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atmospheres of old and cool white dwarfs deliver valuable information on stellar and planetary evolution .
this is because of relatively simple cooling physics that allows for accurate cosmochronometry @xcite and of the stratified nature of a white dwarf star , which allows for tracing the content of acreted planetary - like material @xcite .
however , in order to properly asses this information , atmospheres of these stars must be well characterized and understood .
this is often difficult with the standard models that are based mainly of the ideal - gas description of physics and chemistry .
the atmospheres of helium - rich stars reach densities of fluid ( up to a few g / cc @xcite ) and there are severe problems in proper characterization of such old stars ( e.g. @xcite ) .
impact of the dense atmospheres on stellar spectra are illustrated by the pressure distortion of @xmath1 bands @xcite and also with some problems with fitting the metal lines profiles @xcite .
the former problems in fitting the uv spectra and u and b photometric bands of less extreme hydrogen - rich white dwarfs @xcite , solved by introduction of the ly-@xmath2 red wing opacities @xcite , also show the importance of proper modeling , accurate fitting and characterization of spectra of cool white dwarfs , for correct understanding of these stars @xcite .    in the last two decades various high - pressure improvements
have been introduced to the modeling .
these include the non - ideal equation of state @xcite , the opacities @xcite , the refraction @xcite and the chemical abundances of species @xcite .
the further tests of ly - alpha red wing profiles validated this absorption mechanism for a sample of cool stars @xcite .
while the less extreme atmospheres of hydrogen - rich white dwarfs are now well reproduced by the current models , the modeling of more extreme helium - rich atmospheres still encounters difficulties @xcite . in spite of significant work devoted to improvement of high density chemistry and physics description ,
no conclusive models describing dense , helium - rich atmospheres exist .
in particular , a group of so - called ultracool white dwarfs , which are believed to be helium - rich stars , have rather vague atmospheric parameters assignment @xcite .
we suspect that the pressure - distortion of ir opacities , namely the collision induced absorption ( cia , @xcite ) , contributes to much of these difficulties .    having steady improvement in the performance and availability of the computational power and quantum chemistry simulation software and techniques ,
many dense helium problems could be tackled by atomistic simulations @xcite .
the ir opacities could be directly simulated using _ ab initio _ methods of quantum chemistry @xcite .
recently we attempted such simulations and the first results are briefly discussed here in the context of interpreting the near- and mid - ir spectra of ultracool stars .
recently , using _ ab initio _ molecular dynamics technique , @xcite simulated the cia opacities of pure , dense helium and found significant absorption resulting from triple collisions between helium atoms .
this absorption mechanism was postulated ( e.g. @xcite ) , but had been never before observed experimentally or computed . the obtained absorption coefficient for a representative temperature of 5000 k is given in figure [ f1 ] .
although the intensity of this absorption mechanism is about four orders of magnitude lower that the relevant h@xmath0/h - he cia opacities , because he atoms are the dominant species it becomes a significant ir absorption mechanism in the atmospheres with @xmath3 ( see fig .
[ f2 ] ) . when we applied the new absorption mechanism to the spectrum of lhs1128 star , which is broadly discussed by @xcite ( fig .
[ f2 ] ) , we obtained much better fit to the irac fluxes .
this opacity source thus explains a flat - like behavior of the observed ir fluxes , as opposite to the @xmath4 flux suppression and the @xmath5 flux excess predicted by the current models ( fig .
using the same simulation method as @xcite used for simulation of the he - he - he absorption ( see also @xcite in this volume ) we have simulated the @xmath6 cia opacities . as indicated in fig . 2 of @xcite we found a significant distortion of this absorption mechanism for densities @xmath7 .
the obtained profiles for the low and high densities are compared in figure [ f3 ] . as indicated in the figure
, there are three main pressure effects .
( 1 ) the rototranslational band peak at @xmath8 becomes more pronounced and ( 2 ) shifted towards higher frequencies .
we interpret this as a result of triple collisions @xcite and decrease in the separation between hydrogen atoms in h@xmath0 with increase in density , and related decrease of the h@xmath0 rotational inertia , as illustrated in fig .
[ f4 ] . at the same time , ( 3 ) the @xmath9 main vibrational band shifts to the higher frequencies and splits into two peaks due to an interference between the dipole moments induced in the successive collisions of h@xmath0 and he atoms @xcite .
this splitting has been experimentally observed before @xcite and in figure [ f5 ] we compare those results with these simulated by us .
we obtained larger splitting because simulations were performed at much higher temperature , thus with higher thermal collisions energy than in the experiments performed at ambient conditions .
the obtained results could be used for the interpretation of difference between the modeled and the observed spectra of ultracool white dwarfs , such as lhs3025 .
compared to the synthetic spectrum computed using the standard cia opacity description of @xcite , the observed spectrum does not show the clear minimum at @xmath10 and the flux excess at @xmath11 that are predicted by models , and indicates more absorption in the near - ir .
this is consistent with the simulated broadening and the redistribution of the main vibrational peak and the enhancement of rototranslational peak .
the absorption spectrum simulated at @xmath12 ( fig .
[ f3 ] ) indicates maximum of absorption at @xmath13 and more absorption in the near - ir , which is consistent with the observed spectral energy distribution of lhs3025 , and also other ultracool white dwarfs @xcite .
more elaborate analysis requires more complete set of simulated spectra , including investigation of the temperature dependence , derivation of the relevant distortion model , calculations of a grid of atmosphere models and their usage for subsequent fitting of spectra and analysis of ultracool white dwarfs .
this work is currently in progress .
we have showed the results of simulations of he - he - he and h@xmath0-he cia infrared opacities and their impact for the interpretation of spectra of cool , helium - rich stars , including ultracool white dwarfs .
we discuss the impact of the newly found he - he - he cia opacity from pure helium on the spectra . by improved fit to the spectrum of lhs1128 star we show that this opacity mechanism could be responsible for a flat - like infrared spectral energy distribution of these stars , which could not be reproduced with previous models .
the subsequent simulations of h@xmath0-he cia opacities show significant pressure - distortion of this absorption mechanism with the simulated intensity change being consistent with the one deducted from the mismatch between the current models prediction and the observed spectral energy distribution for star lhs3250 .
however , more complete simulations of the ir opacities and subsequent computation of new models are required for complete analysis and refitting the spectra of cool , helium - rich stars , which is currently in progress .
last but not least , the simulations conducted here and the subsequent analysis show that the modern atomistic modeling methods could be successfully applied to modeling condensed stellar atmospheres and that , on the other hand , the atmospheres of white dwarfs represent excellent laboratories for testing the physics and chemistry of matter at extreme conditions .
abel , m. , frommhold , l. , li , x. , & hunt , k. l. c. 2012 , the journal of chemical physics , 136 , 044319 bergeron , p. , leggett , s. k. , & ruiz , m. t. 2001 , , 133 , 433 bergeron , p. , ruiz , m. t. , & leggett , s. k. 1997 , , 108 , 339 blouin , s. , dufour , p. , & kowalski , p. m. 2017 ( astronomical soc . of the pacific ) ,
this volume dufour , p. , bergeron , p. , liebert , j. , harris , h. c. , knapp , g. r. , anderson , s. f. , hall , p. b. , strauss , m. a. , collinge , m. j. , & edwards , m. c. 2007 , the astrophysical journal , 663 , 1291 farihi , j. , barstow , m. a. , redfield , s. , dufour , p. , & hambly , n. c. 2010 , mon . not .
r. astron .
soc . , 404 , 2123 fontaine , g. , brassard , p. , & bergeron , p. 2001
, , 113 , 409 frommhold , l. 1993 , collision - induced absorption in gases ( cambridge university press ) gianninas , a. , curd , b. , thorstensen , j. r. , kilic , m. , bergeron , p. , andrews , j. j. , canton , p. , & agueros , m. a. 2015 , monthly notices of the royal astronomical society , 449 , 3966 guillot , b. 1991 , the journal of chemical physics , 95 hare , w. f. j. , & welsh , h. l. 1958 , canadian journal of physics , 36 , 88 iglesias , c. a. , rogers , f. j. , & saumon , d. 2002 , the astrophysical journal letters , 569 , l111 jahn , s. , & kowalski , p. 2014
, reviews in mineralogy @xmath14 geochemistry , 78 , 691 kilic , m. , kowalski , p. m. , mullally , f. , reach , w. t. , & von hippel , t. 2008 , , 678 , 1298 kilic , m. , kowalski , p. m. , reach , w. t. , & von hippel , t. 2009 , , 696 , 2094 kowalski , p. 2014
, , 566 , l8 kowalski , p. , saumon , d. , holberg , j. , & leggett , s. 2013 ( san francisco , calif . : astronomical soc . of the pacific ) , 173 kowalski , p. m. 2006 , the astrophysical journal , 641 , 488  2006 , the astrophysical journal , 651 , 1120  2010 , , 519 , l8 kowalski , p. m. , & saumon , d. 2004 , the astrophysical journal , 607 , 970  2006 , , 651 , l137 kranendonk , j. v. 1968
, canadian journal of physics , 46 , 1173 lenzuni , p. , & saumon , d. 1992 , revista mexicana de astronomia y astrofisica , 23 , 223 richer , h. b. , anderson , j. b. , james , d. s. , fahlman , g. g. , hansen , b. m. s. , hurley , j. , kalirai , j. s. , king , i. r. , reitzel , d. , rich , r. m. , shara , m. m. , & stetson , p. b. 2006 , science , 313 , 936 saumon , d. , holberg , j. b. , & kowalski , p. m. 2014 , the astrophysical journal , 790 , 50 saumon , d. , & jacobson , s. b. 1999 , the astrophysical journal letters , 511 , l107 schmidt , g. d. , bergeron , p. , & fegley , b. 1995 , , 443 , 274 silvestrelli , p. l. , bernasconi , m. , & parrinello , m. 1997 , chemical physics letters , 277 , 478 wolff , b. , koester , d. , & liebert , j. 2002 , , 385 , 995
dense , he - rich atmospheres of cool white dwarfs represent a challenge to the modeling . 
 this is because these atmospheres are constituted of a dense fluid in which strong multi - atomic interactions determine their physics and chemistry . 
 therefore , the ideal - gas - based description of absorption is no longer adequate , which makes the opacities of these atmospheres difficult to model . 
 this is illustrated with severe problems in fitting the spectra of cool , he - rich stars . 
 good description of the infrared ( ir ) opacity is essential for proper assignment of the atmospheric parameters of these stars . using methods of computational quantum chemistry 
 we simulate the ir absorption of dense he / h media . 
 we found a significant ir absorption from he atoms ( he - he - he cia opacity ) and a strong pressure distortion of the h@xmath0-he collision - induced absorption ( cia ) . 
 we discuss the implication of these results for interpretation of the spectra of cool stars .