Publications of the Astronomical Society of the Pacific

Wallerstein, George

doi: 10.1086/341698pmid: N/A

The Type II Cepheids include most intrinsic variables with periods between 1 and about 50 days, except for the classical Cepheids and the shortest semiregular variables of type M. The Type II Cepheids may be divided in groups by period, such that the stars with periods beween 1 and 5 days (BL Her class), 10–20 days (W Vir class), and greater than 20 days (RV Tau class) have differing evolutionary histories. The chemical composition of Type II Cepheids reflects the material they were made from as modified by their internal nuclear evolution and mixing. Finally, RV Tau stars are affected by mass loss by dust and species attached to the dust. The populations to which the various classes of Type II Cepheids are assigned constitute important clues to the origin and evolution of the halo of our Galaxy and the dwarf spheroidal systems from which at least part of the halo seems to have been accreted.

Van Dyk, Schuyler D.; Filippenko, Alexei V.; Li, Weidong

doi: 10.1086/341695pmid: N/A

SN 1961V in NGC 1058 was originally classified by Fritz Zwicky as a “Type V” supernova. However, it has been argued that SN 1961V was not a genuine supernova, but instead the superoutburst of an η Carinae–like luminous blue variable star. In particular, Filippenko et al. used pre‐refurbishment Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC) images and the known radio position of SN 1961V to conclude that the star survived the eruption and is likely coincident with a V ≈ 25.6, V - I ≈ 1.9 mag object. Recently, Stockdale et al. recovered the fading SN 1961V at radio wavelengths and argue that its behavior is similar that of some Type II supernovae. We have analyzed post‐refurbishment archival HST WFPC2 data and find that the new radio position is still consistent with the Filippenko et al. object, which has not changed in brightness or color, but is also consistent with an adjacent, fainter (I ≈ 24.3 mag), and very red (V− I>1.0 mag) object. We suggest that this fainter object could be the survivor of SN 1961V. Forthcoming HST observations may settle this issue.

Hillenbrand, Lynne A.; Foster, Jonathan B.; Persson, S. E.; Matthews, K.

doi: 10.1086/341699pmid: N/A

We define and characterize a photometric bandpass (called “Y”) that is centered near 1.035 μm, in between the traditionally classified “optical” and “infrared” spectral regimes. We present Y magnitudes and Y−H and Y−K colors for a sample consisting mostly of photometric and spectral standards, spanning the spectral type range sdO to T5 V. Deep molecular absorption features in the near‐infrared spectra of extremely cool objects are such that the Y−H and Y−K colors grow rapidly with advancing spectral type especially from late M through mid‐L, substantially more rapidly than J−H or H−K, which span a smaller total dynamic range. Consistent with other near‐infrared colors, however, Y−H and Y−K colors turn blueward in the L6–L8 temperature range, with later T‐type objects having colors similar to those of warmer M and L stars. Y−J colors remain constant at 1.0 ± 0.15 mag from early‐L through late‐T dwarfs. The slope of the interstellar reddening vector within this filter is AY = 0.38AV. Reddening moves stars nearly along the YHK dwarf color sequence, making it more difficult to distinguish unambiguously very low mass candidate brown dwarf objects from higher mass stars seen, e.g., through the Galactic plane or toward star‐forming regions. Other diagrams involving the Y band may be somewhat more discriminating.

Patterson, Joseph; Masi, Gianluca; Richmond, Michael W.; Martin, Brian; Beshore, Edward; Skillman, David R.; Kemp, Jonathan; Vanmunster, Tonny; Rea, Robert; Allen, William; Davis, Stacey; Davis, Tracy; Henden, Arne A.; Starkey, Donn; Foote, Jerry; Oksanen, Arto;

Howell, Steve B.; Fried, Robert; Szkody, Paula; Sirk, Martin M.; Schmidt, Gary

doi: 10.1086/341702pmid: N/A

We obtained EUV photometry, optical spectroscopy, and multicolor optical photometry for WX Cet during its 1998 November superoutburst. WX Cet is only the second short‐period, low mass transfer cataclysmic variable (TOAD) to ever be observed in the EUV. Our determined superhump period is consistent with that found by Kato et al. (0.059 day), and we confirm that superhumps are gray in the optical. The optical spectra provide direct evidence that the line emission region is optically thick, and our multiwavelength photometric measurements are used to determine the inner accretion disk and boundary layer temperatures during superoutburst. Using a determined distance to WX Cet of ∼130 pc, we find TID = 21,000 K and TBL ∼ 72,500 K. Both values are in good agreement with that expected by models of the superoutburst continuum being produced by the inner disk and boundary layer.

Hillwig, Todd C.; Gale, Ashley A.; Honeycutt, R. Kent; Rengstorf, Adam W.

doi: 10.1086/341700pmid: N/A

We present orbit‐sampled photometry of the binary system WD 2154+408. The photometric study reveals a period of P = 0.26772 day, which is taken to be the orbital period of the binary. No evidence is seen for either an accretion disk or mass transfer, leading to the conclusion that WD 2154+408 is not a contact or semidetached system but has detached components with a visible irradiation effect. The short orbital period also leads to the conclusion that the system passed through a common‐envelope phase at some time in the past. A single spectrum is shown exhibiting both narrow emission and broad absorption components of the Hβ line. This and the photometric variations are consistent with the system being a post–common‐envelope binary consisting of a hot white dwarf primary and a cool irradiated companion.

Hutchings, J. B.; Stetson, P. B.; Robin, A.; Webb, T.

doi: 10.1086/341704pmid: N/A

We discuss near‐infrared star counts at the Galactic pole with a view to guiding the Next Generation Space Telescope and ground‐based near‐infrared cameras. Star counts from deep K‐band images from the Canada‐France‐Hawaii Telescope are presented and compared with results from the 2MASS survey and some galaxy models. With appropriate corrections for detector artifacts and galaxies, the data agree with the models down to K ∼ 18 but indicate a larger population of fainter red stars. There is also a significant population of compact galaxies that extend to the observational faint limit of K = 20.5. Recent galaxy models agree well down to K ∼ 19 but diverge at fainter magnitudes.

Qian, Shengbang; Liu, Dengliang; Tan, Wenli; Soonthornthum, Boonrucksar

doi: 10.1086/341703pmid: N/A

An analysis of the times of minimum light for the long‐period Algol‐type eclipsing binary RX Gem is presented based on a new linear ephemeris. The O - C curve shows a cyclic oscillation with a period of 55.7 yr and a semiamplitude of 0.0645 day. Assuming the change to be due to the presence of a “third body” revolving around the RX Gem system, the parameters of the third body’s orbit are derived. Since the third‐body assumption is in good agreement with the spectroscopic data from several authors and with published photometric solutions (Gaposchkin, Hall, & Walter; Giuricin et al.), RX Gem is likely to be a triple system. In this case, the third body is an A‐type star in a circular orbit, which is nearly coplanar to the orbit of the eclipsing pair. However, the recent light‐curve analysis by Olson & Etzel does not show any third light, so the third star (M3>2.41 M⊙) may be an unseen neutron star or black hole. Additional eclipse timings over the next decade will be important to verify the presence of the third body.

Li Causi, Gianluca; De Marchi, Guido; Paresce, Francesco

doi: 10.1086/341701pmid: N/A

We have studied the accuracy and reliability of the exposure‐time calculator (ETC) of the Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST) with the objective of determining how well it represents actual observations and therefore how much confidence can be invested in it and in similar software tools. We have found, for example, that the ETC gives, in certain circumstances, very optimistic values for the signal‐to‐noise ratio (S/N) of point sources. These values overestimate by up to a factor of 2 the HST performance when simulations are needed to plan deep‐imaging observations, thus bearing serious implications on observing‐time allocation. For this particular case, we calculate the corrective factors to compute the appropriate S/N and detection limits, and we show how these corrections vary with field crowding and sky background. We also compare the ETC of the WFPC2 with a more general ETC tool, which takes into account the real effects of pixel size and charge diffusion. Our analysis indicates that similar problems may afflict other ETCs in general, showing the limits to which they are bound and the caution with which their results must be taken.

Baltay, C.; Snyder, J. A.; Andrews, P.; Emmet, W.; Schaefer, B.; Sinnott, J.; Bailyn, C.; Coppi, P.; Oemler, A.; Sabbey, C. N.; Sofia, S.; van Altena, W.; Vivas, A. K.; Abad, C.; Bongiovanni, A.; Briceño, C.; Bruzual, G.;