Literature #7: Analyzing [OI] lines

The structure of protoplanetary disks surrounding three young intermediate mass stars

I. Resolving the disk rotation in the [OI] 6300 A line

G. van der Plas, M.E. van den Ancker, D. Fedele, B. Acke, C. Dominik, L. B. F. M. Waters, and J. Bouwman 

HD 172555: detection of 6300 A [OI] emission in a debris disc

P. Riviere-Marichalar, D. Barrado, J.-C. Augereau, W.F. Thi, A. Roberge, C. Eiroa, B. Montesinos, G. Meeus, C. Howard,G . Sandell, G. Duchene, W.R.F. Dent, J. Lebreton, I. Medigutia, N. Huelamo, F. Menard, and C. Pinte.

These two papers caught my eye because the both focus on analysis of [OI] line in circumstellar disks. The van der Plas et al. paper focuses on three Herbig AeBe stars while the Riviere-Marichalar et al. paper focuses on a single star that is a member of the Beta Pictoris moving group. Both papers address the importance of studying the gaseous materials in disk, van der Plas et al. noting that 99% of disk mass is gas, but they diverge in their interests in the [OI] line. It’s interesting to read these two papers together because both teams are essentially studying the same thing, the very beginnings of the formation of planets, but the stars they’re choosing to study are at different evolutionary points. This difference is reflected in their different foci. van der Plas et al. use the [OI] to refine the determination of the disk structures of their three stars while Riviere-Marichalar et al. are focused on determining the composition of the gas in the debris disk.

(I’m going to take an aside and wonder about the disk in the the Riviere-Marichalar paper. The star, HD 17255, is a part of the BMPG and as such as an age around 12-20 Myr so I’m curious to why it has a disk at all. A previous paper I read said that these types of disks dissipate long before that. So why does this star have a disk at all?)

Riviere-Marichalar et al. derived the [OI] mass in the disk by first creating an SED by compiling the observations from numerous sources. By fitting the SED with a blackbody spectrum the authors were able to determine the radius of dust distribution. Then from computing the infrared excess and the mass of the dust disk were able to determine the final dust mass (I really don’t understand how these calculations were done). From there the authors compute the [OI] mass (the paper has an appendix where they go over this computation that I haven’t gone over enough to understand).

The authors of the other paper followed a method created by Acke et al. (2005) which allows for determining disk rotation and distribution of gas in the disk. I haven’t read that paper (it is on my reading list) and van der Plas et al. leave the details of that method out of this paper.  They essentially have expected disk shapes from the SEDs for each star and then they refine the structure of disks using the [OI] line by finding the distance from the star that the [OI] line was emitted.

Both papers discuss the possible origins of the [OI] line – the thing that I’m most interested for my project. van der Plas et al. split the origin of [OI] between broad and narrow lines. According to them, broad lines in T Tauri stars are, “due to a combination of a dense stellar jets and a disk wind or magnetic accretion columns.”  Then they go onto to say the narrow lines in HAeBe stars (I’m not sure if they are saying, in general, T Tauri stars have broad lines and, in general, HAeBe stars have narrow lines or not) are from the photodissociation of OH in the upper layers of the disk.

Both of these papers used the [OI] line in interesting ways. And they both made mention of the difficulty in detecting the line so I’m eager to reduce the data I’ve gotten so far and see if I’ve picked up on [OI]. I will definitely have to think about how each of these terms used the [OI] line and if and how I can apply either of these methods.

Literature Review #6: Parameters Herbig Ae/Be and Vega-type stars

B. Montesinos, C. Eiroa, A. Mora, B. Merin

Previous studies involving the modellin of circumstellar disks, including the previously discussed Manoj et al. (2006), have used basic parameters for the central star such as assuming solar metallicity. This will likely lead to incorrect results since the energy received by the disk from the star, determined by the star’s mass, radius, and effective temperature, affects disk characteristics such as geometry and contribution to the SED. The goal of the paper by Montesino et al. is to determine the stellar characteristics of 27 Herbig Ae/Be stars. The characteristics they found are effective temperature, surface gravity, and metallicity. This was interesting for me to read about because, as the paper points out in its introduction, that the circumstellar disks for T Tauri stars and Herbig Ae/Be stars dissipate at different rates. The disk of a classical T Tauri star will be gone when the star is 5-7 Myr while the disk of a HAeBe star will be gone when the star is 3 Myr. I don’t know, and I think nobody knows, why the lifetime of the disks is so different from the low mass to intermediate mass stars. An accurate model of CSDs based on accurate source parameters is very likely key to understanding this.

The basic method that the authors used to characterize the properties of the stars is to compare observed spectra with synthetic spectra. They observed the sample set using the Calar Alto Faint Object Spectrograph (CAFOS) to get intermediate resolution spectra to use Balmer line profiles in order to estimate stellar gravities. To determine the metallicities of the stars high-resolution echelle spectra was taken with the Utrech Echelle Spectrograph and the William Herschel Telescope.

The first parameter that needed to be found was the effective temperature of each star. To do this the authors compared observed spectral energy distributions with a grid of low-resolution synthetic spectra with different effective temperatures and then choose the best fit.

Stellar gravities, which is the surface gravity of the star not the potential well created by the star, were found by comparing the wings of the Balmer lines with synthetic profiles from Kurucz (1993). Since the wings of the lines are related to how the gas is moving a high surface gravity corresponds to a broader line.

Metallicities were found by comparing synthetic spectra found using SYNTHE (Sbordone et al 2004) with observed high-resolution spectra. Basically the made different spectra with the same effective temperature and surface gravity of the stars computed for different metallicities with the observed spectrum.

In the paper the authors specifically point out RR Tau for the difficulties involved in using the above methods to determine its parameters. Because RR Tau varies in a complex way the authors tried to single out spectral features that had as little variation as possible, features that would originate from the photosphere, to determine the parameters.

The paper presents a single SED for RR Tau and I don’t think it can ever be completely accurate to present a single thing of anything for RR Tau. I understand that they tried to minimize the influence of the variation but I think it would be better to make SEDs of RR Tau at many different places in its brightness. I talked to Hugh about the idea of doing this with HYPERION and my Gemini observations but he said that because my wavelength region is so narrow it’s unlikely that the SED shape will change that much. So he suggested I do full spectral fits instead of SEDs using line indices. So I’m going to start looking into that.

Literature Review #5: Emission Lines and Accretion Disks

P Manoj, H.C. Bhatt, G. Maheswar, S. Muneer

This paper is centered around the question: How long does emission line activity persist in young stars? This is a relevant question because emission line activity is associated with the presence of an active accretion disk around the star in both T Tauri and HAeBe stars. Since the matter that feeds accretion disks is taken from the the gas rich disk that surrounds the star we can characterize how long it takes Circumstellar disks to get to the planet forming phase by looking at when their emission line activity drops off. As the circumstellar disk loses material to the beginning stages of planet formation and other processes the rate of accretion goes down. This paper cites various other papers to say that the most stars lose their inner disks by 5 Myr. They don’t discuss what happens to to the outer disk and they also don’t explicitly say that once the inner disk is gone so is the accretion disk.

The authors compare low-mass T Tauri stars to intermediate mass HAeBe stars. Apparently, the origin of emission lines in pre-main sequence T Tauri stars is understood to be from magnetospheric accretion (Uchida & Shibata 1985, and several other papers). There is no such general understanding for emission lines in HAeBe stars but it is thought to be similar. In Muzerolle et al. (2004) the authors were able to apply the magnetospheric accretion model to HAeBe stars.

The Manoj et al. sample includes 91 HAeBe stars including RR Tau and a couple other UXors such as KK Oph. They found the equivalent width of H-Alpha for each of these stars and compared those values to the stellar age of each star. They found that H-Alpha line strength decreases with increasing stellar age and thus accretion gradually declines during the pre main sequence phase. They also find that inner disks dissipate on a similar timescale which they say suggests that inner disks dissipate after the accretion activity has fallen below a certain level. This seems like a backwards interpretation to me if it’s the inner disk is what feeds the accretion disk.

I’m also curious about the validity of only including one equivalent width value for H-Alpha per star. For UXors like RR Tau the equivalent width of the H-Alpha line varies so it’s curious to me at what point in RR Tau’s variation did the authors get the equivalent width.

Literature Review #4: Flaring and Shadowing

On the interplay between flaring and shadowing in diss around Herbig Ae/Be stars
B. Acke, M. Min, M.E van den Ancker, J. Bouwman, B. Ochsendorf, A. Juhasz, and L. B. F. M Waters

Meeus et al. (2001) used SEDS of a sample of Herbig stars to determine that members of the class can be split up into two groups; group I have much larger far-infrared excess than those of group II. This paper focuses on the proposed physical reason for this discrepancy which is that stars in group II have an outer disk that is protected from stellar radiation from the “puffed up” inner disk. However there has been evidence that if the disks of group II have a large flaring angle (meaning not a steep wall) then the outer disk is not as protected and will have IR excess comparable to the stars of group I.

This paper is actually a letter to the editor with a follow up paper planned. But it presents the preliminary results of 33 Herbig Ae/Be sources with the goal of better understanding the disk geometry and the nature of that geometry.

This was a fun paper to read after the last because it compliments the previous paper in focus but is much more digestible. The last paper was about the changes in dust size and structure as the disks around these stars age and this paper is about how the dust size affects the structure of the disk. The authors did this by comparing their spectra to self-consistent passive-disk models (I don’t know what that means). They found evidence that in the inner disks (~1 AU from the star) and the outer disks (~10s of AU) of the sources in their sample are related to each other. In an attempt to understand the physical cause they made disk models using the radiative transfer code MCMax (Min et al. 2009). Here is a list of their input parameters:
1.) Central star is a main sequence of spectra type A0 (T_eff = 10,000 K, R=2R_sun, M_*=2.5 M_sun)
2.) Assumed hydrostatic equilibrium and thermal coupling between dust and gas
3.) gas-to-dust ratio = 100
4.) Dust is made of astronomical silicates with a grain size of 0.12 microns

They used these parameters to compute models at different dust masses (m). They also varied the surface density power law and the inclination of the disk system (i).

They found that different dust masses gives the most change and spread in the models. The surface density and inclination give either inconclusive pro negligible results.

The authors conclude that the geometry of the outer disk is dependent on the inner disk and that the degree of flaring is determined by the mass of small dust grains and the degree of shadowing is dependent on inner disk scale height.

Literature Review #3: Characterizing dust


As I’ve written about previously, this term I will be working on a model of the circumstellar environment around RR Tau. Both Hugh and Bernadette suggested looking at Spitzer data as a way to inform my work. Spitzer is a space telescope that is run by NASA that observes in the infrared. Apparently infrared is where you want to look when you want to characterize dust.

This is the first paper that I read specifically for my tutorial. A lot of it was a bit over my head and I think that I’ll have to keep coming back to it as I learn more. Overall, though, it had some really great information that I think will help my project greatly.

It’s the first paper in a series of data presenting the data from 40 solar mass T Tauri stars and 7 intermiediate mass Herbig Ae stars in the ~5-35 micron region. And, yes, RR Tau just happens to be one of the stars included in their set. The team was able to resolve both the 10 an 20 micron features. The main goal of the paper is to characterize the dust grains in circumstellar disks and determine properties of the disks from the dust grains. This is important because changes in grain size and composition have been linked to disk properties and planet formation but the rates of grain growth and processing in disks is not understood completely. Not to mention the underlying mechanism instigating all of that.

The authors of the paper linked grain features to disk features by comparing the spectroscopic results with synthetic results given by models and doing a statistical analysis over the whole set. Most of their observational focus seems to be on silicates. It’s not clear to me why silicates are so important other than maybe that just happens to be what circumstellar disks are made out of (?). Since it’s possible to determine grain size, shape, and composition from the strength, peak, and shape of an emission line the authors modeled the opacity of the sample grains various, shapes, sizes, and compositions. They then compared the grain opacities to the observed emission without the influence of continuum.

The authors go on to interpret the changes in the line shape and strengths as source-to-source variations in grain size. This is apparently a unique way of doing this but it seems sensible to me. What (I think) this means is that they cannot find a general rule for the type of grain in certain stars. Anything like, “In this type of star you’ll find this type of dust grain”. They attribute this to fast grain growth and turnover (processing). Basically, whatever the process that makes silicates grow and go from amorphous to crystalline has to be relatively quick in order for there to be such disparate source-to-source variations. Some conclusive things they did find is that the equivalent width of H-alpha is not correlated with disk age. On this they say, “This suggests the importance of turbulence and regeneration of small (micron-sized) grans on the disk surface.” and that M stars show much flatter silicate features, which means larger grain sizes, than A/B stars. They tentatively attribute this to the differences in disk temperature which affects where in the radius the emission line comes from. This suggests that the grain size differs as you move through the radius.

Literature Review #2: Is RR Tau a binary?

On the Nature of Cyclic Light Variations in UX Ori Stars
Grinin et al., Astronomy Letters

Vladimir Grinin is a name that often pops up when looking into UXors. He has been studying them for decades which is quite an advantage in variable star research. He had his his co-authors conducted decades long photometric monitoring on many UXor type stars that produced a series of papers. This paper is the first in the series and includes the results for RR Tau.

They observed RR Tau for 673 nights spanning 30 years. They found evidence of possible periodicity on timescales of ~8.6 and ~3.4 years. As far as I know this is the first discussion of long photometric variability in UXors and something I haven’t really seen discussed until the Maria Mitchell paper. It is interesting to me that our timescales are so different. The long term variability that we discussed in our paper was on the order of 100 days but Grinin et al. are talking about many years. I doubt that this has anything to do with the mechanism of variability that I want to look into for my thesis but it is fascinating to add another layer of complexity to this stellar system.

The change in continuum light over time.

The above figure is from a small review paper they wrote summarizing all their results.

The authors attribute the variation to binarity mostly because it is periodic and because the percentage of binary stars among Herbig Ae/Be is high. From what I can tell from their paper that they don’t directly attribute the periodicity to the companion star passing infront but rather the companion star is disrupting the the column density of the circumstellar disk. They go onto assert that the a binary system accounts for the short term activity that characterizes UXors also because of disruptions. This possibility could be relevant to my investigation on the medium term timescale.

I don’t understand the differing timescales they report, I don’t see that in the light curve. I’m going to read the other papers in the sereis and see if they shed so light on it.

Literature Review #1

Circumstellar disks around Herbig Be stars

T. Alonso-Albi et al., A&A

This paper presents the last of the data from a survey of Herbig Be stars using the Very Large Array (VLA) and the Plateau de Bure (PdB). The point of the survey was to investigate the properties of intermediate mass stars to determine the occurrence, lifetime, and evolution of the disks surrounding them.

Herbig Be, along with Herbig Ae, stars are pre-main sequence. Herbig Ae stars have strong infrared excess have disks that are similar to T Tauri stars. Herbig Be stars tend to be more modest their infrared excess and their disks are flatter than Herbig Ae stars. While no one is quite sure what causes this divergence in disk geometry T. Alonso-Albi et al. put forth the idea that Herbig Be stars lose a large portion of their mass before they hit the pre-main sequence phase.

T. Alonso-Albi et al. report the results of 6 objects studied in mm wavelengths. They chose mm wavelengths for their observations since optical-NIR and mid-IR observations are limited in only providing information about the disk surface and cannot give the disk mass. Also, observations at mm wavelengths allowed them to determine the size and properties of the large grains of dust that partially make up the disk.

The authors claim that a two component model is necessary to fit the SED  since the envelop surrounding the disk influenced so much of the observed flux. Out of the six objects observed, four were found to have disks. They found that the disk mass was usually < 10% of the entire envelope and 5-10 times lower than the disks around Herbig Ae stars.

They propose that photoevaporation is the cause of the dissipation of the disks. From what I understand their argument is that this happens with HAe stars as well, it’s just that the time scale is shorter. I’m not entirely clear why there would be such a significant difference. I’m actually fuzzy on physics of how these star/disk/envelope work on a detailed level. As I work on this project I think it will be necessary to learn more about the astrophysics in order to make any substantial and novel claims.