Studying stellar light : spectroscopyassociation at 600 pc, 3 subgroups; similar to Sco OB2, but more distant 2. Robichon et al. (1999): Cluster at ~520 pc Classical Results: Small,

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Stellar PhotometryStudying stellar light : spectroscopy

Obtaining spectra

A telescope with a spectrograph measures the spectrum of a star and gives the brightness at different wavelengths.

Different stars –different spectra –

different stellar typesObtaining photometry

Telescope + set of colored filters.

The intensity of a star's image is different at different filters, and photometric data can thus be used to study the distribution of the star's radiation over the wavelength's scale.

Studying stellar light : photometry

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Spectroscopy or Photometry

Large samples of stars - stellar photometry is the most efficient technique

About 170 photometric systems are presently available to study different aspects of stellar physics and stellar systems.

band c (Å) WHM (Å)

U 3580 550

B 4390 990

V 5450 850

UBV photometric system- Johnson and Morgan - 1953

The technique of stellar photometry

Stellar Photometry

Contradiction between system’s information power and effectiveness

• Broad band systems - more observations, low classification power • Intermediate band systems – less observations, better classification power• Narrow band observations – good classification power

Stellar Classification

• Spectral classification– surface temperature (T)

• Luminosity class– LC is related to the electron density Ne in the star’s

photosphere (which in turn effects line strengths through ionization and recombination), Enables us to recognize stars of the same size (R) and luminosity (L)

Recall: SB Law: L = 4πR2 σT4

Example: Sun = G2 V star

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Same Tbut ↑ L and ↑ R

Supergiants

Giants

Dwarf

Spectral type and luminosity class uniquely determine a stars position in the HR diagram

High luminositylow Ne

Supergiants

Low luminosityhigh Ne

Main sequence

• photometry (contaminated by absorption)

• stellar classification- find a smart photometric system or use spectra

• need to correct the photometry for the reddening - need calibrations for the Sp&LC- need a sample of nearby non-reddened stars

• intrinsic brightness - need calibrations depending on Sp&LC- need a sample of nearby stars with distance obtained

independently of the physical parameters (trigonometric parallaxes)

Many nearby A, F, G, K, M stars No nearby O, B stars

The technique of stellar photometry

Grounded in distance determinations of bright objects located in the spiral arms

Stellar Photometry Approach

Contradiction between system’s information power and effectiveness

uvbyβ -- both efficient and informative

The spiral structure of the Milky Way

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Spiral Structure of the Milky Way

Spiral arms look strung out along the line of sight Different parts overlapped Need distances to bright spiral structure tracersOur understanding entirely depends on observational data

Star-forming fields -- building blocks of spiral arms

Tracing star-forming fields -- OB stars30 large star-forming fields in MW

Band λ (Å) ∆λ(Å)

u 3500 300

v 4110 190

b 4670 180

y 5470 230

Hβ n 4859 30

Hβ w 4890 145

uvbyβ photometric system - Strömgren and Crawford (1970)

Huge improvement over the UBV photometry.Measure important spectral details Easily to calibrate in terms of temperature, metallicity and intrinsic brightness

The technique of stellar photometry

uvbyβ photometric system - Strömgren and Crawford (1970) The technique of stellar photometry Empirical intrinsic-color

calibrations for O&B stars

Need to know:•LC•Idea about Sp (sub)type

III, IV,V - Crawford (1978) / I, II - Kilkenny & Whittet (1985)

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Empirical luminosity calibrations for O&B type stars

•Crawford (1978)•Eggen (1981)•Philip and Egret (1981) •Balona & Shobbrook (1984) (for MS and evolved stars)•Schönberner & Harmanec (1995)

• No hot stars in the solar vicinity •Low MS of very young clusters - still contracting•Evolution of the upper MS - fast and hard to detect

Schönberner & Harmanec, 1995Philip & Egret, 1980

Crawford, 1978

Balona & Shobbrook, 1984

Empirical Mv calibrations for early-type stars

σπ/π < 20 %LC III, IV, V

Tests of upper MS via Hipparcos

Spectroscopic MV Photometric MV

σπ/π < 40 %

Tests for BI, II via Hipparcos

Spectroscopic MVPhotometric MV

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uvbyβ photometric system – photometry derived parameters

Precise enough to allow studyof the structure of distant (5-6 kpc)

star-forming regions

Consistent with the astrometric distances

Accuracy:15-20 % for a single star

~15 % for a group of stars

uvbyβ photometry – reliable approach for studying MW spiral structure

• Measures important spectral details• Efficient • Well calibrated • Calibrations consistent with space observations• Accurate stellar parameters • A significant amount of data accumulated

Studying star-forming fields Ara – Norma Field

Carina – Crux – CentaurusCarOB1, CarOB2, Tr16, Cr 228, Crux OB1, 2 …

Vela- Puppis-Canis Major Cr 121, NGC 2439

Monoceros OB2

Carina Spiral Feature

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Carina Spiral Feature

NGC 3766

IC 2944

Cr 240NGC 3293

NGC 3114

Cr 228Bo 10

• magnitude-limited (~ 9.3 mag) sample of O and B stars is collated

• the available catalogues are searched in order to avoid stars with peculiar spectra, double and multiple stars

• the photometric diagrams are analyzed to reveal spectral peculiarities and spectral misclassification that can affect the derived stellar physical parameters

Survey of the Carina Field based on uvbyβ photometry

Survey of the Carina Field based on uvbyβ photometry

• color excesses are derived • stellar distances are calculated • spatial distribution is investigated• re-examination and revision of the characteristic of the field is performed

Vela-Puppis-Canis Major

225 OB stars

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Vela-Puppis-Canis Major

Vela Group

CMa OB1Pup OB1/2?

Gal Long

NGC2439

“The 4SG”

Vela GroupSpatial structure and membership to 2.5 kpc

CMa OB1

Separation of CMaOB1 and background structures

Norma Spiral Arm

Norma-Scutum Arm

Carina-SagittariusArm

The discovery of Norma-Scutum arm

First radio-wave map of the MW

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Norma-Scutum Arm

SagittariusArm (-1)

(-2)

NormaAra

Triangulum

• Several prominent groups of young stars have been distinguished

• Four of the groups are within the Sagittarius arm

• Some are clearly beyond the Sagittarius Arm

• May belong to the other edge of the Norma Arm

• May belong to interarm links or bridges between the two arms

Young clusters and associations toward Normaplotted at their estimated distances

Tracing Norma arm using young stars:structure of the field - results from literature

mid 80s - first map toward Norma

The membership to different groups indicated:

Associations of stars:• Ara OB1 and AraOB2• Norma OB1Groups connected with Supernova remnants:• R 103• R 105Clusters of stars:• Lodden 2158• Lynga 6 • NGC 6087

Ara OB1/2

Norma OB1

R103

Lynga 6

Lod 2158

NGC 6087

R105

0

4

-4

-8

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Norma Field: the uvbyβ sample (~150 stars)

clusters of young stars (filled light-blue circles)

All low-massive groups are consistent with the local arm.

associations of young stars (filled dark-blue circles)

All intrinsically luminous groups are back to the Sagittarius arm (where they should belong).

uvbyβ photometry of Norma: results

The newly obtained locations of young stellar groups in Norma

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Collinder 121

Hipparcos results1. de Zeeuw et al. (1999):Large unbound OB association at 600 pc, 3 subgroups; similar toSco OB2, but more distant

2. Robichon et al. (1999):Cluster at ~520 pc

Classical Results:Small, compact, low reddened,~1.3 kpc

Collinder 121

loose nearby structure atmean distance 660±730 pc

compact more distant group1085±41(s.e.) pc

uvbyβ sample (~150 stars)

Strömgren classification diagram