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.

September Project Update

I got the proposal submitted on August 18th! I will put the full proposal up when I get the chance to add a wiki element to this site

O I narrowband filters are commercially made so I had to give up on that idea.

The primary mirror at the Bennington observatory is having Issues. The coating doesn’t seem to be taking at all. Worst case scenario it has to be repolished. I bet we could get some volunteers at Stellafane to help us out.

Speaking of Stellafane I should get in touch with Wayne again.

Maria Mitchell as been MIA as far a communication. I’m hoping to hear back from Gemini soon so I have something new to tell the MMO Folks.

Official start of the term is tomorrow!

Disks and things

Paddling has been wearing me out. We have practice every day for about an hour and half. On Thursday and Friday last week we paddled 4.5 and 3.5 miles respectively and now we’ve gotten into to doing half mile sprints. I have gotten to the point where whileI’m not horribly sore the next day which is really satisfying it still takes a lot out of me. Our regatta is this Sunday and then that’s the end of it. It’s been a ton of fun be out on the bay every day but I’m looking forward to be back to normal energy levels.

I spoke with Bernadette again on Monday. I updated her on the papers I have been reading and the conversation I had with Vladimir. She was particularly interested in the Grinin paper that proposed the binarity of RR Tau. I need to email her the reference so she can take a look at it. I have a few questions on that paper and it would be good if she can answer them.

For most of our conversation we talked about what kind of data I want to get focusing on what wavelength region I should be interested in. In her 2002 paper Bernadette detected about 15 features over a large wavelength range. We decided we wanted to focus on a smaller wavelength range, and thus less features, so we can get higher resolution data. My job is to play with the Integration Time Calculator and decide the observing parameters: the best grating, wavelength range, etc…Then I’m going to write the proposal for time despite the fact that it doesn’t need to be submitted until August when RR Tau finally comes back up. I’ve never written a proposal before so it seems best to start as soon as possible so I can get feedback and make sure everything is correct so when it’s time of submission I can do it immediately.

Bernadette sent me a research statement she wrote a few years back about some high resolution spectra of RR Tau she obtained but never published.

Two Fe II lines overlaid in velocity space.

In the statement Bernadette reiterates the key point made in her 2002 paper, that the spectral signature of RR Tau changes with continuum variations. When the star is at its brightest there are strong metallic absorption lines and at its dimmest there are faint metal emission lines that are associated with the stellar photosphere and an optically-thin disk atmosphere respectively. Understanding this is completely tied to understanding the nature of the obscuring material. What is it made of? How does it move? Evolve? What is it’s structure?

The general outline of discerning the fundamentals of the obscuring material is to use the characteristics of the light curve with spectral information to constrain models which has been fairly rudimentary. I think it will be a real step up to use narrow band filters along with spectroscopy. Also, depending on the resolution we’re able to obtain we might be able to resolve the disk in greater detail than what has been previously managed.

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.