity is about 40 of that of the Milky Way while the SMC s is about 10 Finding extinction curves in both the LMC and SMC which are similar to those found in the Milky Way 29 and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC 35 and in the SMC Bar 36 has given rise to a new interpretation The variations in the curves seen in the Magellanic Clouds and Milky Way may instead be caused by processing of the dust grains by nearby star formation This interpretation is supported by work in starburst galaxies which are undergoing intense star formation episodes which shows that their dust lacks the 2175 A bump 37 38 Atmospheric extinction EditAtmospheric extinction gives the rising or setting Sun an orange hue and varies with location and altitude Astronomical observatories generally are able to characterise the local extinction curve very accurately to allow observations to be corrected for the effect Nevertheless the atmosphere is completely opaque to many wavelengths requiring the use of satellites to make observations This extinction has three main components Rayleigh scattering by air molecules scattering by particulates and molecular absorption Molecular absorption is often referred to as telluric absorption as it is caused by the Earth telluric is a synonym for terrestrial The most important sources of telluric absorption are molecular oxygen and ozone which strongly absorb radiation near ultraviolet and water which strongly absorbs infrared The amount of such extinction is lowest at the observer s zenith and highest near the horizon A given star preferably at solar opposition reaches its greatest celestial altitude and optimal time for observation when the star is near the local meridian around solar midnight and if the star has a favorable declination i e similar to the observer s latitude thus the seasonal time due to axial tilt is key Extinction is approximated by multiplying the standard atmospheric extinction curve plotted against each wavelength by the mean air mass calculated over the duration of the observation A dry atmosphere reduces infrared extinction significantly References Edit Trumpler R J 1930 Preliminary results on the distances dimensions and space distribution of open star clusters Lick Observatory Bulletin 14 420 154 188 Bibcode 1930LicOB 14 154T doi 10 5479 ADS bib 1930LicOB 14 154T Karttunen Hannu 2003 Fundamental astronomy Physics and Astronomy Online Library Springer p 289 ISBN 978 3 540 00179 9 Struve F G W 1847 St Petersburg Tip Acad Imper 1847 IV 165 p in 8 DCCC 4 211 1 Whittet Douglas C B 2003 Dust in the Galactic Environment Series in Astronomy and Astrophysics 2nd ed CRC Press p 10 ISBN 978 0750306249 See Binney and Merrifeld Section 3 7 1998 ISBN 978 0 691 02565 0 Carroll and Ostlie Section 12 1 2007 ISBN 978 0 8053 0402 2 and Kutner 2003 ISBN 978 0 521 52927 3 for applications in astronomy Interstellar Reddening Extinction and Red Sunsets Astro virginia edu 2002 04 22 Retrieved 2017 07 14 Gottlieb D M Upson W L 1969 Local Interstellar Reddening Astrophysical Journal 157 611 Bibcode 1969ApJ 157 611G doi 10 1086 150101 Milne D K Aller L H 1980 An average model for the galactic absorption Astrophysical Journal 85 17 21 Bibcode 1980AJ 85 17M doi 10 1086 112628 Lynga G 1982 Open clusters in our Galaxy Astronomy amp Astrophysics 109 213 222 Bibcode 1982A amp A 109 213L Schlegel David J Finkbeiner Douglas P Davis Marc 1998 Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds Astrophysical Journal 500 2 525 553 arXiv astro ph 9710327 Bibcode 1998ApJ 500 525S doi 10 1086 305772 S2CID 59512299 a b Cardelli Jason A Clayton Geoffrey C Mathis John S 1989 The relationship between infrared optical and ultraviolet extinction Astrophysical Journal 345 245 256 Bibcode 1989ApJ 345 245C doi 10 1086 167900 Valencic Lynne A Clayton Geoffrey C Gordon Karl D 2004 Ultraviolet Extinction Properties in the Milky Way Astrophysical Journal 616 2 912 924 arXiv astro ph 0408409 Bibcode 2004ApJ 616 912V doi 10 1086 424922 S2CID 119330502 Mathis John S Cardelli Jason A 1992 Deviations of interstellar extinctions from the mean R dependent extinction law Astrophysical Journal 398 610 620 Bibcode 1992ApJ 398 610M doi 10 1086 171886 T K Fritz S Gillessen K Dodds Eden D Lutz R Genzel W Raab T Ott O Pfuhl F Eisenhauer and F Yusuf Zadeh 2011 Line Derived Infrared Extinction toward the Galactic Center The Astrophysical Journal 737 2 73 arXiv 1105 2822 Bibcode 2011ApJ 737 73F doi 10 1088 0004 637X 737 2 73 S2CID 118919927 a href Template Cite journal html title Template Cite journal cite journal a CS1 maint uses authors parameter link Schultz G V Wiemer W 1975 Interstellar reddening and IR excess of O and B stars Astronomy and Astrophysics 43 133 139 Bibcode 1975A amp A 43 133S a b Majaess Daniel David Turner Istvan Dekany Dante Minniti Wolfgang Gieren 2016 Constraining dust extinction properties via the VVV survey Astronomy and Astrophysics 593 A124 arXiv 1607 08623 Bibcode 2016A amp A 593A 124M doi 10 1051 0004 6361 201628763 S2CID 54218060 R Ks is mathematically likewise A Ks E J Ks Nishyiama Shogo Motohide Tamura Hirofumi Hatano Daisuke Kato Toshihiko Tanabe Koji Sugitani Tetsuya Nagata 2009 Interstellar Extinction Law Toward the Galactic Center III J H KS Bands in the 2MASS and the MKO Systems and 3 6 4 5 5 8 8 0 mm in the Spitzer IRAC System The Astrophysical Journal 696 2 1407 1417 arXiv 0902 3095 Bibcode 2009ApJ 696 1407N doi 10 1088 0004 637X 696 2 1407 S2CID 119205751 Predehl P Schmitt J H M M 1995 X raying the interstellar medium ROSAT observations of dust scattering halos Astronomy and Astrophysics 293 889 905 Bibcode 1995A amp A 293 889P Bohlin Ralph C Blair D Savage J F Drake 1978 A survey of interstellar H I from L alpha absorption measurements II Astrophysical Journal 224 132 142 Bibcode 1978ApJ 224 132B doi 10 1086 156357 Diplas Athanassios Blair D Savage 1994 An IUE survey of interstellar H I LY alpha absorption 2 Interpretations Astrophysical Journal 427 274 287 Bibcode 1994ApJ 427 274D doi 10 1086 174139 Guver Tolga Ozel Feryal 2009 The relation between optical extinction and hydrogen column density in the Galaxy Monthly Notices of the Royal Astronomical Society 400 4 2050 2053 arXiv 0903 2057 Bibcode 2009MNRAS 400 2050G doi 10 1111 j 1365 2966 2009 15598 x Marshall Douglas J Robin A C Reyle C Schultheis M Picaud S Jul 2006 Modelling the Galactic interstellar extinction distribution in three dimensions Astronomy and Astrophysics 453 2 635 651 arXiv astro ph 0604427 Bibcode 2006A amp A 453 635M doi 10 1051 0004 6361 20053842 S2CID 16845046 Robin Annie C Reyle C Derriere S Picaud S Oct 2003 A synthetic view on structure and evolution of the Milky Way Astronomy and Astrophysics 409 2 523 540 arXiv astro ph 0401052 Bibcode 2003A amp A 409 523R doi 10 1051 0004 6361 20031117 Cardelli Jason A Sembach Kenneth R Mathis John S 1992 The quantitative assessment of UV extinction derived from IUE data of giants and supergiants Astronomical Journal 104 5 1916 1929 Bibcode 1992AJ 104 1916C doi 10 1086 116367 ISSN 0004 6256 Stecher Theodore P 1965 Interstellar Extinction in the Ultraviolet Astrophysical Journal 142 1683 Bibcode 1965ApJ 142 1683S doi 10 1086 148462 Stecher Theodore P 1969 Interstellar Extinction in the Ultraviolet II Astrophysical Journal 157 L125 Bibcode 1969ApJ 157L 125S doi 10 1086 180400 Bradley John Dai ZR et al 2005 An Astronomical 2175 A Feature in Interplanetary Dust Particles Science 307 5707 244 247 Bibcode 2005Sci 307 244B doi 10 1126 science 1106717 PMID 15653501 S2CID 96858465 a b Gordon Karl D Geoffrey C Clayton Karl A Misselt Arlo U Landolt Michael J Wolff 2003 A Quantitative Comparison of the Small Magellanic Cloud Large Magellanic Cloud and Milky Way Ultraviolet to Near Infrared Extinction Curves Astrophysical Journal 594 1 279 293 arXiv astro ph 0305257 Bibcode 2003ApJ 594 279G doi 10 1086 376774 S2CID 117180437 Fitzpatrick Edward L 1986 An average interstellar extinction curve for the Large Magellanic Cloud Astronomical Journal 92 1068 1073 Bibcode 1986AJ 92 1068F doi 10 1086 114237 Misselt Karl A Geoffrey C Clayton Karl D Gordon 1999 A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud Astrophysical Journal 515 1 128 139 arXiv astro ph 9811036 Bibcode 1999ApJ 515 128M doi 10 1086 307010 S2CID 14175478 Lequeux J Maurice E Prevot Burnichon M L Prevot L Rocca Volmerange B 1982 SK 143 an SMC star with a galactic type ultraviolet interstellar extinction Astronomy and Astrophysics 113 L15 L17 Bibcode 1982A amp A 113L 15L Prevot M L Lequeux J Prevot L Maurice E Rocca Volmerange B 1984 The typical interstellar extinction in the Small Magellanic Cloud Astronomy and Astrophysics 132 389 392 Bibcode 1984A amp A 132 389P Gordon Karl D Geoffrey C Clayton 1998 Starburst like Dust Extinction in the Small Magellanic Cloud Astrophysical Journal 500 2 816 824 arXiv astro ph 9802003 Bibcode 1998ApJ 500 816G doi 10 1086 305774 S2CID 18090417 Clayton Geoffrey C Karl D Gordon Michael J Wolff 2000 Magellanic Cloud Type Interstellar Dust along Low Density Sight Lines in the Galaxy Astrophysical Journal Supplement Series 129 1 147 157 arXiv astro ph 0003285 Bibcode 2000ApJS 129 147C doi 10 1086 313419 S2CID 11205416 Valencic Lynne A Geoffrey C Clayton Karl D Gordon Tracy L Smith 2003 Small Magellanic Cloud Type Interstellar Dust in the Milky Way Astrophysical Journal 598 1 369 374 arXiv astro ph 0308060 Bibcode 2003ApJ 598 369V doi 10 1086 378802 S2CID 123435053 Calzetti Daniela Anne L Kinney Thaisa Storchi Bergmann 1994 Dust extinction of the stellar continua in starburst galaxies The ultraviolet and optical extinction law Astrophysical Journal 429 582 601 Bibcode 1994ApJ 429 582C doi 10 1086 174346 hdl 10183 108843 Gordon Karl D Daniela Calzetti Adolf N Witt 1997 Dust in Starburst Galaxies Astrophysical Journal 487 2 625 635 arXiv astro ph 9705043 Bibcode 1997ApJ 487 625G doi 10 1086 304654 S2CID 2055629 Further reading EditBinney J amp Merrifield M 1998 Galactic Astronomy Princeton Princeton University Press ISBN 978 0 691 00402 0 Howarth I D 1983 LMC and galactic extinction Monthly Notices of the Royal Astronomical Society 203 2 301 304 Bibcode 1983MNRAS 203 301H doi 10 1093 mnras 203 2 301 King D L 1985 Atmospheric Extinction at the Roque de los Muchachos Observatory La Palma RGO La Palma Technical Note 31 McCall M L 2004 On Determining Extinction from Reddening The Astronomical Journal 128 2144 2169 http adsabs harvard edu abs 2004AJ 128 2144M Rouleau F Henning T Stognienko R 1997 Constraints on the properties of the 2175A interstellar feature carrier Astronomy and Astrophysics 322 633 645 arXiv astro ph 9611203 Bibcode 1997A amp A 322 633R Retrieved from https en wikipedia org w index php title Extinction astronomy amp oldid 1125217582, wikipedia, wiki, book, books, library,

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