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Maurizio Paolillo » 9.X-rays from low-mass and PMS stars


Definition: low mass stars

Schematic representation of the Herzprung Russel diagram with the Sun compared to a young and bright B star.

Schematic representation of the Herzprung Russel diagram with the Sun compared to a young and bright B star.


Initial Mass Function

Stars form with an initial mass distribution represented by the Initial Mass Function or IMF.

The most common parametrization of the IMF is the Sapeter IMF, which has a powe-law slope.

Thus most of the stars in the Galaxy are low-mass stars, with few giants.

Salpeter Initial Mass Function

Salpeter Initial Mass Function


X-rays in main-sequence stars

A proof of the “dynamo” model is represented by the close link between stellar rotation and X-ray emission: stars that rotate quickly have higher levels of X-ray emission than stars that rotate slowly.

This relationship is linear for slowly rotating stars (at normal velocities for stars on the main sequence).

X-ray activity is also related to age: this evidence can be explained if the loss of angular momentum is due to mass loss over the life of the star.

X-ray luminosity vs rotation period: stars that rotate faster are also X-ray brighter

X-ray luminosity vs rotation period: stars that rotate faster are also X-ray brighter

Comparison of X-ray luminosity of star clusters of different ages: the luminosity decreases with time

Comparison of X-ray luminosity of star clusters of different ages: the luminosity decreases with time


Ultrafast rotators

The X-ray behaviour of ultrafast rotating stars is peculiar with respect to the trends observed in low-mass stars:

  • P < 0.5 days;
  • v sin i = 100 – 200 km/s;
  • X-ray emission is below the saturation limit.

Possible reasons:

  1. rotation is so rapid that it contains magnetic flux near the poles;
  2. coronal temperatures are too hot to have been detected by survey instruments.

Close binaries

Close binaries have enhanced X-ray emission.

  • Binary period < a few days.
  • Tidal locking leads to synchronization of rotation and orbital periods.
  • Thus, binary stars are more rapidly rotating than other main-sequence stars of the same age and spectral type.
RS CV systems mat be tidally locked with very stron magnetic fields.

RS CV systems mat be tidally locked with very stron magnetic fields.

Image of a visual bynary sistem.

Image of a visual bynary sistem.


RS CVn Stars

  • Magnetic field are connected.
  • Hot plasma is confined in the region between the two stars.
  • Temperature could reach >107 oK.
The schematic representation of connected magnetic fields in a RS CVn system, where the X-ray plasma can be trapped between the two stars.

The schematic representation of connected magnetic fields in a RS CVn system, where the X-ray plasma can be trapped between the two stars.


X-rays in pre-main-sequence stars

Young stars are known to have copious X-ray activity, well in excess of the one predicted from the dynamo model which explains main sequence stars.

Multiwavelength observations over the last two decades have shown magnetic activity in young pre-main sequence (pre-MS) stars or T Tauri stars (TTs) is greatly enhanced compared to the main sequence Sun.

Why?

The star forming region in the Orion nebula. Optical image. Courtesy of NASA

The star forming region in the Orion nebula. Optical image. Courtesy of NASA

Chandra X-ray image of young stars in the Orion region. Feigelson et al, 2003

Chandra X-ray image of young stars in the Orion region. Feigelson et al, 2003


Characteristics of pre-main-sequence (PMS) stars

  • Rapid rotation (10 – 100 km/s; main-sequence stars have rotational velocities of ~2 km/s).
  • Circumstellar disk material.
  • Entirely convective interior structure.
  • Large surface area (by spectral type).

Correlation with rotation period in PMS stars

Young stars show no correlation between rotational period and X-ray luminosity.

This is in direct contrast to the trend in main-sequence stars, and contradicts the commonly-held perception that high X-ray emission in young stars is a result of faster rotational velocities.

X-ray luminosity vs rotational period for pre-main-sequence stars.

X-ray luminosity vs rotational period for pre-main-sequence stars.


Correlation with presence of a disk in PMS stars

Young stars show no correlation between disk presence and X-ray luminosity.

“Box plots” encompass distributed data. Box centers indicate median values.

X-ray luminosity vs presence of a disk in pre-main –sequence stars.

X-ray luminosity vs presence of a disk in pre-main –sequence stars.


Correlation with bolometric lum. in PMS stars

Young stars show correlation between bolometric and X-ray luminosity.

There is a correlation between bolometric and luminosity and X-ray luminosity, but this is likely due to the larger surface area of stars with greater bolometric luminosity.

X-ray luminosity vs bolometric (total) luminosity in pre-main-sequence stars.

X-ray luminosity vs bolometric (total) luminosity in pre-main-sequence stars.


Alternative sources of X-ray activity in PMS stars

  • Distributed dynamo, solely due to turbulence in the convection zone. Rotation is not a necessary factor.
  • “Fossil magnetic fields” left over from star formation processes, not the product of dynamo activity.
  • Reconnection at interface between stellar surface and disk material. Unlikely due to non-correlation between disk presence and X-rays.
  • Another possibility: young stars are saturated in X-ray surface activity, so that rotation can no longer have an effect on total X-ray emission.

Summary

  • X-rays in low-mass stars are the product of magnetic dynamo activity and magnetic reconnection in the stellar coronae.
  • In “normal” main-sequence stars, X-ray emission is closely related to rotational period.
  • In young stars, the precise X-ray emission mechanism is still a mystery.

I materiali di supporto della lezione

Feigelson et al. 2003. “X-rays in the Orion Nebula cluster: constraints on the origins of magnetic activity in pre-main-sequence stars.”

Feigelson & Montmerle, 1999. “High-energy processes in young stellar objects.”

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