Model Atmospheres From Very Low Mass To Brown Dwarfs & Young Planets The project is based on 5 or >5 journal papers (each for 2 points) that are related to

Model Atmospheres From Very Low Mass To Brown Dwarfs & Young Planets The project is based on 5 or >5 journal papers (each for 2 points) that are related to solar, wind, or PEM fuel cell technology (Dr. Wang will provide 1-2 papers for each topic. You select 1 paper according to your own interest or background and find additional 5 relevant papers. Use google scholar for search.). The selected literature articles or topic will be critically evaluated (summarize and comment on their work). An example is “Wang et al. A review of polymer electrolyte membrane fuel cells: technology, applications, and needs on fundamental research”. Please follow the ASME conference paper format (see the sample).under there is the requirment for the research paper, i already choose the topic paper and put it in the files. i also put the requirement file. please read it carefully and wirte the paper with at least 5 journal paper as references. MAE 117– SOLAR AND RENEWABLE ENERGY SYSTEMS
Class Project 1
(Due: TBD)
The project is based on 5 or >5 journal papers (each for 2 points) that are related to
solar, wind, or PEM fuel cell technology (Dr. Wang will provide 1-2 papers for
each topic. You select 1 paper according to your own interest or background and
find additional 5 relevant papers. Use google scholar for search.). The selected
literature articles or topic will be critically evaluated (summarize and comment on
their work). An example is “Wang et al. A review of polymer electrolyte
membrane fuel cells: technology, applications, and needs on fundamental
research”. Please follow the ASME conference paper format (see the sample).
The report must include the following:
1.
2.
3.
4.
5.
6.
Abstract (one paragraph, no more than 200 words): Describe the problem
very briefly, highlight the main results and conclusions (5 points).
Introduction: Describe the problem very carefully and provide schematics
(you need to draw one schematic yourself) if necessary. (20 points).
Methods: Provide a concise description of the methods in selected
papers…. (30 points)
Results and Discussion: Please read a standard review paper on how to
write this part. Include graphs and/or tables. Number all the graphs/tables
(as Figure 1, Figure 2, …, or Table 1, Table 2, …) and refer to them in your
discussions accordingly. Provide captions for each figure or table that you
use in your report. (30 points).
Conclusion: (5 points).
References: Number all the references that you have used in the report and
list them here.
* ASME Paper Format (10 points); Easy to follow, clearly presented, … (TAs
only have 5-8 minutes to evaluate your report.). Make the entire report T eff > 400 K, 5.5 > logg > -0.5, and [M/H]= +0.5 to
-1.5, and the reference solar abundances of Asplund et al. (2009). We found that the
new solar abundances allow an improved (close to perfect) reproduction of the photometric and spectroscopic VLMs properties, and, for the first time, a smooth transition
between stellar and substellar regimes — unlike the transition between the NextGen
models from Hauschildt et al. 1999a,b, and the AMES-Dusty models from Allard et
al. 2001). In the BDs regime, the BT-Settl models propose an improved explanation
for the M-L-T spectral transition. In this paper, we therefore present the new BT-Settl
model atmosphere grid, which explains the entire transition from the stellar to planetary
mass regimes.
1.
The Impact of the new Solar Abundances on VLMs spectral properties
The modeling of the atmospheres of very low mass stars (hereafter VLMs) has evolved
with the development of computing capacities from an analytical treatment of the transfer equation using moments of the radiation field (Allard 1990), to a line-by-line opacity sampling in spherical symmetry (Allard et al. 1997; Hauschildt et al. 1999a,b), and
finally 3D radiation transfer (Seelmann et al. 2010). In parallel to detailed radiative
transfer in an assumed static environment, hydrodynamical simulations have been de1
2
Allard, F., Freytag, B., and Homeier, D.
veloped to reach a realistic representation of the granulation and the line profiles shifted
and shaped by the hydrodynamical flow of the sun and sun-like stars (see for details the
review in a special isssue of the Journal of Computational Physics by Freytag et al.
2011) by using a non-grey (multi-group binning of opacities) radiative transfer using a
pure blackbody source function (scattering is neglected).
Figure 1. Model atmosphere synthetic spectrum versus VB10 IR SED, using different water opacity profiles over the years (Ludwig 1971; Jørgensen et al. 2001;
Schryber et al. 1995; Partridge & Schwenke 1997).
The model atmospheres and synthetic spectra have also been made possible thanks
to the development of realistic opacities calculated often ab initio for the needs of an
accurate account of their cooling and heating effects in the internal atmospheric layers where temperatures close to 3000 K can prevail. For some time, the remaining
discrepancies in the model synthetic spectra were believed to be due to incomplete water vapor line lists, both in temperature and in the rotation quantum number J of the
molecular simulations. In Fig.1, model atmosphere synthetic spectra are shown using
different published water vapor opacity profiles such as hot flames laboratory experiments (Ludwig 1971), empirical calculations (Jørgensen et al. 2001), and ab initio cal-
From Very Low Mass Stars to Brown Dwarfs
3
culations (Schryber et al. 1995; Partridge & Schwenke 1997) based on independently
measured interaction potential surface. As can be seen from Fig.1 where the models
are compared to the infrared spectrum of an M8 dwarf (VB10), the water vapor opacity profile which shape this part of the spectrum has strongly change over time with
the improvement of computational capacities and a better knowledge of the interaction
potential surface. But in general, all opacity profiles converge in predicting an overopacity (or lack of flux in the model) in the K bandpass. The UCL opacity profile —
likely because of its incompleteness — could allow a seemingly correct J − K color
while not allowing a detailed spectral comparison of the NextGen models (Allard et al.
1994). It became however clear that the more recent versions of the water vapor profile
(Partridge & Schwenke 1997; Barber et al. 2006) all agree in establishing a systematic
lack of flux in the K bandpass in the models. This is the reason why the Allard et al.
(2001) AMES-Cond/Dusty model grids, based on the Partridge & Schwenke (1997)
water vapor opacity profile were never proposed for the study of VLMs, and reserved
for the study of the limiting properties of brown dwarfs.
Figure 2. A BT-Settl synthetic spectrum with Teff=2900K, logg=5.0, and solar
metallicity by Asplund et al. (2009) is shown in yellow compared to the SED of
the red dwarf GJ 866 (cyan curve). Both are plotted in absolute flux, i.e no flux
adjustment at a specific wavelength is needed. The infrared SED (top panel) is now
perfectly reproduced by the model. The agreement is particularly good in the Wing
Ford FeH bands near 0.99 µm while some discrepancy prevail in the missing CaOH
bands around 6000Å. Telluric absorption is corrected from the observations in red.
In this regard, the new solar abundances based on radiation hydrodynamical simulations of the solar atmosphere (Asplund et al. 2009; Caffau et al. 2010) help as they
Allard, F., Freytag, B., and Homeier, D.
4
predict an oxygen abundance of 0.3 dex (a factor of 2) lower than the previously used
solar abundances of Grevesse et al. (1993). The results are shown in Fig.2, where the
new BT-Settl model atmosphere synthetic spectra for a T eff = 2900 K, logg=5.0 and
solar abundances according Asplund et al. (2009) are compared to the spectrum of an
M6 dwarfs, GJ866 (kindly provided by M. Bessel, Mt-Stromlo Obs.). For the first time,
we find a perfectly fitting spectra distribution across the near-IR to infrared spectral region (the model is the yellow line). The agreement is also excellent in the optical to red
part of the spectrum in particularly in the FeH Wing Ford bands near 0.99 µm, and in
the VO bands thanks to line lists provided by B. Plez (GRAAL, Montpellier, France).
Missing opacities are, however, affecting still the spectral distribution (CaOH bands),
and the Allard et al. (2000) TiO line list becomes now too corse compared to progress
made with water vapor.
4500
4000

3500
3000
2500
0.7
0.8
0.9
1.0
1.1

Figure 3. Estimated Teff for M dwarfs by Casagrande et al. (2008) and brown
dwarfs by Golimowski et al. (2004) are reported as a function of J-K short. Overplotted are the NextGen model isochrones for 5 Gyrs (Baraffe et al. 1997, 1998) using varius generations of model atmospheres, starting with the NextGen (black line),
pursuing with the limiting case AMES-Cond/Dusty grids by Allard et al. (2001)
(blue and red line respectively), and finishing with the BT-Settl models using the
Asplund et al. (2009) solar abundances (green line).
One can see from Fig.3 that the NextGen models systematically and increasingly
overestimate T eff through the lower main sequence, while the AMES-Cond/Dusty
models were underestimating T eff compared to the averaged empirical determinations
of T eff of individual stars (Casagrande et al. 2008). This situation is relieved when using
the newer Asplund et al. (2009) abundances, and the BT-Settl models now agree fairly
From Very Low Mass Stars to Brown Dwarfs
5
well with most of the empirical estimations of T eff . Evolution models are currently
being prepared using the BT-Settl model atmosphere grid.
2.
Dust formation in late type VLMs and Brown Dwarf Atmospheres
One of the most important challenge in modeling the atmospheres and spectral properties of VLMs and brown dwarfs is the formation of dust clouds and its associated greenhouse effects making the infrared colors of late M and early L dwarfs extremely red
compared to colors of low mass stars. The cloud composition, according to equilibrium
chemistry, is going from zirconium oxide (ZrO2 ), to refractory ceramics (perovskite
and corundum; CaTiO3 , Al2 O3 ), to silicates (forsterite; Mg2 SiO4 ), to salts (CsCl, RbCl,
NaCl), and finally to ices (H2 O, NH3 , NH4 SH) as brown dwarfs cools down with time
from M through L, T spectral types and beyond (Allard et al. 2001; Lodders & Fegley
2006). Many cloud models have been constructed to address this problem in brown
dwarfs over the past decade (see Helling et al. 2008, for a review on the subject). However, none treated the mixing properties of the atmosphere, and the resulting diffusion
mechanism realistically enough to reproduce the properties of the spectral transition
from M through L and T spectral types without changing cloud parameters (for example Ackerman & Marley 2001). It is in this context that we have decided to address
the issue of mixing and diffusion by 2D Radiation HydroDymanic (hereafter RHD)
simulations of VLMs and brown dwarfs atmospheres, using the Phoenix opacities in a
multi-group binning, and forsterite geometric cross-sections (Freytag et al. 2010). We
found that gravity waves have a decisive role in clouds formation in brown dwarfs,
while around T eff ≤ 2200 K the cloud layers become optically thick enough to initiate
cloud convection, which participate in the global mixing. Overshoot is also important
in the mixing of the largest dust particles (see paper by D. Homeier in this book). In
Fig.4, 3D RHD simulations (a 350 × 350 × 170 km3 box at the surface of the star) are
shown for dwarfs with T eff = 2600 K, 2200 K, and 1500 K from top to bottom. For each
simulation, two snapshots are shown side-by-side to illustrate the intensity variation
due to cloud formation and granulation. The 2600 K case shows no or negligible dust
formation, while dust formation progresses to reach optically thick density at around
2200K, before sedimenting out again towards the 1300K regime. The T eff =1500 K
case illustrate the importance of gravity waves, where the minima of the waves reach
condensation levels while the maxima remain in condensed phase. The box of simulations are too small compared to the radius of the star to show adequately the variability
of these objects, however. See Freytag et al. (2010) and B. Freytag’s poster paper in
these proceedings for details.
These simulations permitted to account in Phoenix for the advective forces bringing fresh condensible material from the hotter lower layers to the cloud forming layers.
Our cloud model is based on the condensation and sedimentation timescales from a
study of Earth, Venus, Mars and Jupiter atmospheres by Rossow (1978). However we
had to compute the supersaturation pressure from our pre-tabulated equilibrium chemistry in order to obtain the correct amount of dust formation (as opposed to use an
approximate value cited in Rossow, 1978). We then solved the cloud model and equilibrium chemistry in turn layer by layer inside out to account for the sequence of grain
species formation as a function of cooling of the gas. One can see from Fig.5 that the
late-type M and early type L dwarfs behave as if dust is formed nearly in equilibrium
with the gas phase with extremely red colors in some agreement with the BT-Dusty
Allard, F., Freytag, B., and Homeier, D.
6
time=3660sec ∆Irms= 1.093%
400
300
300
y [km]
y [km]
time=3600sec ∆Irms= 0.975%
400
200
100
200
100
100
200
x [km]
300
400
100
time=4000sec ∆Irms= 1.561%
400
300
y [km]
y [km]
300
time=4020sec ∆Irms= 1.476%
300
200
100
200
100
100
200
x [km]
300
100
time=14000sec ∆Irms=44.539%
300
300
250
250
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100
50
50
100
150
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x [km]
250
300
300
150
100
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x [km]
time=14015sec ∆Irms=40.414%
y [km]
y [km]
200
x [km]
50
100
150
200
x [km]
250
300
Figure 4. 3D HRD simulations using CO5BOLD (Freytag et al. 2010) of a 350 ×
350 × 170 km3 of atmosphere at the surface of, from top to bottom, 2600K, 2200K
and 1500K VLMs and brown dwarfs of logg=5.0, and solar metallicity. For each
model two intensity snapshots are shown in time to illustrate the intensity variability.
models. At the low Teff regime dominated by T-type dwarfs, the AMES-Cond models also appear to provide a good limitation for the brown dwarf colors. However, the
chemistry of T dwarfs can be very far from the equilibrium in the models. The BT-Settl
From Very Low Mass Stars to Brown Dwarfs
7
6000
5000
4000

3000
2000
1000
0.5
0.0
0.5
1.0
1.5
2.0
Figure 5. Same plot as Fig.3 but zooming out and extending into the brown dwarfs
region of the diagram. This region below 2500K is dominated by dust formation
(essentially forsterite and other silicates). The limiting case AMES-Cond/Dusty
models atmosphere provide a description of the span in colors of the brown dwarfs
in this diagram.
models, which account for a cloud model, dynamical mixing from RHD simulations,
a supersaturation computed from pre-tabulated equilibrium chemistry calculations, and
the Asplund et al. (2009) solar abundances, manage to reproduce the main sequence
down to the L-type brown dwarf regime, before turning to the blue in the late-L and
T dwarf regime. The models used an age of 5 Gyrs, and in the case of the BT-Settl
models, a younger age of a few Gyrs would easily reproduce the reddest brown dwarfs.
3.
Summary and Futur Prospects
We propose, in this paper, a new model atmosphere grid, named BT-Settl, computed using the atmosphere code Phoenix which has been updated, compared to the Allard et al.
(2001) AMES-Cond/Dusty models, for: i) the Barber et al. (2006) BT2 water opacity line list, ii) the solar abundances revised by Asplund et al. (2009), and iii) a cloud
model accounting for supersaturation and RHD mixing. It is covering the whole range
of VLMs and brown dwarfs and beyond: 1000,000 K < T eff < 400 K; -0.5 < logg < 5.5; and +0.5 < [M/H] < -4.0, including various values of the alpha element enhancement. Only the confrontation of the models using spectral synthesis will allow to define the content of oxygen and alpha elements of VLMs and brown dwarfs. But it is clear that these objects are excellent constraint for the solar oxygen abundance. The models 8 Allard, F., Freytag, B., and Homeier, D. are available at the Phoenix simulator website ”http://phoenix.ens-lyon.fr/simulator/” and are in preparation for publication. However, the interior and evolution models are expected for the second half of 2011. In order to say something about the spectral variability of VLMs and brown dwarfs, 3D RHD simulations of ”the star in the box” with rotation will be required. This is our current project supported by the French ”Agence Nationale de la Recherche” for the period 2010-2015. Rotation is already modeled for a scaled down model of the Sun using CO5BOLD (Steffen & Freytag 2007) and can be applied to brown dwarf simulations. Acknowledgments. We would like to thank specifically Mickael Bessel (Mt Stromlo Obs.) for his visit to CRAL and the fruitful discussions, as well as Robert Barber (UCL) for his generous support. We thank the french ”Agence Nationale de la Recherche” (ANR) and ”Programme National de Physique Stellaire” (PNPS) of CNRS (INSU) for their financial support. The computations of dusty M dwarf and brown dwarf models were performed at the Pôle Scientifique de Modélisation Numérique (PSMN) at the École Normale Supérieure (ENS) in Lyon. References Ackerman, A. S., & Marley, M. S. 2001, ApJ, 556, 872 Allard, F. 1990, Ph.D. thesis, PhD thesis. Ruprecht Karls Univ. Heidelberg, (1990) Allard, F., Hauschildt, P. H., Alexander, D. R., & Starrfield, S. 1997, ARA&A, 35, 137 Allard, F., Hauschildt, P. H., Alexander, D. R., Tamanai, A., & Schweitzer, A. 2001, ApJ, 556, 357 Allard, F., Hauschildt, P. H., Miller, S., & Tennyson, J. 1994, ApJ, 426, L39 Allard, F., Hauschildt, P. H., & Schwenke, D. 2000, ApJ, 540, 1005 Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481. 0909.0948 Baraffe, I., Chabrier, G., Allard, F., & Hauschildt, P. H. 1997, A&A, 327, 1054 — 1998, A&A, 337, 403 Barber, R. J., Tennyson, J., Harris, G. J., & Tolchenov, R. N. 2006, MNRAS, 368, 1087 Caffau, E., Ludwig, H., Steffen, M., Freytag, B., & Bonifacio, P. 2010, Solar Phys., 66. 1003.1190 Casagrande, L., Flynn, C., & Bessell, M. 2008, MNRAS, 389, 585. 0806.2471 Freytag, B., Allard, F., Ludwig, H., Homeier, D., & Steffen, M. 2010, A&A, 513, A19+. 1002.3437 Golimowski, D. A., Leggett, S. K., Marley, M. S., Fan, X., Geballe, T. R., Knapp, G. R., Vrba, F. J., Henden, A. A., Luginbuhl, C. B., Guetter, H. H., Munn, J. A., Canzian, B., Zheng, W., Tsvetanov, Z. I., Chiu, K., Glazebrook, K., Hoversten, E. A., Schneider, D. P., & Brinkmann, J. 2004, AJ, 127, 3516 Grevesse, N., Noels, A., & Sauval, A. J. 1993, A&A, 271, 587 Hauschildt, P. H., Allard, F., & Baron, E. 1999a, ApJ, 512, 377. arXiv:astro-ph/9807286 Hauschildt, P. H., Allard, F.,... Purchase answer to see full attachment