BeACoN Meettings

Xuxu - Pseudo-photospheric models

Or disk brightness profile.

τ_z ≥ 1 (vertical optical depth) sets the transition between thin and thick optical regime for each λ.

Effective radius ^‒R(λ, n₀, i) is defined at τ ∼ 1 at midplane. This is paper I.

For paper II, Rodrigo is proposing fitting AKARI, WISE and X infrared measurements in and statistical analysis.

Leandro - Fitting Be stars lightcurves

MACHO survey has a spectroscopic correspondent.

OGLE does not have. But they are likely Be stars, strong candidates. B and I filters observed.

Diffusion equation depends on M, T, α

Assuming to exponential components: a growing one and another for dissipation.

The fitting is done in a grid of singleBe models.

Robert and radio

Robert continued a project to observe stars on radio. The observations (~5 stars) show that disks extends up to 200 200R_eq. Fitting the SED, from visible to radio wavelengths, shows a systematic slope of n ≈ 3.1 − 3.2 ≠ 3.5 from the steady-state solution, a pure Hydrogen disk.

This is likely the result of disk temperature decreasing with radius. MAYBE hydrodynamic considerations yield to lower disk temperature to a slope of 3.0.

Other explanation is the present of Helium as a disk coolant. The presence of Helium in the radiative transfer would change the disk from a isothermal structure to a decreasing temperatures yielding 3.0. This was suggested by earlier studies (~1970's) on stellar WINDS that could be applicable to Be disks.

Dai and the B[e]

Dai is fitting and B[e] star (RS ??) in one of the Magellanic Clouds. The distance is 49.97+/-1.11 kpc (Pietzyński, ??). For Dai master, the Ṁ value used for fitting was 1 − 10×10^− 6M_⊙/yr/sr. She showed the results for 5×10^− 5M_⊙/yr/sr, and the results appeared to do not converge...

Alex suggested Dai to present in a future meeting the thesis of Will Robson. One of the main results of his thesis is that, the absence of 9.7 microns feature in some proto-planetary disks can be explained by the creation of a ice mantel around small grains in the disk. This conclusion was based on high-energy particle collisions with dust particles, and offers an alternative explanation to the absence of the feature due to an average bigger size of the disk grains (not likely).

André and the Polarization as a diagnostic tool

Summary of Bjorkman & Bjorkman, 1994, (BJ94) on spectropolarimetry around the Balmer jump (BJ; UV observations). The lower level of polarization before the jump could be explained by the central-star high-rotation. This was compared to the polarized SED of the star Zeta Tau.

A positive slope in the polarized spectrum in visible was not expected and was discovered by Klement et al., 2015.

André showed images and surface brightness curves of the unpolarized and polarized regions of the Be disks. Conclusion is that polarization is coming from regions very close to the stellar surface.

The conclusion of the positive slope only occurs for low-density disks (Σ₀ < 0.1 g/cm2) and stars with high-rotation (R_eq ⁄ R_p ≳ 1.4).

A new window will be open by the Spektr-RG (SXG or SRG) satellite (https://en.wikipedia.org/wiki/Spektr-RG), having a spectropolarimeter at UV wavelengths. Other UV satellites are Astrosat (https://en.wikipedia.org/wiki/Astrosat) and Spektr-UV (https://en.wikipedia.org/wiki/Spektr-UV; but it appears to be just another name of the SRG).

Question to understood: Why the depolarization is stronger in the UV (i.e., before the BJ) than in the visible (after BJ)? Answer in BJ94.

According to Brown & McLean 1977, the polarization of Be disks can be understood as function of a shape factor (γ) and ^‒τ, the mean scattering optical depth. According to Alex, the rotation necessarily would decrease the mean scattering optical depth.

Alex did not agreed with the models André showed with a increasing polarization with rotation in the BeAtlas. Rodrigo emphasized that the goal was not the level, but the slope. He answered saying that the slope was a function of the level, so the directed comparison between models was not accurate.

Something that wasn't discussed in the meeting: the polarization in the BeAtlas models decrease with rotation, as "expected", if a inclination angle o i ≲ 60 deg is considered (at least for low-density disks). Rodrigo told me that me models shown were for i ∼ 70 deg, were the maximum of polarization occurs. IMHO, for i > 60 deg effects of self absorption become important so that the polarization can increase.

I took a quick look in BJ94, and they predicted I quite strong polarization decrease for i = 60 deg. On the other hand, they used a quite different model for the star (β = 0.25) and a strange density distribution for the envelope:

It appears that, while André is exploring the diagnostic potential of the phenomenon, Alex wants to explore the theoretical foundations of it. IMHO, it should be assessed be possibility of "redo" BJ94 with more precise envelope models and Rodrigo's opacity description...

Rodrigo's Papers (I and II)

Rodrigo develop an analytic model to the emission of Be systems: star + optical thick + optical thin disk components. It is possible to predict 2D brightness distributions to (roughly) any inclination angle not edge-on (equator-on).

The model works good for mid/distant-IR, because of gauss factors.

Rodrigo took archive IR data (IRAS, AKARI, etc) and compared the best fit using Frémat 2004 atmospheric modeling (2MASS, due to short wavelengths, is not good to model). He fitted n₀ and m, the volumetric density exponent, to infer the Ṁ. His Ṁ determinations are 2 order of magnitudes lower than the ones predicted by Waters 1987. This agrees with Anahi's work on late 2014.

He found:

• 3 peaks in n₀ distribution: hotter stars tends to create denser disks (Teff>18000; 15000<Teff<18000; Teff<15000)!

• NO DEPENDENCE of Teff with m: MYTH of late-B types have more stable disks busted!

• It is possible to do m-age relations. Ages of disks, and metallicity correlations.

• No m < 3.5 and high n₀: limits on correlations.

Bednarski: Polarimetric reduction

Discussion with Leandro, Rodrigo and Alex

SPH connected with Xuxu's models (Despina+Rodrigo). Leandro is working on SingleBe and Xuxu's models.

Bruno's talk: emcee and UV bump at 2200Å

emcee: better results fixing one parameter (e.g., oblateness). Otherwise, parameters are kind of undetermined.

UV bump (2200 Å) = 4.54 μm^− 1. Whole bump between 3 to 6 μm^− 1.

3 kinds of extinction? = bump strength can change depending on something(?) and on the kind of grains present. This is also related with R_V.

Moser's talk: Aeri ------------------^ No further details (see log, perhaps).