Star formation in young embedded clusters: New observational results using near-infrared detector arrays

Infrared Phys. Technol. Vol. 35, No. 213.pp. 167-174,1994 CopyrIght Q 1994 ElsewerScienceLtd PrmtedIIIGreatBritam.All rights reserved 1350-4495/94 $6.00 + 0.00

Pergamon

STAR FORMATION IN YOUNG EMBEDDED CLUSTERS: NEW OBSERVATIONAL RESULTS USING NEAR-INFRARED DETECTOR ARRAYS KLAUS-WERNER HODAPP Institute

for Astronomy,

University

of Hawaii,

2680 Woodlawn

Drive, Honolulu, HI 96822, U.S.A.

(Received 29 May 1993)

Abstract-A significant fraction of all recently formed stars in the Galaxy are found in embedded

groups and clusters of young stars. Infrared imaging, photometry, and spectroscopy can be used to study the evolutionary history of such young clusters. Initial observations indicate that star formation in clusters is going on for an extended time (x IO6 yr) with only a few stars in the main accretion phase at any given time.

I.

INTRODUCTION

Ever since the discovery of gravitationally unbound associations of young stars it has been assumed that even future field stars are forming in groups or clusters. Near-infrared imaging has confirmed this conjecture by directly showing such clusters of young embedded stars in many of the well-known star-forming regions. A preliminary confirmation that these embedded stars are indeed in the pre-main-sequence phase of their evolution is now usually obtained by multi-color photometry, usually in the J, H and K bands, that typically shows K-band radiation in excess of what would be expected from a reddened main-sequence star. Examples of this type of work are the studies of S106,“’ of Lick Ha lOl,‘*’ and of Serpens. (3) Some of these studies also begin to address the question of the prevalence of clusters and the relative importance of isolated vs clustered star formation. While the existence of embedded clusters of young stars has now been confirmed, many detailed questions remain to be answered. The role that star formation in clusters plays on a galactic scale is not sufficiently understood. Are most new stars currently being formed in clusters? Are there classes of stars, e.g. the most massive ones, that are only being formed in clusters? Is there any evidence for bimodal star formation? On the scale of the individual star, the question must be addressed whether the star-formation process is different for cluster-members and for isolated stars. This question is partly related to the history of star formation in a cluster. If all stars in a cluster form essentially at the same time, the star-formation process may indeed be very different from that forming isolated stars, due to interaction between the gas disks in the accretion phase. On the other hand, if stars in a cluster basically form one-at-a-time, then the individual accretion processes will hardly interfere with each other. Observationally, star-formation in clusters is best studied at NIR wavelengths. Most of the young clusters are deeply embedded in molecular clouds, so that studies at optical wavelengths tend to be restricted to the surface of the cloud and consequently suffer from bias effects. On the other hand, ground-based observations at wavelengths longer than 3 pm are hampered by the background radiation in the optical path. Recent advances in the technology of infrared detector arrays, mostly with HgCdTe and InSb as IR sensitive materials, have made all of the classic optical methods of observation available in the NIR: imaging, photometry, polarimetry, and spectroscopy. In this contribution we will present some preliminary results of a K/-band imaging survey of all bipolar outflow sources listed by FukuP4’ plus additional sources reported during 1990. As an example of the prospects for obtaining more quantitative information from NIR spectroscopy we will also discuss results obtained by us’ on L 1641 North. 167

KLAUS-WERNERHODAPP

168 II.

A detailed

study

of a young

INSTRUMENTS

embedded

cluster

USED

requires

a broad

range

of observational

data.

Thanks to the development of infrared detector arrays, most of the classical methods of optical astronomy can now also be applied in the NIR. The observations summarized in this contribution and other ongoing work in this field were obtained the current state of the art in IR instrumentation:

with the following

instruments

that all represent

l

Infrared imaging and polarimetry were done with the UH IR Camera using a 256 x 256 NICMOS3 HgCdTe device. (6) For polarimetry, this camera was used in combination with a rotating external half-wave plate and fixed wire-grid polarizer in the Dewar.

l

Infrared spectroscopy was obtained using CGS4 on UKIRT with a 58 x 62 InSb detector array. This instrument, in its low resolution mode, allows long slit spectroscopy covering the full K band in one exposure. Ongoing spectroscopic work is done using KSPEC, a recently completed cross-dispersed spectrograph covering the l-2.5 pm region at a spectral resolution of R z 700 in one exposure. Optical photometry, mostly in the R and I bands, was obtained with CCD cameras, most recently with a 2048 x 2048 Tektronix device. For optical point-source polarimetry, a Savart plate was used in front of the CCD cameras.“’

l

l

l

III.

THE

K’ SURVEY

In an effort to study the prevalence of clusters and their typical properties, all sources of molecular outflow listed by Fukui,‘4’ as well as additions to this list published during 1990, were imaged in the K’ band (8) at the UH 2.2 m telescope. Basing the sample on sources of molecular outflow selects regions that have formed at least one new star in the recent past (Z 105yr). Assuming, as is now commonly done, that outflow is a necessary phase in the early evolution of a star, an outflow selected sample is expected to give a complete census of all currently active star-forming regions within the search and sensitivity limits of the outflow surveys. At least for some completely surveyed nearby molecular clouds,(4’ this sample can also be expected to be reasonably complete. The outflow-selected sample avoids the bias towards regions of massive star formation that is intrinsic to samples based on HI1 regions, but it may omit some regions of low present star-formation activity. A total of 164 outflow fields were imaged; the survey field was 8’ x 3’, providing adequate sky coverage to identify most clusters against the background star density. While survey data were also taken under less than photometric conditions, the photometric precision obtained allows to plot useful brightness distribution histograms. Depending on field crowding and nebulosity. the magnitude histograms typically drop to 50% detection at a K’ magnitude of 17. To illustrate some of the relevant cluster properties found in this survey, we will present a few selected examples. The full survey will be submitted to the Astrophysical Journal Supplement Series in a few months.

IV.

EXAMPLES

OF

YOUNG

EMBEDDED

CLUSTERS

Clusters come in different sizes and the lower limit of what one classifies as a cluster is necessarily arbitrary, in particular when considering the limited area coverage of the survey fields and the problem of foreground and background stars. For the purpose of this work we define as a cluster any group of 5 or more stars clearly distinguished from the background or foreground star field by local number density, number density in certain magnitude bins, or physical features such as localized nebulosity.

Young

Fig. I The region

The criteria l

l

l

for identifying

of the L 1535 molecular

a cluster

clusters

169

outflow

source at a distance

may best be illustrated

of 140

pc?

by some examples:

The L 1535 molecular outflow (Fig. I) seems to be a clear case of isolated star formation. The bright outflow source and its associated nebula are the only bright objects visible in the field. Note however that the physical field size is much smaller for this nearby source than for other, more distant ones. The field associated with the Haro 4-255 outflow source (Fig. 2) shows 3 relatively bright stars associated with nebulosity against a background field of uniformly distributed stars. This is clearly a group of young stars, but since it consists of only 3 likely members, it is not called a cluster. Such small groups of stars have been named19) a stellar density enhancement. The region around the outflow source in L 1228 (Fig. 3) represents a different case. There are about 6 roughly equally bright stars in the field, some of them associated with localized nebulosity. A condensation towards the center of the frame is indicated. This is an example of the low end of what we identified as a cluster in this paper.

* .t* f

.

,,

.

. ,*

.

.

”

*

Fig. 2 The region of Haro 4-255 molecular outflow source at a distance of 500 pc.14’ This is a small group of young stars (3) that may be called a stellar density enhancement, but is not classified as a cluster m this work

KLAUS-WERNERHODAPP

170

Fig. 3. K’ survey image of the L 1228 molecular outflow region at a distance of 150 pc w This 1s a very small cluster. basically defining the lower end of what was classified as a cluster.

l

l

l

Figure 4 shows the cluster associated with the molecular outflow L 1641 North. By the number of stars in this field and the contrast to the sky east and west of the cluster, this region clearly qualifies as a cluster. Figure 5 shows the cluster in the L 1654 dark cloud. This cluster is rich in stars, but only very few stars near its center are associated with nebulosity and are therefore regarded as extremely young. In contrast to this, the cluster associated with GGC 12-15 (Fig. 6) and also the L 1641 North cluster (Fig. 4) are much richer in nebulous objects, indicating a higher rate of star formation in the recent past.

By inspection of the images, using the necessarily subjective criteria discussed above. 54 outflow sources (or 33% of all 164 fields) were found to be associated with clusters. The prevalence of nebulosity detectable in the K band can be a potentially useful criterium for a study of the star-formation rate in different molecular clouds. In order to serve this purpose, the “nebulosity criterium” must be calibrated against other, better understood evolutionary indicators.

Fig. 4. K’ survey

Image of the young embedded cluster associated with the L 1641 North outflow source at a distance of 480 pc.

molecular

Young

clusters

Fig. 5. K’ survey image of the young embedded cluster associated with the L 1654 molecular relatively old cluster. source at a distance of 1.1 Kpc. w This is a large and apparently

Assuming as the clusters and that timescale in both cluster to cluster than others (e.g.

outflow

simplest model that the individual star-formation process is the same in the processes leading to the dispersion of localized nebulosity have the cases, one is led to the conclusion that the present star-formation rate differs and that some clusters (e.g. L 1654) have been forming stars for a longer L 1641 North or GGD 12-15).

V. THE

HR DIAGRAM

OF YOUNG

EMBEDDED

both same from time

CLUSTERS

The track of a star in the HR diagram during its pre-main-sequence evolution is relatively well understood, even though questions remain, in particular about the proper definition of the age zero point. Observationally, determining the locus of a star in the HR diagram requires the measurement of its effective temperature and bolometric luminosity. In stars of well understood physical state, the spectral type can be used to determine the effective temperature. In star-forming regions, only stars with a classifiable absorption line spectrum, i.e. among the low-mass stars only the weak line T Tauri stars, are suitable for this purpose. Most

I

I

Fig. 6. K’ survey

I

I

Image of the GGD

I

12-15 embedded

’

cluster

I

at a distance

’

”

I ““““‘I

of 1.0 Kpc.lJJ

172

KLAUS-WERNER HODAPP

1.2

I/II

IltIIIIiI

1.1

IlIIIlIII

L 1641 North

IIllllllijilll

# 32

1.0 0.9 5

0.8

E 0.7 d .jj 0.6 xi 2 2

O5 0.4

EW EW EW EW EW

0.3 0.2

(2.171 (2.211 (2.26) (2.301 12.361 Slope

Continuum

0.1 I

0.0

I

I

I

It

I

I

I

I

I

I

I

7. A

I

I

I

I

I

I

2.20

2.10 Fig

H_bg No I Co-1 CO-20 CO142

Wavelength

ty~lcal low resolution

I

I

I

I

I

I

I

I

I

I

III

III

III

2.40

2.30

[microns]

K band

spectrum

I

Cnml Cnml Cnml [nml Enml [nml

= = = = = =

spectrum of a late type star m a young was obtamed using CGS4 at UKIRT

embedded

cluster

This

spectral classification work on young stars has been done in the K window which is welt suited for this purpose. A broader spectral coverage. say the l-2.5 pm spectral range. is clearly advantageous however. since more spectral features are available. Also, while the extinction is certainty higher at the shorter wavelengths, these are also less affected by thermal emission from warm dust particles near the young star. From the instrumentation point of view, even restricting

I/II/

IgI~~lIII

L 1641 North

,II,f,,lI

1,,11,11,

,,II,

# 31

0.6

2.171 2.211 2.261

. ;*33:1 Slope 2.20 Wavelength Fig 8.

A typlcal

continuum

2.30 [microns]

-0.375 0.025 0.004 -0.104 -0.047 1.561

[nml Cnml Cnml Cnml Enml Cnml

2.40

low resolution K band spectrum of a young star whose spectrum 1s veiled by dust cm~ss~on and rhows Br;, in ernl\slon as the only spectral feature This spectrum was ohtamed using CGSJ at UKIRT.

173

Young clusters

the observations to the l-l.8 pm regions may be an interesting wavelengths are not affected by the ambient temperature radiation

option, since these shorter background, so that existing

(warm) multi-object spectrographs, combined with IR detector arrays, could be used. The measurement of the bolometric luminosity of a star is more difficult. Considering only stars with clearly identified absorption spectra (e.g. Fig. 7) excludes those stars whose NIR luminosity is dominated by accretion disk luminosity and that therefore show continuum or emission-line spectra (e.g. Fig. 8). The (presumed) disk around absorption line stars may still dissipate energy but must radiate this energy at longer (h > 2.5 pm) wavelengths than studied here, so that such effects are separate from the luminosity of the star itself and can therefore be neglected when placing the star in the HR diagram. Depending on the orientation of the disk, it may absorb a fraction of the near IR pre-main-sequence can be corrected

radiation of the star. Within the uncertainties of the intrinsic colors of stars and the extinction laws in dense molecular cloud cores, this absorption for by multicolor photometry and the usual dereddening methods. With the

known spectral type and dereddened magnitude at a certain wavelength, the bolometric magnitude can be computed within the uncertainty of our present knowledge of this correction for pre-main-sequence stars.

VI.

THE

CASE

OF

L 1641

NORTH

In the one case where we have completed a study of this kind, (5)in the small cluster associated with the L 1641 North outflow source, the spectroscopy was obtained at low resolution in the K band. This band contains atomic (H, Ca, Na) and molecular (CO) absorption features that allow both a temperature and luminosity measurement. (lo) Further, the K band is less crowded with lines than, by comparison, the H band, so that the spectral classification can be done on spectra of relatively low resolution (R z 300) that we obtained with CGS4 on UKIRT. The results(5) can only be summarized here: there is a strong correlation between localized nebulosity found around a star and spectra showing strong dust emission continua and Bry emission. The nebulosity found around the stars could be confirmed to be reflection nebulosity by polarimetric measurements. Stars with these characteristics are interpreted as still being surrounded by substantial amounts of matter and as still accreting this matter onto their surfaces and therefore are thought to be very young. This allows to use a simple census of localized reflection nebulosity as a rough indicator for the star-formation rate in the past z 1 x 10h yr. About half of all stars in our sample have classifiable absorption spectra dominated by lines, line-blends, and bandheads of H, Na, Ca and CO, respectively, similar to what was found for sources in p Ophiuchus. (“I Stars for which both spectroscopy and multi-color photometry were available were placed in the HR diagram. It should be noted that most of the stars in the upper right corner of the HR diagram, corresponding to extremely young age and very low mass, have actually been identified as double stars on high resolution I-band images. The other stars. hopefully mostly single stars or double stars with insignificant companion luminosity, lie on evolutionary isochrones between z lo5 yr and a few lo6 yr, with a median age of approx. 0.3 x lo6 yr. Independent of the spectral classification, the K’-band magnitude histogram can give at least a rough estimate of the typical age of stars in a cluster. The pre-main sequence contraction of stars along their Hayashi tracks is temporarily halted by the onset of Deuterium burning, so that stars accumulate in a certain luminosity bin. In a strictly coeval stellar population, stars of different mass reach this Deuterium-burning phase at different times so that. over time, stars accumulate in different K magnitude bins. The peak of the K magnitude histogram (dereddened ideally) after subtraction of the background contribution therefore contains information on the typical age found in a young, almost coeval cluster. (I’) Acknowledging all the difficulties of this method it should be mentioned that it indicates a typical age between 0.3 x lo6 yr and 0.7 x 10’ yr for the L 1641 North cluster, consistent with the result given by the HR diagram.

KLAUS-WERNER HODAPP

174

lsochrones

A B

3 x 104YR 3 x lo5

-

F

_

G

6x106 1 x10' 2 x10'

-

H -3 -CL

’

I

I 4.0

I 3.8

3.6

3.4

Log (T,,/K) Fig. 9. The HR diagram

of all stars with classifiable spectra in L 1641 North.“) are adapted from Cohen and Kuhi.“”

VII.

The evolutionary

tracks

CONCLUSION

The results presented here serve only to illustrate the potential of near infrared observations for the study of star-formation in young clusters. Clearly, more work needs to be done, in the field of instrumentation, in the calibration of observational methods such as spectral classification, and in the theoretic understanding of pre-main sequence evolution and of the observable characteristics of young stars. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

K.-W. Hodapp and J. Rayner. Astron. J. 102, 1108 (1991). M. Barsony, J. M. Schombert and K. Kis-Halas. Astrophys. J. 379. 221 (1991). C. Eiroa and M. M. Casah, Asrron. Astrophys. 262, 468 (1992) Y. Fukui, Proc. ES0 Workshop on Low-Mass Star Formalion and Pre-Main Sequence Objects (Edtted by B. Reipurth), p. 95. ESO, Garching, (1989), and references therem. K.-W. Hodapp and J. Deane, Astrophys. J. Suppl. 88, 119 (1993). K.-W. Hodapp. J. Rayner and E. Irwin. PASP 104, 441 (1992). K.-W. Hodapp. J. L. Hora, E. Irwin and T. Young, PASP In press (1994). R. J. Wainscoat and L. L. Cowie, Asfron. J 103, 332 (1992). H. Chen. A T Tokunaga, K. M. Strom and K.-W. Hodapp, Astrophys J. 407, 639 (1993). S. G. Kleinmann and D. N. B. Hall, Astrophys. J. Suppl. 62, 501 (1986). M. M. Casali and H. E. Matthews, Mon. Not R. Astron. Sot. 258, 399, (1992). H. Zmnecker, M. J. McCaughrean and B. Wilking, Profostars and Planers III, (Edited by E. H. Levy, J. I. Lumne and M S. Matthews). Umversity of Arizona, Tucson. In press. M. Cohen and L. V. Kuhn, Astrophys. J. Suppl. 41, 743 (1979).