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Sci TR-21

Updates: 03.05.2012 (Ünal)

2.1 Stars and Stellar Evolution

1. Young Neutron Star Populations

Several young neutron star populations have been identified in the last two decades. These isolated neutron star systems, namely, anomalous X-ray pulsars (AXPs), soft gamma-ray repeaters (SGRs), dim isolated thermal neutron stars (XDINs), rotating radio transients (RRATs), and compact central objects (CCOs) show striking similarities and also rather different peculiarities. For instance, AXPs, SGRs, and XDINs all have spin periods clustered to a narrow range (2–12 s). AXP and SGRs (also known as magnetars) show super-Eddington soft gamma-ray bursts that are not observed from the other populations. In comparison with other systems, XDINs are older (characteristic age ~106 yr) and have lower X-ray luminosities (1030–1032 erg/s) and have not been detected in radio bands. All known XDINs lie within a distance of 500 pc, which indicates that XDINs could have birth rates higher than radio pulsars. RRATs have the unique property of irregular short radio bursts lasting much less than the periods of the sources. Based on the observed number of sources, it was speculated that RRATs could be progenitors of XDINs.

Different evolutionary paths creating different neutron star populations are likely to be due to different initial conditions: magnetic dipole moment, initial spin period and presence or absence (and properties) of fallback disks around the neutron stars (Alpar 2001). Physical properties of these young systems are not clear yet. AXP and SGR bursts require strong (magnetar, >1014 G) fields, nevertheless it is a matter of debate whether these fields are stored in dipole or higher multipole fields of the star. In the magnetar model (Thompson and Duncans 1995), neutron stars in AXP/SGRs are assumed to rotate in vacuum slowing down with magnetic dipole torques. This requires that the magnetar fields should be in the dipole component. This model cannot explain some basic properties of AXP and SGRs, like the period clustering of the sources in 2-12 s range. On the other hand, in the fallback disk model (Chatterjee et al. 2000,Alpar 2001) the strength of the dipole field is required to be <1013 G in order explain rotational, optical, IR and X-ray properties of the sources. In this model, the rotational evolution of the neutron stars are governed by the disk torques acting on the dipole component of the magnetic field.

First evidence for the presence of fallback disk around a young neutron star was found through MIR and NIR observations of AXP 4U 0142+61 (Wang et al. 2006). Most of the other AXP and SGRs were detected in different NIR bands. These sources show variations in X-ray and IR luminosities. Observation of these sources in X-ray enhancement phases in different luminosity regimes are very important to understand the physical mechanisms producing the observed broad data. In FDM, the source of the IR radiation is the irradiated active disk, while in the magnetar model it is attributed to magnetospheric emission from a dipole magnetar field. In the frame of the fallback disk model, it is estimated that not only AXP and SGRs, but also XDINs, possibly RRATs are the sources evolving with fallback disk with conventional magnetic fields. So far, XDINs could not be detected in IR bands. Present upper limits in the IR bands are low and consistent with the expected IR fluxes from these sources. The faintness of XDINs in the IR bands could be explained by their relatively low X-ray luminosities and the corresponding weak X-ray illumination of the disk. Nevertheless, the known XDINs are all at small distances and likely to be the old XDINs. The number of these sources at the ages of AXP and SGRs (~103 – several 104 yrs) are expected to be about an order magnitude higher than that of AXP and SGRs. These younger XDINs, are probably have higher X-ray luminosities and, when identified in X-rays by means of rotational properties, they could also be detected in the NIR and MIR bands.

Among these young neutron star populations, some of them could have evolutionary links and others might completely different evolutionary paths. The aim is to explain the evolution of all these systems including radio pulsars (Çalışkan, Ertan and Alpar 2012) within a unified model with different initial conditions. This could also help in understanding the details of source properties like dipole moments and initial disk masses. It was shown that the general rotational and X-ray properties of AXP and SGRs can be accounted for by the evolution of the neutron stars evolving with fallback disks for certain ranges of initial disk masses and with dipole fields less than 1013 G on the surface of the neutron star (Ertan et al. 2009). A similar work on the evolution of XDINs is in preparation (Ertan et al. 2012). Our model results on the disk properties indicate that a significant fraction of all young neutron star systems, in different evolutionary phases, could be detected in the near and mid IR bands depending on the distances of the souces.

Updates: 13.05.2012 (Mükremin Kılıç)

2. White Dwarfs

A 4m telescope with an infrared imaging camera and Y, J, H, and K filters would be extremely useful for studying white dwarf atmospheres and cosmochronology, their stellar/substellar companions and debris disks.

Atmospheres: Cool M dwarfs, brown dwarfs, and white dwarfs show near-infrared flux deficits due to collision induced absorption (CIA) from molecular hydrogen. Imaging in the near-infrared will provide spectral energy distributions of cool white dwarfs that can be used to constrain the effects of CIA opacity. There are clear problems with the current CIA opacity calculations; observations of a large number of cool WDs in the infrared can help constrain the models for cool WDs.

White dwarf cosmochronology is a powerful method to constrain the ages of the oldest stars in the solar neighborhood, as well as in open and globular clusters, and the Galactic halo. A precise temperature measurement for a white dwarf can be obtained from modeling its optical and infrared energy distribution. This temperature measurement can then be used to estimate the age of a white dwarf, if the distance is known. An infrared camera on a 4m telescope can be used to study a large number of white dwarfs in the Galactic disk and halo to constrain their spectral energy distributions, temperatures, and ages.

Substellar companions: Brown dwarf companions to WDs are important benchmarks for the spectral models for brown dwarfs and massive planets. WDs have well determined cooling ages, which also constrain the ages of their substellar companions. For WDs hotter than 5000 K, Balmer lines are visible in the optical and these lines can be used to estimate Teff and logg for these objects, which give an age estimate. In fact, the first candidate brown dwarf was found around a WD more than two decades ago (Becklin & Zuckerman 1988). Even though only a few more WD + Brown dwarf systems have been discovered since then (Farihi & Christopher 2004; Steele et al. 2009; Day-Jones et al. 2011), the discoveries to date include the coolest known brown dwarf candidate (Luhman et al. 2011, 2012) and brown dwarfs that survived common envelope evolution (Maxted et al. 2006; Debes et al. 2006). New discoveries from a near-infrared imaging campaign on a large number of WDs would have a significant impact on this field and would enable us to calibrate the models for substellar objects.

Debris disks around WDs: Recent discoveries of debris disks around WDs suggest that at least a few per cent of WDs have remnant planetary systems. Since the progenitor main-sequence stars of the WDs in the solar neighborhood range from 1 to 7 Msun, the frequency of disks around WDs can be used to constrain the frequency of planets around intermediate-mass stars. A large near-infrared imaging survey (including the K-band) on a 4m telescope can identify many new WDs with debris disks and constrain the frequency of remnant planetary systems around their progenitor stars. We still do not know if 3-7 Msun stars form planets or not. This could be one way to answer that question.

Updates: 30.09.2012 (Sacit Ozdemir)

3. Eclipsing Binaries

Eclipsing binaries (EB) are composed of gravitationally bound two stellar components which are orbiting around their common center of masses to each other and exhibit eclipses during every orbital period if the inclination angle is fit. Some of them may house more than two componets which are called multiple stellar systems. Owing to mutual eclipces, i.e., transit and occultation events, and radial velocity measurements, the eclipsing binary systems provide much more precise parameters of masses, radius, gravities, densities, etc., than single or non-eclipsing stellar systems. These information are very essential to check the theoretical models on stellar evolution. There are, depending on evolutionary satages, few sub-types of binary systems such as Algol, Beta Lyrae, W UMa, X-ray binaries (XRBs), cataclysmic binaries (CVs).

Updates: 15.10.2012 (Ayse Ulubay Siddiki)

4. Stellar Associations

Stellar associations are groups of loosely bound young stars that share a common origin. They are mostly populated by O and B stars (OB associations), or T-Tauri stars (T-associations). Altough the member stars sparsely fill in the region they occupy, they move with similar velocities. When an association is identified through the common velocities of its stars, it is sometimes called a co-moving group. Since the system as a whole is not gravitationally bound, it is expexted that stellar associations have a relatively short lifetime, after which its stars disperse through the galaxy.

The current effort in the studies of stellar associations can be summarized as follows:

- Identification of new stellar associations

- Studying the early evolution of circumstellar disks at a stage when the planetary systems are believed to form (Gautier et al. 2008)

- Determining the shape of the initial mass function

NOTE: The topics, hence the requirements from DAG, for this subsection mostly overlap with those under subsections 1.2 Exoplanets, and 2.3 ISM and star formation. Please consider to merge this subsection with the ones listed above.

Updates: 01.10.2012 (Sacit Özdemir, Timur Şahin, Yücel Kılıç)

5. Asymptotic Giant Branch (AGB) Stars

The asymptotic giant branch (AGB) stars can be described by evolved low and medium-mass stars which have a central and inert core of carbon and oxygen, a shell of helium burning into carbon, another shell where hydrogen is undergoing fusion forming helium. However, they show spectral composition that is similar to normal stars.

Since AGB stars show large amount of mass ejection, as a result, they are surrounded by a circumstellar (CS) shell of gas and dust. CS shell has an important affect on converting stellar spectrum into a shifted towards infrared wavelengths. This makes IR observations very esential to study the structures of CS shells. In addition, since AGB stars exhibit variability periods up to thousands of days, they should be monitored for long time intervals.

References

Alpar 2001
Becklin & Zuckerman 1988
Chatterjee et al. 2000
Çalışkan, Ertan and Alpar 2012, in preperation
Day-Jones et al. 2011
Debes et al. 2006
Ertan et al. 2009
Ertan et al. 2012
Farihi & Christopher 2004
Luhman et al. 2011, 2012
Maxted et al. 2006
Steele et al. 2009
Thompson and Duncans 1995
Wang et al. 2006
Gautier,Thomas.N., III; et. al. (2008), ApJ, 683, 813

Updates: 15.10.2012 (Sacit Özdemir, Timur Şahin, Yücel Kılıç, Ayşe Ulubay Sıddıki)

What can be done with DAG

Observational Studies with DAG regarding Young Stellar Associations

Determining stellar membership: Requires the measurement of radial velocities through high resolution (R~30000) spectra.

Circumstellar discs: Determining the properties of the disc requires optical and IR photometry (down to a few microns), as well as high spectral resolution for which the contrast between the star and the planet becomes larger.

• Most of the astronomers and astrophysicists in Turkey have been experienced on eclipsing binaries. Main requirement of these colleagues is to have supplementary radial velocity curves of eclipsing binaries for obtaining a full solution of light curves. Therefore radial velocity curves extracted from either optical or NIR spectra of eclipsing binary systems will largely satisfy the Turkish scientific communitiy.

• Algol type systems have a special importance since they are one of the test laboratory of apsidal motion predicted by the theory of general relativity. Existence of apsidal motion in Algols can be only verified by timing analysis of secondary minima which is difficult to recognize because of the large difference in surface temperatures of the components. However, Planckian energy distribution of the stars offers a chance to have an evident secondary minima. On this occasion, NIR photometry carried out by DAG facilities is going to be useful to determine secondary minima of Algol type systems.

• As the number and sensitivity of high energy experiments in orbit increase, new members of XRBs and CVs, a very small but amazing group of binary stars, are continuously added to the catalogues. Therefore, follow-up observations of such energetic binaries in optical and IR bands will have great impacts on scientific arena.

Observational Studies with DAG regarding AGB and post-AGB stars

In last twenty years, the availability of large IR databases from space-borne telescopes like ISO, IRTS, MSX, SPITZER etc. has substantially improved our knowledge on the AGB stars in general. For instance, their distances are now more reliable than before (see Bergeat & Chevallier 2005 for corrected Hipparcos distances of AGB stars). However, observations from space-borne telescopes in the IR present also disadvantages. In particular, duration of the operational period is quite limited and AGB stars are generally studied with a single-epoch observations (regardless of their variability) therefore our understanding of basic physical parameters is hindered. Solution is ground-based observations at IR wavelengths. Here we list some fundamental topics regarding AGB stars in general and brief information on selected example IR instrumentation.

Mass-loss (efficiency): For mass-loss, its duration and the chemical composition of the matter ejected by is fundamental to understand the AGB phase.

8.5 and 12.5 microns are good indicators, also permits a classification of the chemical properties of the circumstellar envelopes (see also Ciprini & Busso 2003).

Aim: simple relations connecting the strength of the stellar winds to the photometric properties and then to the chemical properties (e.g. enhancement of newly produced carbon and s-elements in the photosphere) of AGB stars.

Extra info.: Photometric observations of AGB stars in mid-infrared, at 10 micron, is important since cool dust normally dominates over the photospheric flux (Corti et al. (2003), Marengo et al. (1999), Busso et al. (1996)).

ISOGal sample, consisting of spectroscopy of 107 candidate AGB stars, could be scrutinized as a start (Schulteis et al. 2003) and sample could be extented to include LMC and SMC AGB stars.

Population synthesis: AGB stars are pulsating variables: one fundamental question awaits for an answer is how their average spectra differ from those of non-variable red-giants and of red supergiants since population synthesis techniques rely on stellar libraries with a broad spectral type and wavelength coverage (for stars of known physical parameters).

Spectral Energy distribution: Necessity for re-analysis of Galactic AGB stars at Infra Red wavelengths to determine their energy distribution, mass losses, and absolute magnitudes (see van Loon et al. 2001, 2005 for planned database).

Example: TIRCAM2 mounted on TIRGO, an italian infrared telescope (locate at 3200m) in Switzerland (Persi et al. 1994). TIRCAM2 consists of five narrow band filters (10% bandwidth) between 8 and 13 m, with the N broadband filter and a circular variable filter having a spectral resolution of 3% in the 8–14 m range.

Near-IR: Cryogenic Array Spectrometer and Imager (CASPIR), a 256x256 InSb detector, mounted on the Australian NationalUniversity 2.3-m telescope, is another example instrument providing direct imaging and spectroscopic capabilities in the 1 – 5 micron wavelength range (Table 1.).

Grism Wavelength range
J 0.99 – 1.32
H 1.50 – 1.81
K 1.98 – 2.47

Table.1 Overview of grisms for CASPIR.

Studying (O-rich) Galactic Bulge AGB stars:

  • Metallicities are in the range -1 < (:cell PQA(PSS(Fe/H):) < 1.
  • They're suitable for follow up observations.
  • They're all oxigen rich with silicate dust features

Measuring Luminosities of AGB stars:

Another problem awaits for a solution for AGB stars is the bolometric variability which can be clearly distinguished in the region of the mid-IR of AGB star spectral energy distributions (observed at different epochs). These kind of observations performed during variability show that period and luminosity is not only shifted at different wavelengths bur also subject to intrinsic variations (possible reason could be a non-thermal energy source). There is still no consensus on the origin of variations in the bolometric magnitudes of the AGB stars.

Photometric Monitoring of AGB stars:

  • As shown by Le Bertre et al (1994), a seperation between C-rich and O-rich AGB stars can be performed on the basis of infrared colour diagrams. By combining near IR observations with mid-IR colours taken from satellite databases (such as IRAS, ISO, SPITZER), much larger samples of sub-types of AGB stars, including also extra-galactic targets, can be studied.
  • In a series of papers, Guandalini et al (2006), Guandalini & Busso (2008) and Guandalini (2010) showed that near to mid infrared colours are crucial to study stellar winds, luminosities and mass loss rates among the sub-types of AGB stars, as M, S, SC, Mira types. Studying such physical parameters seems possible with the IR instrumental facilities of DAG telescope.
  • The AGB stars also include Mira type variables which have pulsation periods of hundreds of days. Phase lags between optical and IR light curves of Miras are very well recognized (see e.g., Price et al. 2010). The Miras as well as other pulsating AGB stars can be photometrically monitored, in both optical and IR regions, to investigate the nature of CS envelope.
  • Asteroseismology is one of the essential tools to investigate stellar interiors. There are some orbiting satellites such as Kepler, CoRoT, etc., to preciselly monitor the possible exoplanet transits. Precise photometric observatins of these missions are very convenient to study stellar pulsations. However, their relatively limited life in orbit, restricted sensitivity interval in wavelength, and finite orientation on the sky all restrict to perform any efficient stellar asteroseismology. The significance of red giants in asteroseismological investigations were emphasized by Christensen-Dalsgaard (2011). The observational instruments of DAG will serve a good opportunity to perform long term monitoring of AGB stars, as well as others, in the context of asteroseismologal studies.

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