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Abstract.We have pursued two different methods to analyze the old stellar population near the Galactic plane, using data from the 2MASS survey. The first method is based on the isolation of the red clump giant population in the color-magnitude diagrams and the inversion of its star counts to obtain directly the density distribution along the line of sight. The second method fits the parameters of a disc model to the star counts in 820 regions. Results from both independent methods are consistent with each other. The qualitative conclusions are that the disc is well fitted by an exponential distribution in both the galactocentric distance and height. There is not an abrupt cut-off in the stellar disc (at least within R < 15 kpc). There is a strong flare (i.e. an increase of scale-height towards the outer Galaxy) which begins well inside the solar circle, and hence there is a decrease of the scale-height towards the inner Galaxy. Another notable feature is the existence of a warp in the old stellar population whose amplitude is coincident with the amplitude of the gas warp. It is shown for low latitude stars (mean height: |z| ∼ 300 pc) in the outer disc (galactocentric radius R > 6 kpc) that: the scaleheight in the solar circle is h z (R ) = 3.6 × 10 −2 R , the scale-length of the surface density is h R = 0.42 R and the scale-length of the space density in the plane (i.e. including the effect of the flare) is H = 0.25 R . The variation of the scale-height due to the flare follows roughly a law h z (R) ≈ h z (R ) exp R−R [12−0.6R(kpc)] kpc (for R < ∼ 15 kpc; R = 7.9 kpc). The warp moves the mean position of the disc to a height z w = 1.2 × 10 −3 R(kpc) 5.25 sin(φ + 5 • ) pc (for R < ∼ 13 kpc; R = 7.9 kpc).

We present near-IR colour--magnitude diagrams and star counts for a number of regions along the Galactic plane. It is shown that along the l=27 b=0 line of sight there is a feature at 5.7 +-0.7kpc with a density of stars at least a factor two and probably more than a factor five times that of the disc at the same position. This feature forms a distinct clump on an H vs. J-H diagram and is seen at all longitudes from the bulge to about l=28, but at no longitude greater than this. The distance to the feature at l=20 is about 0.5kpc further than at l=27 and by l=10 it has merged with, or has become, the bulge. Given that at l=27 and l=21 there is also a clustering of very young stars, the only component that can reasonably explain what is seen is a bar with half length of around 4kpc and a position angle of about 43+-7.Comment: 5 pages, 5 figures accepted as a letter in MNRA

We describe the technique that will be used to develop a set of on-orbit calibrators for IRAC and demonstrate the validity of the method for stars with spectral types either K0-M0III or A0-A5V. For application to SIRTF, the approach is intended to operate with all available optical, near-infrared (NIR), and mid-infrared (MIR) photometry, and to yield complete absolute spectra from UV to MIR. One set of stars is picked from Landolt's extensive network of optical (UBV RI) calibrators; the other from the Carter-Meadows set of faint infrared standards. Traceability to the "Cohen-Walker-Witteborn" framework of absolute photometry and stellar spectra is assured (Cohen, Walker, & Witteborn 1992a).The method is based on the use of either "supertemplates", that represent the intrinsic shapes of the spectra of K0-M0III stars from far-ultraviolet (1150Å) to MIR (35 µm) wavelengths, or Kurucz synthetic spectra for A0-5V stars. Each supertemplate or Kurucz model is reddened according to the individual star's extinction and is normalized using available characterized optical photometry. This paper tests our capability to predict NIR (JHK) magnitudes using supertemplates or models constrained by Hipparcos/Tycho or precision ground-based optical data. We provide absolutely calibrated 0.275-35.00 µm spectra of thirty three Landolt or Carter-Meadows optical standard stars to demonstrate the viability of this technique, and to offer a set of IR calibrators 100-1000 times fainter than those we have previously published. As an indication of what we can expect for actual IRAC calibration stars, we have calculated the absolute uncertainties associated with predicting the IRAC magnitudes for the faintest cool giant and hot dwarf in this new set of calibration stars.

Context. Over the last decade a series of results have lent support to the hypothesis of the existence of a long thin bar in the Milky Way with a half-length of 4.5 kpc and a position angle of around 45• . This is apparently a very different structure from the triaxial bulge of the Galaxy. Aims. In this paper, we analyse the stellar distribution in the inner 4 kpc of the Galaxy to see if there is clear evidence for two triaxial or barlike structures, or whether there is only one. Methods. By using the red-clump population as a tracer of the structure of the inner Galaxy we determine the apparent morphology of the inner Galaxy. Star counts from 2MASS are used to provide additional support for this analysis. Results. We show that there are two very different large-scale triaxial structures coexisting in the inner Galaxy: a long thin stellar bar constrained to the Galactic plane (|b| < 2• ) with a position angle of 43.• 0 ± 1.• 8, and a distinct triaxial bulge that extends to at least |b| ≤ 7.5• with a position angle of 12.• 6 ± 3.• 2. The scale height of the bar source distribution is around 100 pc, whereas for the bulge the value of this parameter is five times larger.